Dry-Farming : A System of Agriculture for Countries under a Low Rainfall
by John Andreas Widtsoe · English · Gutenberg #4924 · 472,341 chars · public domain (US)
The Project Gutenberg eBook of Dry-Farming : A System of Agriculture for Countries under a Low Rainfall
This eBook is for the use of anyone anywhere in the United States and
most other parts of the world at no cost and with almost no restrictions
whatsoever. You may copy it, give it away or re-use it under the terms
of the Project Gutenberg License included with this eBook or online
at www.gutenberg.org. If you are not located in the United States,
you will have to check the laws of the country where you are located
before using this eBook.
Title: Dry-Farming : A System of Agriculture for Countries under a Low Rainfall
Author: John Andreas Widtsoe
Release date: January 1, 2004 [eBook #4924]
Most recently updated: July 17, 2025
Language: English
Other information and formats: www.gutenberg.org/ebooks/4924
Credits: Charles Aldarondo
*** START OF THE PROJECT GUTENBERG EBOOK DRY-FARMING : A SYSTEM OF AGRICULTURE FOR COUNTRIES UNDER A LOW RAINFALL ***
DRY-FARMING
A SYSTEM OF AGRICULTURE FOR COUNTRIES UNDER LOW RAINFALL
BY JOHN A. WIDTSOE, A.M., Ph. D
PRESIDENT OF THE AGRICULTURAL COLLEGE OF UTAH
NEW YORK
1920
TO
LEAH
THIS BOOK IS INSCRIBED
JUNE 1, 1910
PREFACE
Nearly six tenths of the earth's land surface receive an annual
rainfall of less than twenty inches, and can be reclaimed for
agricultural purposes only by irrigation and dry-farming. A
perfected world-system of irrigation will convert about one tenth of
this vast area into an incomparably fruitful garden, leaving about
one half of the earth's land surface to be reclaimed, if at all, by
the methods of dry-farming. The noble system of modern agriculture
has been constructed almost wholly in countries of abundant
rainfall, and its applications are those demanded for the
agricultural development of humid regions. Until recently irrigation
was given scant attention, and dry-farming, with its world problem
of conquering one half of the earth, was not considered. These facts
furnish the apology for the writing of this book.
One volume, only, in this world of many books, and that less than a
year old, is devoted to the exposition of the accepted dry-farm
practices of to-day.
The book now offered is the first attempt to assemble and organize
the known facts of science in their relation to the production of
plants, without irrigation, in regions of limited rainfall. The
needs of the actual farmer, who must understand the principles
before his practices can be wholly satisfactory, have been kept in
view primarily; but it is hoped that the enlarging group of dry-farm
investigators will also be helped by this presentation of the
principles of dry-farming. The subject is now growing so rapidly
that there will soon be room for two classes of treatment: one for
the farmer, and one for the technical student.
This book has been written far from large libraries, and the
material has been drawn from the available sources. Specific
references are not given in the text, but the names of investigators
or institutions are found with nearly all statements of fact. The
files of the Experiment Station Record and Der Jahresbericht der
Agrikultur Chemie have taken the place of the more desirable
original publications. Free use has been made of the publications of
the experiment stations and the United States Department of
Agriculture. Inspiration and suggestions have been sought and found
constantly in the works of the princes of American soil
investigation, Hilgard of California and King of Wisconsin. I am
under deep obligation, for assistance rendered, to numerous friends
in all parts of the country, especially to Professor L. A. Merrill,
with whom I have collaborated for many years in the study of the
possibilities of dry-farming in Western America.
The possibilities of dry-farming are stupendous. In the strength of
youth we may have felt envious of the great ones of old; of Columbus
looking upon the shadow of the greatest continent; of Balboa
shouting greetings to the resting Pacific; of Father Escalante,
pondering upon the mystery of the world, alone, near the shores of
America's Dead Sea. We need harbor no such envyings, for in the
conquest of the nonirrigated and nonirrigable desert are offered as
fine opportunities as the world has known to the makers and shakers
of empires. We stand before an undiscovered land; through the
restless, ascending currents of heated desert air the vision comes
and goes. With striving eyes the desert is seen covered with
blossoming fields, with churches and homes and schools, and, in the
distance, with the vision is heard the laughter of happy children.
The desert will be conquered.
JOHN A. WIDTSOE.
June 1, 1910.
CHAPTER I
INTRODUCTION
DRY-FARMING DEFINED
Dry-farming, as at present understood, is the profitable production
of useful crops, without irrigation, on lands that receive annually
a rainfall of 20 inches or less. In districts of torrential rains,
high winds, unfavorable distribution of the rainfall, or other
water-dissipating factors, the term "dry-farming" is also properly
applied to farming without irrigation under an annual precipitation
of 25 or even 30 inches. There is no sharp demarcation between
dry-and humid-farming.
When the annual precipitation is under 20 inches, the methods of
dry-farming are usually indispensable. When it is over 30 inches,
the methods of humid-farming are employed; in places where the
annual precipitation is between 20 and 30 inches, the methods to be
used depend chiefly on local conditions affecting the conservation
of soil moisture. Dry-farming, however, always implies farming under
a comparatively small annual rainfall.
The term "dry-farming" is, of course, a misnomer. In reality it is
farming under drier conditions than those prevailing in the
countries in which scientific agriculture originated. Many
suggestions for a better name have been made. "Scientific
agriculture" has-been proposed, but all agriculture should be
scientific, and agriculture without irrigation in an arid country
has no right to lay sole claim to so general a title. "Dry-land
agriculture," which has also been suggested, is no improvement over
"dry-farming," as it is longer and also carries with it the idea of
dryness. Instead of the name "dry-farming" it would, perhaps, be
better to use the names, "arid-farming." "semiarid-farming,"
"humid-farming," and "irrigation-farming," according to the climatic
conditions prevailing in various parts of the world. However, at the
present time the name "dry-farming" is in such general use that it
would seem unwise to suggest any change. It should be used with the
distinct understanding that as far as the word "dry" is concerned it
is a misnomer. When the two words are hyphenated, however, a
compound technical term--"dry-farming"--is secured which has a
meaning of its own, such as we have just defined it to be; and
"dry-farming," therefore, becomes an addition to the lexicon.
Dry-versus humid-farming
Dry-farming, as a distinct branch of agriculture, has for its
purpose the reclamation, for the use of man, of the vast unirrigable
"desert" or "semi-desert" areas of the world, which until recently
were considered hopelessly barren. The great underlying principles
of agriculture are the same the world over, yet the emphasis to be
placed on the different agricultural theories and practices must be
shifted in accordance with regional conditions. The agricultural
problem of first importance in humid regions is the maintenance of
soil fertility; and since modern agriculture was developed almost
wholly under humid conditions, the system of scientific agriculture
has for its central idea the maintenance of soil fertility. In arid
regions, on the other hand, the conservation of the natural water
precipitation for crop production is the important problem; and a
new system of agriculture must therefore be constructed, on the
basis of the old principles, but with the conservation of the
natural precipitation as the central idea. The system of dry-farming
must marshal and organize all the established facts of science for
the better utilization, in plant growth, of a limited rainfall. The
excellent teachings of humid agriculture respecting the maintenance
of soil fertility will be of high value in the development of
dry-farming, and the firm establishment of right methods of
conserving and using the natural precipitation will undoubtedly have
a beneficial effect upon the practice of humid agriculture.
The problems of dry-farming
The dry-farmer, at the outset, should know with comparative accuracy
the annual rainfall over the area that he intends to cultivate. He
must also have a good acquaintance with the nature of the soil, not
only as regards its plant-food content, but as to its power to
receive and retain the water from rain and snow. In fact, a
knowledge of the soil is indispensable in successful dry-farming.
Only by such knowledge of the rainfall and the soil is he able to
adapt the principles outlined in this volume to his special needs.
Since, under dry-farm conditions, water is the limiting factor of
production, the primary problem of dry-farming is the most effective
storage in the soil of the natural precipitation. Only the water,
safely stored in the soil within reach of the roots, can be used in
crop production. Of nearly equal importance is the problem of
keeping the water in the soil until it is needed by plants. During
the growing season, water may be lost from the soil by downward
drainage or by evaporation from the surface. It becomes necessary,
therefore, to determine under what conditions the natural
precipitation stored in the soil moves downward and by what means
surface evaporation may be prevented or regulated. The soil-water,
of real use to plants, is that taken up by the roots and finally
evaporated from the leaves. A large part of the water stored in the
soil is thus used. The methods whereby this direct draft of plants
on the soil-moisture may be regulated are, naturally, of the utmost
importance to the dry-farmer, and they constitute another vital
problem of the science of dry-farming.
The relation of crops to the prevailing conditions of arid lands
offers another group of important dry-farm problems. Some plants use
much less water than others. Some attain maturity quickly, and in
that way become desirable for dry-farming. Still other crops, grown
under humid conditions, may easily be adapted to dry-farming
conditions, if the correct methods are employed, and in a few
seasons may be made valuable dry-farm crops. The individual
characteristics of each crop should be known as they relate
themselves to a low rainfall and arid soils.
After a crop has been chosen, skill and knowledge are needed in the
proper seeding, tillage, and harvesting of the crop. Failures
frequently result from the want of adapting the crop treatment to
arid conditions.
After the crop has been gathered and stored, its proper use is
another problem for the dry-farmer. The composition of dry-farm
crops is different from that of crops grown with an abundance of
water. Usually, dry-farm crops are much more nutritious and
therefore should command a higher price in the markets, or should be
fed to stock in corresponding proportions and combinations.
The fundamental problems of dry-farming are, then, the storage in
the soil of a small annual rainfall; the retention in the soil of
the moisture until it is needed by plants; the prevention of the
direct evaporation of soil-moisture during; the growing season; the
regulation of the amount of water drawn from the soil by plants; the
choice of crops suitable for growth under arid conditions; the
application of suitable crop treatments, and the disposal of
dry-farm products, based upon the superior composition of plants
grown with small amounts of water. Around these fundamental problems
cluster a host of minor, though also important, problems. When the
methods of dry-farming are understood and practiced, the practice is
always successful; but it requires more intelligence, more implicit
obedience to nature's laws, and greater vigilance, than farming in
countries of abundant rainfall.
The chapters that follow will deal almost wholly with the problems
above outlined as they present themselves in the construction of a
rational system of farming without irrigation in countries of
limited rainfall.
CHAPTER II
THE THEORETICAL BASIS OF DRY-FARMING
The confidence with which scientific investigators, familiar with
the arid regions, have attacked the problems of dry-farming rests
largely on the known relationship of the water requirements of
plants to the natural precipitation of rain and snow. It is a most
elementary fact of plant physiology that no plant can live and grow
unless it has at its disposal a sufficient amount of water.
The water used by plants is almost entirely taken from the soil by
the minute root-hairs radiating from the roots. The water thus taken
into the plants is passed upward through the stem to the leaves,
where it is finally evaporated. There is, therefore, a more or less
constant stream of water passing through the plant from the roots to
the leaves.
By various methods it is possible to measure the water thus taken
from the soil. While this process of taking water from the soil is
going on within the plant, a certain amount of soil-moisture is also
lost by direct evaporation from the soil surface. In dry-farm
sections, soil-moisture is lost only by these two methods; for
wherever the rainfall is sufficient to cause drainage from deep
soils, humid conditions prevail.
Water for one pound dry matter
Many experiments have been conducted to determine the amount of
water used in the production of one pound of dry plant substance.
Generally, the method of the experiments has been to grow plants in
large pots containing weighed quantities of soil. As needed, weighed
amounts of water were added to the pots. To determine the loss of
water, the pots were weighed at regular intervals of three days to
one week. At harvest time, the weight of dry matter was carefully
determined for each pot. Since the water lost by the pots was also
known, the pounds of water used for the production of every pound of
dry matter were readily calculated.
The first reliable experiments of the kind were undertaken under
humid conditions in Germany and other European countries. From the
mass of results, some have been selected and presented in the
following table. The work was done by the famous German
investigators, Wollny, Hellriegel, and Sorauer, in the early
eighties of the last century. In every case, the numbers in the
table represent the number of pounds of water used for the
production of one pound of ripened dry substance:
Pounds Of Water For One Pound Of Dry Matter
Wollny Hellreigel Sorauer
Wheat 338 459
Oats 665 376 569
Barley 310 431
Rye 774 353 236
Corn 233
Buckwheat 646 363
Peas 416 273
Horsebeans 282
Red clover 310
Sunflowers 490
Millet 447
It is clear from the above results, obtained in Germany, that the
amount of water required to produce a pound of dry matter is not the
same for all plants, nor is it the same under all conditions for the
same plant. In fact, as will be shown in a later chapter, the water
requirements of any crop depend upon numerous factors, more or less
controllable. The range of the above German results is from 233 to
774 pounds, with an average of about 419 pounds of water for each
pound of dry matter produced.
During the late eighties and early nineties, King conducted
experiments similar to the earlier German experiments, to determine
the water requirements of crops under Wisconsin conditions. A
summary of the results of these extensive and carefully conducted
experiments is as follows:--
Oats 385
Barley 464
Corn 271
Peas 477
Clover 576
Potatoes 385
The figures in the above table, averaging about 446 pounds, indicate
that very nearly the same quantity of water is required for the
production of crops in Wisconsin as in Germany. The Wisconsin
results tend to be somewhat higher than those obtained in Europe,
but the difference is small.
It is a settled principle of science, as will be more fully
discussed later, that the amount of water evaporated from the soil
and transpired by plant leaves increases materially with an increase
in the average temperature during the growing season, and is much
higher under a clear sky and in districts where the atmosphere is
dry. Wherever dry-farming is likely to be practiced, a moderately
high temperature, a cloudless sky, and a dry atmosphere are the
prevailing conditions. It appeared probable therefore, that in arid
countries the amount of water required for the production of one
pound of dry matter would be higher than in the humid regions of
Germany and Wisconsin. To secure information on this subject,
Widtsoe and Merrill undertook, in 1900, a series of experiments in
Utah, which were conducted upon the plan of the earlier
experimenters. An average statement of the results of six years'
experimentation is given in the subjoined table, showing the number
of pounds of water required for one pound of dry matter on fertile
soils:--
Wheat 1048
Corn 589
Peas 1118
Sugar Beets 630
These Utah findings support strongly the doctrine that the amount of
water required for the production of each pound of dry matter is
very much larger under arid conditions, as in Utah, than under humid
conditions, as in Germany or Wisconsin. It must be observed,
however, that in all of these experiments the plants were supplied
with water in a somewhat wasteful manner; that is, they were given
an abundance of water, and used the largest quantity possible under
the prevailing conditions. No attempt of any kind was made to
economize water. The results, therefore, represent maximum results
and can be safely used as such. Moreover, the methods of
dry-farming, involving the storage of water in deep soils and
systematic cultivation, were not employed. The experiments, both in
Europe and America, rather represent irrigated conditions. There are
good reasons for believing that in Germany, Wisconsin, and Utah the
amounts above given can be materially reduced by the employment of
proper cultural methods.
The water in the large bottle would be required to produce the grain
in the small bottle.
In view of these findings concerning the water requirements of
crops, it cannot be far from the truth to say that, under average
cultural conditions, approximately 750 pounds of water are required
in an arid district for the production of one pound of dry matter.
Where the aridity is intense, this figure may be somewhat low, and
in localities of sub-humid conditions, it will undoubtedly be too
high. As a maximum average, however, for districts interested in
dry-farming, it can be used with safety.
Crop-producing power of rainfall
If this conclusion, that not more than 750 pounds of water are
required under ordinary dry-farm conditions for the production of
one pound of dry matter, be accepted, certain interesting
calculations can be made respecting the possibilities of
dry-farming. For example, the production of one bushel of wheat will
require 60 times 750, or 45,000 pounds of water. The wheat kernels,
however, cannot be produced without a certain amount of straw, which
under conditions of dry-farming seldom forms quite one half of the
weight of the whole plant. Let us say, however, that the weights of
straw and kernels are equal. Then, to produce one bushel of wheat,
with the corresponding quantity of straw, would require 2 times
45,000, or 90,000 pounds of water. This is equal to 45 tons of water
for each bushel of wheat. While this is a large figure, yet, in many
localities, it is undoubtedly well within the truth. In comparison
with the amounts of water that fall upon the land as rain, it does
not seem extraordinarily large.
One inch of water over one acre of land weighs approximately 226,875
pounds. or over 113 tons. If this quantity of water could be stored
in the soil and used wholly for plant production, it would produce,
at the rate of 45 tons of water for each bushel, about 2-1/2 bushels
of wheat. With 10 inches of rainfall, which up to the present seems
to be the lower limit of successful dry-farming, there is a maximum
possibility of producing 25 bushels of wheat annually.
In the subjoined table, constructed on the basis of the discussion
of this chapter, the wheat-producing powers of various degrees of
annual precipitation are shown:--
One acre inch of water will produce 2-1/2 bushels of wheat.
Ten acre inches of water will produce 25 bushels of wheat.
Fifteen acre inches of water will produce 37-1/2 bushels of wheat.
Twenty acre inches of water will produce 50 bushels of wheat.
It must be distinctly remembered, however, that under no known
system of tillage can all the water that falls upon a soil be
brought into the soil and stored there for plant use. Neither is it
possible to treat a soil so that all the stored soil-moisture may be
used for plant production. Some moisture, of necessity, will
evaporate directly from the soil, and some may be lost in many other
ways. Yet, even under a rainfall of 12 inches, if only one half of
the water can be conserved, which experiments have shown to be very
feasible, there is a possibility of producing 30 bushels of wheat
per acre every other year, which insures an excellent interest on
the money and labor invested in the production of the crop.
It is on the grounds outlined in this chapter that students of the
subject believe that ultimately large areas of the "desert" may be
reclaimed by means of dry-farming. The real question before the
dry-farmer is not, "Is the rainfall sufficient?" but rather, "Is it
possible so to conserve and use the rainfall as to make it available
for the production of profitable crops?"
CHAPTER III
DRY-FARM AREAS--RAINFALL
The annual precipitation of rain and snow determines primarily the
location of dry-farm areas. As the rainfall varies, the methods of
dry-farming must be varied accordingly. Rainfall, alone, does not,
however, furnish a complete index of the crop-producing
possibilities of a country.
The distribution of the rainfall, the amount of snow, the
water-holding power of the soil, and the various
moisture-dissipating causes, such as winds, high temperature,
abundant sunshine, and low humidity frequently combine to offset the
benefits of a large annual precipitation. Nevertheless, no one
climatic feature represents, on the average, so correctly
dry-farming possibilities as does the annual rainfall. Experience
has already demonstrated that wherever the annual precipitation is
above 15 inches, there is no need of crop failures, if the soils are
suitable and the methods of dry-farming are correctly employed. With
an annual precipitation of 10 to 15 inches, there need be very few
failures, if proper cultural precautions are taken. With our present
methods, the areas that receive less than 10 inches of atmospheric
precipitation per year are not safe for dry-farm purposes. What the
future will show in the reclamation of these deserts, without
irrigation, is yet conjectural.
Arid, semiarid, and sub-humid
Before proceeding to an examination of the areas in the United
States subject to the methods of dry-farming it may be well to
define somewhat more clearly the terms ordinarily used in the
description of the great territory involved in the discussion.
The states lying west of the 100th meridian are loosely spoken of as
arid, semiarid, or sub-humid states. For commercial purposes no
state wants to be classed as arid and to suffer under the handicap
of advertised aridity. The annual rainfall of these states ranges
from about 3 to over 30 inches.
In order to arrive at greater definiteness, it may be well to assign
definite rainfall values to the ordinarily used descriptive terms of
the region in question. It is proposed, therefore, that districts
receiving less than 10 inches of atmospheric precipitation annually,
be designated arid; those receiving between 10 and 20 inches,
semiarid; those receiving between 20 and 30 inches, sub-humid, and
those receiving over 30 inches, humid. It is admitted that even such
a classification is arbitrary, since aridity does not alone depend
upon the rainfall, and even under such a classification there is an
unavoidable overlapping. However, no one factor so fully represents
varying degrees of aridity as the annual precipitation, and there is
a great need for concise definitions of the terms used in describing
the parts of the country that come under dry-farming discussions. In
this volume, the terms "arid," "semiarid," "sub-humid" and "humid"
are used as above defined.
Precipitation over the dry-farm territory
Nearly one half of the United States receives 20 inches or less
rainfall annually; and that when the strip receiving between 20 and
30 inches is added, the whole area directly subject to reclamation
by irrigation or dry-farming is considerably more than one half (63
per cent) of the whole area of the United States.
Eighteen states are included in this area of low rainfall. The areas
of these, as given by the Census of 1900, grouped according to the
annual precipitation received, are shown below:--
Arid to Semi-arid Group
Total Area Land Surface (Sq. Miles)
Arizona 112,920
California 156,172
Colorado 103,645
Idaho 84,290
Nevada 109,740
Utah 82,190
Wyoming 97,545
TOTAL 746,532
Semiarid to Sub-Humid Group
Montana 145,310
Nebraska 76,840
New Mexico 112,460
North Dakota 70,195
Oregon 94,560
South Dakota 76,850
Washington 66,880
TOTAL 653,095
Sub-Humid to Humid Group
Kansas 81,700
Minnesota 79,205
Oklahoma 38,830
Texas 262,290
TOTAL 462,025
GRAND TOTAL 1,861,652
The territory directly interested in the development of the methods
of dry-farming forms 63 per cent of the whole of the continental
United States, not including Alaska, and covers an area of 1,861,652
square miles, or 1,191,457,280 acres. If any excuse were needed for
the lively interest taken in the subject of dry-farming, it is amply
furnished by these figures showing the vast extent of the country
interested in the reclamation of land by the methods of dry-farming.
As will be shown below, nearly every other large country possesses
similar immense areas under limited rainfall.
Of the one billion, one hundred and ninety-one million, four hundred
and fifty-seven thousand, two hundred and eighty acres
(1,191,457,280) representing the dry-farm territory of the United
States, about 22 per cent, or a little more than one fifth, is
sub-humid and receives between 20 and 30 inches of rainfall,
annually; 61 per cent, or a little more than three fifths, is
semiarid and receives between 10 and 20 inches, annually, and about
17 per cent, or a little less than one fifth, is arid and receives
less than 10 inches of rainfall, annually.
These calculations are based upon the published average rainfall
maps of the United States Weather Bureau. In the far West, and
especially over the so-called "desert" regions, with their sparse
population, meteorological stations are not numerous, nor is it easy
to secure accurate data from them. It is strongly probable that as
more stations are established, it will be found that the area
receiving less than 10 inches of rainfall annually is considerably
smaller than above estimated. In fact, the United States Reclamation
Service states that there are only 70,000,000 acres of desert-like
land; that is, land which does not naturally support plants suitable
for forage. This area is about one third of the lands which, so far
as known, at present receive less than 10 inches of rainfall, or
only about 6 per cent of the total dry-farming territory.
In any case, the semiarid area is at present most vitally interested
in dry-farming. The sub-humid area need seldom suffer from drouth,
if ordinary well-known methods are employed; the arid area,
receiving less than 10 inches of rainfall, in all probability, can
be reclaimed without irrigation only by the development of more
suitable. methods than are known to-day. The semiarid area, which is
the special consideration of present-day dry-farming represents an
area of over 725,000,000 acres of land. Moreover, it must be
remarked that the full certainty of crops in the sub-humid regions
will come only with the adoption of dry-farming methods; and that
results already obtained on the edge of the "deserts" lead to the
belief that a large portion of the area receiving less than 10
inches of rainfall, annually, will ultimately be reclaimed without
irrigation.
Naturally, not the whole of the vast area just discussed could be
brought under cultivation, even under the most favorable conditions
of rainfall. A very large portion of the territory in question is
mountainous and often of so rugged a nature that to farm it would be
an impossibility. It must not be forgotten, however, that some of
the best dry-farm lands of the West are found in the small mountain
valleys, which usually are pockets of most fertile soil, under a
good supply of rainfall. The foothills of the mountains are almost
invariably excellent dry-farm lands. Newell estimates that
195,000,000 acres of land in the arid to sub-humid sections are
covered with a more or less dense growth of timber. This timbered
area roughly represents the mountainous and therefore the nonarable
portions of land. The same authority estimates that the desert-like
lands cover an area of 70,000,000 acres. Making the most liberal
estimates for mountainous and desert-like lands, at least one half
of the whole area, or about 600,000,000 acres, is arable land which
by proper methods may be reclaimed for agricultural purposes.
Irrigation when fully developed may reclaim not to exceed 5 per cent
of this area. From any point of view, therefore, the possibilities
involved in dry-farming in the United States are immense.
Dry-farm area of the world
Dry-farming is a world problem. Aridity is a condition met and to be
overcome upon every continent. McColl estimates that in Australia,
which is somewhat larger than the continental United States of
America, only one third of the whole surface receives above 20
inches of rainfall annually; one third receives from 10 to 20
inches, and one third receives less than lO inches. That is, about
1,267,000,000 acres in Australia are subject to reclamation by
dry-farming methods. This condition is not far from that which
prevails in the United States, and is representative of every
continent of the world. The following table gives the proportions of
the earth's land surface under various degrees of annual
precipitations:--
Annual Precipitation Proportion of Earth's Land Surface
Under 10 inches 25.0 per cent
From 10 to 20 inches 30.0 per cent
From 20 to 40 inches 20.0 per cent
From 40 to 60 inches 11.0 per cent
From 60 to 80 inches 9.0 per cent
From 100 to 120 inches 4.0 per cent
From 120 to 160 inches 0.5 per cent
Above 160 inches 0.5 per cent
Total 100 per cent
Fifty-five per cent, or more than one half of the total land surface
of the earth, receives an annual precipitation of less than 20
inches, and must be reclaimed, if at all, by dry-farming. At least
10 per cent more receives from 20 to 30 inches under conditions that
make dry-farming methods necessary. A total of about 65 per cent of
the earth's land surface is, therefore, directly interested in
dry-farming. With the future perfected development of irrigation
systems and practices, not more than 10 per cent will be reclaimed
by irrigation. Dry-farming is truly a problem to challenge the
attention of the race.
CHAPTER IV
DRY-FARM AREAS.--GENERAL CLIMATIC FEATURES
The dry-farm territory of the United States stretches from the
Pacific seaboard to the 96th parallel of longitude, and from the
Canadian to the Mexican boundary, making a total area of nearly
1,800,000 square miles. This immense territory is far from being a
vast level plain. On the extreme east is the Great Plains region of
the Mississippi Valley which is a comparatively uniform country of
rolling hills, but no mountains. At a point about one third of the
whole distance westward the whole land is lifted skyward by the
Rocky Mountains, which cross the country from south to northwest.
Here are innumerable peaks, canons, high table-lands, roaring
torrents, and quiet mountain valleys. West of the Rockies is the
great depression known as the Great Basin, which has no outlet to
the ocean. It is essentially a gigantic level lake floor traversed
in many directions by mountain ranges that are offshoots from the
backbone of the Rockies. South of the Great Basin are the high
plateaus, into which many great chasms are cut, the best known and
largest of which is the great Canon of the Colorado. North and east
of the Great Basin is the Columbia River Basin characterized by
basaltic rolling plains and broken mountain country. To the west,
the floor of the Great Basin is lifted up into the region of eternal
snow by the Sierra Nevada Mountains, which north of Nevada are known
as the Cascades. On the west, the Sierra Nevadas slope gently,
through intervening valleys and minor mountain ranges, into the
Pacific Ocean. It would be difficult to imagine a more diversified
topography than is possessed by the dry-farm territory of the United
States.
Uniform climatic conditions are not to be expected over such a
broken country. The chief determining factors of climate--latitude,
relative distribution of land and water, elevation, prevailing
winds--swing between such large extremes that of necessity the
climatic conditions of different sections are widely divergent.
Dry-farming is so intimately related to climate that the typical
climatic variations must be pointed out.
The total annual precipitation is directly influenced by the land
topography, especially by the great mountain ranges. On the east of
the Rocky Mountains is the sub-humid district, which receives from
20 to 30 inches of rainfall annually; over the Rockies themselves,
semiarid conditions prevail; in the Great Basin, hemmed in by the
Rockies on the east and the Sierra Nevadas on the west, more arid
conditions predominate; to the west, over the Sierras and down to
the seacoast, semiarid to sub-humid conditions are again found.
Seasonal distribution of rainfall
It is doubtless true that the total annual precipitation is the
chief factor in determining the success of dry-farming. However, the
distribution of the rainfall throughout the year is also of great
importance, and should be known by the farmer. A small rainfall,
coming at the most desirable season, will have greater
crop-producing power than a very much larger rainfall poorly
distributed. Moreover, the methods of tillage to be employed where
most of the precipitation comes in winter must be considerably
different from those used where the bulk of the precipitation comes
in the summer. The successful dry-farmer must know the average
annual precipitation, and also the average seasonal distribution of
the rainfall, over the land which he intends to dry-farm before he
can safely choose his cultural methods.
With reference to the monthly distribution of the precipitation over
the dry-farm territory of the United States, Henry of the United
States Weather Bureau recognizes five distinct types; namely: (1)
Pacific, (2) Sub-Pacific, (3) Arizona, (4) the Northern Rocky
Mountain and Eastern Foothills, and (5) the Plains Type:--
_"The Pacific Type.--_This type is found in all of the territory
west of the Cascade and Sierra Nevada ranges, and also obtains in a
fringe of country to the eastward of the mountain summits. The
distinguishing characteristic of the Pacific type is a wet season,
extending from October to March, and a practically rainless summer,
except in northern California and parts of Oregon and Washington.
About half of the yearly precipitation comes in the months of
December, January, and February, the remaining half being
distributed throughout the seven months--September, October,
November, March, April, May, and June."
_"Sub-Pacific Type.--_The term 'Sub-Pacific' has been given to that
type of rainfall which obtains over eastern Washington, Nevada, and
Utah. The influences that control the precipitation of this region
are much similar to those that prevail west of the Sierra Nevada and
Cascade ranges. There is not, however, as in the eastern type, a
steady diminution in the precipitation with the approach of spring,
but rather a culmination in the precipitation."
_"Arizona Type.--_The Arizona Type, so called because it is more
fully developed in that territory than elsewhere, prevails over
Arizona, New Mexico, and a small portion of eastern Utah and Nevada.
This type differs from all others in the fact that about 35 per cent
of the rain falls in July and August. May and June are generally the
months of least rainfall."
_"The Northern Rocky Mountain and Eastern Foothills Type.--_This
type is closely allied to that of the plains to the eastward, and
the bulk of the rain falls in the foothills of the region in April
and May; in Montana, in May and June."
_"The Plains Type.--_This type embraces the greater part of the
Dakotas, Nebraska, Kansas; Oklahoma, the Panhandle of Texas, and all
the great corn and wheat states of the interior valleys. This region
is characterized by a scant winter precipitation over the northern
states and moderately heavy rains during the growing season. The.
bulk of the rains comes in May, June, and July."
This classification emphasizes the great variation in distribution
of rainfall over the dry-farm territory of the country. West of the
Rocky Mountains the precipitation comes chiefly in winter and
spring, leaving the summers rainless; while east of the Rockies, the
winters are somewhat rainless and the precipitation comes chiefly in
spring and summer. The Arizona type stands midway between these
types. This variation in the distribution of the rainfall requires
that different methods be employed in storing and conserving the
rainfall for crop production. The adaptation of cultural methods to
the seasonal distribution of rainfall will be discussed hereafter.
Snowfall
Closely related to the distribution of the rainfall and the average
annual temperature is the snowfall. Wherever a relatively large
winter precipitation occurs, the dry-farmer is benefited if it comes
in the form of snow. The fall-planted seeds are better protected by
the snow; the evaporation is lower and it appears that the soil is
improved by the annual covering of snow. In any case, the methods of
culture are in a measure dependent upon the amount of snowfall and
the length of time that it lies upon the ground.
Snow falls over most of the dry-farm territory, excepting the
lowlands of California, the immediate Pacific coast, and other
districts where the average annual temperature is high. The heaviest
snowfall is in the intermountain district, from the west slope of
the Sierra Nevadas to the east slope of the Rockies. The degree of
snowfall on the agricultural lands is very variable and dependent
upon local conditions. Snow falls upon all the high mountain ranges.
Temperature
With the exceptions of portions of California, Arizona, and Texas
the average annual surface temperature of the dry-farm territory of
the United States ranges from 40 deg to 55 deg F. The average is not
far from 45 deg F. This places most of the dry-farm territory in the
class of cold regions, though a small area on the extreme east
border may be classed as temperate, and parts of California and
Arizona as warm. The range in temperature from the highest in summer
to the lowest in winter is considerable, but not widely different
from other similar parts of the United States. The range is greatest
in the interior mountainous districts, and lowest along the
seacoast. The daily range of the highest and lowest temperatures for
any one day is generally higher over dry-farm sections than over
humid districts. In the Plateau regions of the semiarid country the
average daily variation is from 30 to 35 deg F., while east of the
Mississippi it is only about 20 deg F. This greater daily range is
chiefly due to the clear skies and scant vegetation which facilitate
excessive warming by day and cooling by night.
The important temperature question for the dry-farmer is whether the
growing season is sufficiently warm and long to permit the maturing
of crops. There are few places, even at high altitudes in the region
considered, where the summer temperature is so low as to retard the
growth of plants. Likewise, the first and last killing frosts are
ordinarily so far apart as to allow an ample growing season. It must
be remembered that frosts are governed very largely by local
topographic features, and must be known from a local point of view.
It is a general law that frosts are more likely to occur in valleys
than on hillsides, owing to the downward drainage of the cooled air.
Further, the danger of frost increases with the altitude. In
general, the last killing frost in spring over the dry-farm
territory varies from March 15 to May 29, and the first killing
frost in autumn from September 15 to November 15. These limits
permit of the maturing of all ordinary farm crops, especially the
grain crops.
Relative humidity
At a definite temperature, the atmosphere can hold only a certain
amount of water vapor. When the air can hold no more, it is said to
be saturated. When it is not saturated, the amount of water vapor
actually held by the air is expressed in percentages of the quantity
required for saturation. A relative humidity of 100 per cent means
that the air is saturated; of 50 per cent, that it is only one half
saturated. The drier the air is, the more rapidly does the water
evaporate into it. To the dry-farmer, therefore, the relative
humidity or degree of dryness of the air is of very great
importance. According to Professor Henry, the chief characteristics
of the geographic distribution of relative humidity in the United
States are as follows:--
(1) Along the coasts there is a belt of high humidity at all
seasons, the percentage of saturation ranging from 75 to 80 per
cent.
(2) Inland, from about the 70th meridian eastward to the Atlantic
coast, the amount varies between 70 and 75 per cent.
(3) The dry region is in the Southwest, where the average annual
value is not over 50 per cent. In this region are included Arizona,
New Mexico, western Colorado, and the greater portion of both Utah
and Nevada. The amount of annual relative humidity in the remaining
portion of the elevated district, between the 100th meridian on the
east to the Sierra Nevada and the Cascades on the west, varies
between 55 and 65 per cent. In July, August, and September, the mean
values in the Southwest sink as low as 20 to 30 per cent, while
along the Pacific coast districts they continue about 80 per cent
the year round. In the Atlantic coast districts, and generally east
from the Mississippi River, the variation from month to month is not
great. April is probably the driest month of the year.
The air of the dry-farm territory, therefore, on the whole, contains
considerably less than two thirds the amount of moisture carried by
the air of the humid states. This means that evaporation from plant
leaves and soil surfaces will go on more rapidly in semiarid than in
humid regions. Against this danger, which cannot he controlled, the
dry-farmer must take special precautions.
Sunshine
The amount of sunshine in a dry-farm section is also of importance.
Direct sunshine promotes plant growth, but at the same time it
accelerates the evaporation of water from the soil. The whole
dry-farm territory receives more sunshine than do the humid
sections. In fact, the amount of sunshine may roughly be said to
increase as the annual rainfall decreases. Over the larger part of
the arid and semiarid sections the sun shines over 70 per cent of
the time.
Winds
The winds of any locality, owing to their moisture-dissipating
power play an important part in the success of dry-farming. A
persistent wind will offset much of the benefit of a heavy rainfall
and careful cultivation. While great general laws have been
formulated regarding the movements of the atmosphere, they are of
minor value in judging the effect of wind on any farming district.
Local observations, however, may enable the farmer to estimate the
probable effect of the winds and thus to formulate proper cultural
means of protection. In general, those living in a district are able
to describe it without special observations as windy or quiet. In
the dry-farm territory of the United States the one great region of
relatively high and persistent winds is the Great Plains region east
of the Rocky Mountains. Dry-farmers in that section will of
necessity be obliged to adopt cultural methods that will prevent the
excessive evaporation naturally induced by the unhindered wind, and
the possible blowing of well-tilled fallow land.
Summary
The dry-farm territory is characterized by a low rainfall, averaging
between 10 and 20 inches, the distribution of which falls into two
distinct types: a heavy winter and spring with a light summer
precipitation, and a heavy spring and summer with a light winter
precipitation. Snow falls over most of the territory, but does not
lie long outside of the mountain states. The whole dry-farm
territory may be classed as temperate to cold; relatively high and
persistent winds blow only over the Great Plains, though local
conditions cause strong regular winds in many other places; the air
is dry and the sunshine is very abundant. In brief, little water
falls upon the dry-farm territory, and the climatic factors are of a
nature to cause rapid evaporation.
In view of this knowledge, it is not surprising that thousands of
farmers, employing, often carelessly agricultural methods developed
in humid sections, have found only hardships and poverty on the
present dry-farm empire of the United States.
Drouth
Drouth is said to be the arch enemy of the dry-farmer, but few agree
upon its meaning. For the purposes of this volume, drouth may be
defined as a condition under which crops fail to mature because of
an insufficient supply of water. Providence has generally been
charged with causing drouths, but under the above definition, man is
usually the cause. Occasionally, relatively dry years occur, but
they are seldom dry enough to cause crop failures if proper methods
of farming have been practiced. There are four chief causes of
drouth: (1) Improper or careless preparation of the soil; (2)
failure to store the natural precipitation in the soil; (3) failure
to apply proper cultural methods for keeping the moisture in the
soil until needed by plants, and (4) sowing too much seed for the
available soil-moisture.
Crop failures due to untimely frosts, blizzards, cyclones,
tornadoes, or hail may perhaps be charged to Providence, but the
dry-farmer must accept the responsibility for any crop injury
resulting from drouth. A fairly accurate knowledge of the climatic
conditions of the district, a good understanding of the principles
of agriculture without irrigation under a low rainfall, and a
vigorous application of these principles as adapted to the local
climatic conditions will make dry-farm failures a rarity.
CHAPTER V
DRY-FARM SOILS
Important as is the rainfall in making dry-farming successful, it is
not more so than the soils of the dry-farms. On a shallow soil, or
on one penetrated with gravel streaks, crop failures are probable
even under a large rainfall; but a deep soil of uniform texture,
unbroken by gravel or hardpan, in which much water may be stored,
and which furnishes also an abundance of feeding space for the
roots, will yield large crops even under a very small rainfall.
Likewise, an infertile soil, though it be deep, and under a large
precipitation, cannot be depended on for good crops; but a fertile
soil, though not quite so deep, nor under so large a rainfall, will
almost invariably bring large crops to maturity.
A correct understanding of the soil, from the surface to a depth of
ten feet, is almost indispensable before a safe Judgment can be
pronounced upon the full dry-farm possibilities of a district.
Especially is it necessary to know (a) the depth, (b) the uniformity
of structure, and (c) the relative fertility of the soil, in order
to plan an intelligent system of farming that will be rationally
adapted to the rainfall and other climatic factors.
It is a matter of regret that so much of our information concerning
the soils of the dry-farm territory of the United States and other
countries has been obtained according to the methods and for the
needs of humid countries, and that, therefore, the special knowledge
of our arid and semiarid soils needed for the development of
dry-farming is small and fragmentary. What is known to-day
concerning the nature of arid soils and their relation to cultural
processes under a scanty rainfall is due very largely to the
extensive researches and voluminous writings of Dr. E. W. Hilgard,
who for a generation was in charge of the agricultural work of the
state of California. Future students of arid soils must of necessity
rest their investigations upon the pioneer work done by Dr. Hilgard.
The contents of this chapter are in a large part gathered from
Hilgard's writings.
The formation of soils
"Soil is the more or less loose and friable material in which, by
means of their roots, plants may or do find a foothold and
nourishment, as well as other conditions of growth." Soil is formed
by a complex process, broadly known as _weathering, _from the rocks
which constitute the earth's crust. Soil is in fact only pulverized
and altered rock. The forces that produce soil from rocks are of two
distinct classes, _physical and chemical. _The physical agencies of
soil production merely cause a pulverization of the rock; the
chemical agencies, on the other hand, so thoroughly change the
essential nature of the soil particles that they are no longer like
the rock from which they were formed.
Of the physical agencies, _temperature changes _are first in order
of time, and perhaps of first importance. As the heat of the day
increases, the rock expands, and as the cold night approaches,
contracts. This alternate expansion and contraction, in time, cracks
the surfaces of the rocks. Into the tiny crevices thus formed water
enters from the falling snow or rain. When winter comes, the water
in these cracks freezes to ice, and in so doing expands and widens
each of the cracks. As these processes are repeated from day to day,
from year to year, and from generation to generation, the surfaces
of the rocks crumble. The smaller rocks so formed are acted upon by
the same agencies, in the same manner, and thus the process of
pulverization goes on.
It is clear, then, that the second great agency of soil formation,
which always acts in conjunction with temperature changes, is
_freezing water. _The rock particles formed in this manner are often
washed down into the mountain valleys, there caught by great rivers,
ground into finer dust, and at length deposited in the lower
valleys. _Moving water _thus becomes another physical agency of soil
production. Most of the soils covering the great dry-farm territory
of the United States and other countries have been formed in this
way.
In places, glaciers moving slowly down the canons crush and grind
into powder the rock over which they pass and deposit it lower down
as soils. In other places, where strong winds blow with frequent
regularity, sharp soil grains are picked up by the air and hurled
against the rocks, which, under this action, are carved into
fantastic forms. In still other places, the strong winds carry soil
over long distances to be mixed with other soils. Finally, on the
seashore the great waves dashing against the rocks of the coast
line, and rolling the mass of pebbles back and forth, break and
pulverize the rock until soil is formed._ Glaciers, winds, _and
_waves _are also, therefore, physical agencies of soil formation.
It may be noted that the result of the action of all these agencies
is to form a rock powder, each particle of which preserves the
composition that it had while it was a constituent part of the rock.
It may further be noted that the chief of these soil-forming
agencies act more vigorously in arid than in humid sections. Under
the cloudless sky and dry atmosphere of regions of limited rainfall,
the daily and seasonal temperature changes are much greater than in
sections of greater rainfall. Consequently the pulverization of
rocks goes on most rapidly in dry-farm districts. Constant heavy
winds, which as soil formers are second only to temperature changes
and freezing water, are also usually more common in arid than in
humid countries. This is strikingly shown, for instance, on the
Colorado desert and the Great Plains.
The rock powder formed by the processes above described is
continually being acted upon by agencies, the effect of which is to
change its chemical composition. Chief of these agencies is _water,
_which exerts a solvent action on all known substances. Pure water
exerts a strong solvent action, but when it has been rendered impure
by a variety of substances, naturally occurring, its solvent action
is greatly increased.
The most effective water impurity, considering soil formation, is
the gas, _carbon dioxid. _This gas is formed whenever plant or
animal substances decay, and is therefore found, normally, in the
atmosphere and in soils. Rains or flowing water gather the carbon
dioxid from the atmosphere and the soil; few natural waters are free
from it. The hardest rock particles are disintegrated by carbonated
water, while limestones, or rocks containing lime, are readily
dissolved.
The result of the action of carbonated water upon soil particles is
to render soluble, and therefore more available to plants, many of
the important plant-foods. In this way the action of water, holding
in solution carbon dioxid and other substances, tends to make the
soil more fertile.
The second great chemical agency of soil formation is the oxygen of
the air. Oxidation is a process of more or less rapid burning, which
tends to accelerate the disintegration of rocks.
Finally, the _plants _growing in soils are powerful agents of soil
formation. First, the roots forcing their way into the soil exert a
strong pressure which helps to pulverize the soil grains; secondly,
the acids of the plant roots actually dissolve the soil, and third,
in the mass of decaying plants, substances are formed, among them
carbon dioxid, that have the power of making soils more soluble.
It may be noted that moisture, carbon dioxid, and vegetation, the
three chief agents inducing chemical changes in soils, are most
active in humid districts. While, therefore, the physical agencies
of soil formation are most active in arid climates, the same cannot
be said of the chemical agencies. However, whether in arid or humid
climates, the processes of soil formation, above outlined, are
essentially those of the "fallow" or resting-period given to
dry-farm lands. The fallow lasts for a few months or a year, while
the process of soil formation is always going on and has gone on for
ages; the result, in quality though not in quantity, is the
same--the rock particles are pulverized and the plant-foods are
liberated. It must be remembered in this connection that climatic
differences may and usually do influence materially the character of
soils formed from one and the same kind of rock.
Characteristics of arid soils
The net result of the processes above described Is a rock powder
containing a great variety of sizes of soil grains intermingled with
clay. The larger soil grains are called sand; the smaller, silt, and
those that are so small that they do not settle from quiet water
after 24 hours are known as clay.
Clay differs materially from sand and silt, not only in size of
particles, but also in properties and formation. It is said that
clay particles reach a degree of fineness equal to 1/2500 of an
inch. Clay itself, when wet and kneaded, becomes plastic and
adhesive and is thus easily distinguished from sand. Because of
these properties, clay is of great value in holding together the
larger soil grains in relatively large aggregates which give soils
the desired degree of filth. Moreover, clay is very retentive of
water, gases, and soluble plant-foods, which are important factors
in successful agriculture. Soils, in fact, are classified according
to the amount of clay that they contain. Hilgard suggests the
following classification:--
Very sandy soils 0.5 to 3 per cent clay
Ordinary sandy soils 3.0 to 10 per cent clay
Sandy loams 10.0 to 15 per cent clay
Clay loams 15.0 to 25 per cent clay
Clay soils 25.0 to 35 per cent clay
Heavy clay soils 35.0 per cent and over
Clay may be formed from any rock containing some form of _combined
silica _(quartz). Thus, granites and crystalline rocks generally,
volcanic rocks, and shales will produce clay if subjected to the
proper climatic conditions. In the formation of clay, the extremely
fine soil particles are attacked by the soil water and subjected to
deep-going chemical changes. In fact, clay represents the most
finely pulverized and most highly decomposed and hence in a measure
the most valuable portion of the soil. In the formation of clay,
water is the most active agent, and under humid conditions its
formation is most rapid.
It follows that dry-farm soils formed under a more or less rainless
climate contain less clay than do humid soils. This difference is
characteristic, and accounts for the statement frequently made that
heavy clay soils are not the best for dry-farm purposes. The fact
is, that heavy clay soils are very rare in arid regions; if found at
all, they have probably been formed under abnormal conditions, as in
high mountain valleys, or under prehistoric humid climates.
_Sand.--_The sand-forming rocks that are not capable of clay
production usually consist of _uncombined silica _or quartz, which
when pulverized by the soil-forming agencies give a comparatively
barren soil. Thus it has come about that ordinarily a clayey soil is
considered "strong" and a sandy soil "weak." Though this distinction
is true in humid climates where clay formation is rapid, it is not
true in arid climates, where true clay is formed very slowly. Under
conditions of deficient rainfall, soils are naturally less clayey,
but as the sand and silt particles are produced from rocks which
under humid conditions would yield clay, arid soils are not
necessarily less fertile.
Experiment has shown that the fertility in the sandy soils of arid
sections is as large and as available to plants as in the clayey
soils of humid regions. Experience in the arid section of America,
in Egypt, India, and other desert-like regions has further proved
that the sands of the deserts produce excellent crops whenever water
is applied to them. The prospective dry-farmer, therefore, need not
be afraid of a somewhat sandy soil, provided it has been formed
under arid conditions. In truth, a degree of sandiness is
characteristic of dry-farm soils.
The _humus _content forms another characteristic difference between
arid and humid soils. In humid regions plants cover the soil
thickly; in arid regions they are bunched scantily over the surface;
in the former case the decayed remnants of generations of plants
form a large percentage of humus in the upper soil; in the latter,
the scarcity of plant life makes the humus content low. Further,
under an abundant rainfall the organic matter in the soil rots
slowly; whereas in dry warm climates the decay is very complete. The
prevailing forces in all countries of deficient rainfall therefore
tend to yield soils low in humus.
While the total amount of humus in arid soils is very much lower
than in humid soils, repeated investigation has shown that it
contains about 3-1/2 times more nitrogen than is found in humus
formed under an abundant rainfall. Owing to the prevailing sandiness
of dry-farm soils, humus is not needed so much to give the proper
filth to the soil as in the humid countries where the content of
clay is so much higher. Since, for dry-farm purposes, the nitrogen
content is the most important quality of the humus, the difference
between arid and humid soils, based upon the humus content, is not
so great as would appear at first sight.
_Soil and subsoil.--_In countries of abundant rainfall, a great
distinction exists between the soil and the subsoil. The soil is
represented by the upper few inches which are filled with the
remnants of decayed vegetable matter and modified by plowing,
harrowing, and other cultural operations. The subsoil has been
profoundly modified by the action of the heavy rainfall, which, in
soaking through the soil, has carried with it the finest soil
grains, especially the clay, into the lower soil layers.
In time, the subsoil has become more distinctly clayey than the
topsoil. Lime and other soil ingredients have likewise been carried
down by the rains and deposited at different depths in the soil or
wholly washed away. Ultimately, this results in the removal from the
topsoil of the necessary plant-foods and the accumulation in the
subsoil of the fine clay particles which so compact the subsoil as
to make it difficult for roots and even air to penetrate it. The
normal process of weathering or soil disintegration will then go on
most actively in the topsoil and the subsoil will remain unweathered
and raw. This accounts for the well-known fact that in humid
countries any subsoil that may have been plowed up is reduced to a
normal state of fertility and crop production only after several
years of exposure to the elements. The humid farmer, knowing this,
is usually very careful not to let his plow enter the subsoil to any
great depth.
In the arid regions or wherever a deficient rainfall prevails, these
conditions are entirely reversed. The light rainfall seldom
completely fills the soil pores to any considerable depth, but it
rather moves down slowly as a him, enveloping the soil grains. The
soluble materials of the soil are, in part at least, dissolved and
carried down to the lower limit of the rain penetration, but the
clay and other fine soil particles are not moved downward to any
great extent. These conditions leave the soil and subsoil of
approximately equal porosity. Plant roots can then penetrate the
soil deeply, and the air can move up and down through the soil mass
freely and to considerable depths. As a result, arid soils are
weathered and made suitable for plant nutrition to very great
depths. In fact, in dry-farm regions there need be little talk about
soil and subsoil, since the soil is uniform in texture and usually
nearly so in composition, from the top down to a distance of many
feet.
Many soil sections 50 or more feet in depth are exposed in the
dry-farming territory of the United States, and it has often been
demonstrated that the subsoil to any depth is capable of producing,
without further weathering, excellent yields of crops. This
granular, permeable structure, characteristic of arid soils, is
perhaps the most important single quality resulting from rock
disintegration under arid conditions. As Hilgard remarks, it would
seem that the farmer in the arid region owns from three to four
farms, one above the other, as compared with the same acreage in the
eastern states.
This condition is of the greatest importance in developing the
principles upon which successful dry-farming rests. Further, it may
be said that while in the humid East the farmer must be extremely
careful not to turn up with his plow too much of the inert subsoil,
no such fear need possess the western farmer. On the contrary, he
should use his utmost endeavor to plow as deeply as possible in
order to prepare the very best reservoir for the falling waters and
a place for the development of plant roots.
_Gravel seams.--_It need be said, however, that in a number of
localities in the dry-farm territory the soils have been deposited
by the action of running water in such a way that the otherwise
uniform structure of the soil is broken by occasional layers of
loose gravel. While this is not a very serious obstacle to the
downward penetration of roots, it is very serious in dry-farming,
since any break in the continuity of the soil mass prevents the
upward movement of water stored in the lower soil depths. The
dry-farmer should investigate the soil which he intends to use to a
depth of at least 8 to 10 feet to make sure, first of all, that he
has a continuous soil mass, not too clayey in the lower depths, nor
broken by deposits of gravel.
_Hardpan.--_Instead of the heavy clay subsoil of humid regions, the
so-called hardpan occurs in regions of limited rainfall. The annual
rainfall, which is approximately constant, penetrates from year to
year very nearly to the same depth. Some of the lime found so
abundantly in arid soils is dissolved and worked down yearly to the
lower limit of the rainfall and left there to enter into combination
with other soil ingredients. Continued through long periods of time
this results in the formation of a layer of calcareous material at
the average depth to which the rainfall has penetrated the soil. Not
only is the lime thus carried down, but the finer particles are
carried down in like manner. Especially where the soil is poor in
lime is the clay worked down to form a somewhat clayey hardpan. A
hardpan formed in such a manner is frequently a serious obstacle to
the downward movement of the roots, and also prevents the annual
precipitation from moving down far enough to be beyond the influence
of the sunshine and winds. It is fortunate, however, that in the
great majority of instances this hardpan gradually disappears under
the influence of proper methods of dry-farm tillage. Deep plowing
and proper tillage, which allow the rain waters to penetrate the
soil, gradually break up and destroy the hardpan, even when it is 10
feet below the surface. Nevertheless, the farmer should make sure
whether or not the hardpan does exist in the soil and plan his
methods accordingly. If a hardpan is present, the land must be
fallowed more carefully every other year, so that a large quantity
of water may be stored in the soil to open and destroy the hardpan.
Of course, in arid as in humid countries, it often happens that a
soil is underlaid, more or less near the surface, by layers of rock,
marl deposits, and similar impervious or hurtful substances. Such
deposits are not to be classed with the hardpans that occur normally
wherever the rainfall is small.
_Leaching.--_Fully as important as any of the differences above
outlined are those which depend definitely upon the leaching power
of a heavy rainfall. In countries where the rainfall is 30 inches or
over, and in many places where the rainfall is considerably less,
the water drains through the soil into the standing ground water.
There is, therefore, in humid countries, a continuous drainage
through the soil after every rain, and in general there is a steady
downward movement of soil-water throughout the year. As is clearly
shown by the appearance, taste, and chemical composition of drainage
waters, this process leaches out considerable quantities of the
soluble constituents of the soil.
When the soil contains decomposing organic matter, such as roots,
leaves, stalks, the gas carbon dioxid is formed, which, when
dissolved in water, forms a solution of great solvent power. Water
passing through well-cultivated soils containing much humus leaches
out very much more material than pure water could do. A study of the
composition of the drainage waters from soils and the waters of the
great rivers shows that immense quantities of soluble soil
constituents are taken out of the soil in countries of abundant
rainfall. These materials ultimately reach the ocean, where they are
and have been concentrated throughout the ages. In short, the
saltiness of the ocean is due to the substances that have been
washed from the soils in countries of abundant rainfall.
In arid regions, on the other hand, the rainfall penetrates the soil
only a few feet. In time, it is returned to the surface by the
action of plants or sunshine and evaporated into the air. It is true
that under proper methods of tillage even the light rainfall of arid
and semiarid regions may he made to pass to considerable soil
depths, yet there is little if any drainage of water through the
soil into the standing ground water. The arid regions of the world,
therefore, contribute proportionately a small amount of the
substances which make up the salt of the sea.
_Alkali soils.--_Under favorable conditions it sometimes happens
that the soluble materials, which would normally be washed out of
humid soils, accumulate to so large a degree in arid soils as to
make the lands unfitted for agricultural purposes. Such lands are
called alkali lands. Unwise irrigation in arid climates frequently
produces alkali spots, but many occur naturally. Such soils should
not be chosen for dry-farm purposes, for they are likely to give
trouble.
_Plant-food content.--_This condition necessarily leads at once to
the suggestion that the soils from the two regions must differ
greatly in their fertility or power to produce and sustain plant
life. It cannot be believed that the water-washed soils of the East
retain as much fertility as the dry soils of the West. Hilgard has
made a long and elaborate study of this somewhat difficult question
and has constructed a table showing the composition of typical soils
of representative states in the arid and humid regions. The
following table shows a few of the average results obtained by
him:--
Partial Percentage Composition
Source of soil Humid Arid
Number of samples analyzed 696 573
Insoluble residue 84.17 69.16
Soluble silica 4.04 6.71
Alumina 3.66 7.61
Lime 0.13 1.43
Potash 0.21 0.67
Phos. Acid 0.12 0.16
Humus 1.22 1.13
Soil chemists have generally attempted to arrive at a determination
of the fertility of soil by treating a carefully selected and
prepared sample with a certain amount of acid of definite strength.
The portion which dissolves under the influence of acids has been
looked upon as a rough measure of the possible fertility of the
soil.
The column headed "Insoluble Residue" shows the average proportions
of arid and humid soils which remain undissolved by acids. It is
evident at once that the humid soils are much less soluble in acids
than arid soils, the difference being 84 to 69. Since the only
plant-food in soils that may be used for plant production is that
which is soluble, it follows that it is safe to assume that arid
soils are generally more fertile than humid soils. This is borne out
by a study of the constituents of the soil. For instance, potash,
one of the essential plant foods ordinarily present in sufficient
amount, is found in humid soils to the extent of 0.21 per cent,
while in arid soils the quantity present is 0.67 per cent, or over
three times as much. Phosphoric acid, another of the very important
plant-foods, is present in arid soils in only slightly higher
quantities than in humid soils. This explains the somewhat
well-known fact that the first fertilizer ordinarily required by
arid soils is some form of phosphorus:
The difference in the chemical composition of arid and humid soils
is perhaps shown nowhere better than in the lime content. There is
nearly eleven times more lime in arid than in humid soils.
Conditions of aridity favor strongly the formation of lime, and
since there is very little leaching of the soil by rainfall, the
lime accumulates in the soil.
The presence of large quantities of lime in arid soils has a number
of distinct advantages, among which the following are most
important: (1) It prevents the sour condition frequently present in
humid climates, where much organic material is incorporated with the
soil. (2) When other conditions are favorable, it encourages
bacterial life which, as is now a well-known fact, is an important
factor in developing and maintaining soil fertility. (3) By somewhat
subtle chemical changes it makes the relatively small percentages of
other plant-foods notably phosphoric acid and potash, more available
for plant growth. (4) It aids to convert rapidly organic matter into
humus which represents the main portion of the nitrogen content of
the soil.
Of course, an excess of lime in the soil may be hurtful, though less
so in arid than in humid regions. Some authors state that from 8 to
20 per cent of calcium carbonate makes a soil unfitted for plant
growth. There are, however, a great many agricultural soils covering
large areas and yielding very abundant crops which contain very much
larger quantities of calcium carbonate. For instance, in the Sanpete
Valley of Utah, one of the most fertile sections of the Great Basin,
agricultural soils often contain as high as 40 per cent of calcium
carbonate, without injury to their crop-producing power.
In the table are two columns headed "Soluble Silica" and "Alumina,"
in both of which it is evident that a very much larger per cent is
found in the arid than in the humid soils. These soil constituents
indicate the condition of the soil with reference to the
availability of its fertility for plant use. The higher the
percentage of soluble silica and alumina, the more thoroughly
decomposed, in all probability, is the soil as a whole and the more
readily can plants secure their nutriment from the soil. It will be
observed from the table, as previously stated, that more humus is
found in humid than in arid soils, though the difference is not so
large as might be expected. It should be recalled, however, that the
nitrogen content of humus formed under rainless conditions is many
times larger than that of humus formed in rainy countries, and that
the smaller per cent of humus in dry-farming countries is thereby
offset.
All in all, the composition of arid soils is very much more
favorable to plant growth than that of humid soils. As will be shown
in Chapter IX, the greater fertility of arid soils is one of the
chief reasons for dry-farming success. Depth of the soil alone does
not suffice. There must be a large amount of high fertility
available for plants in order that the small amount of water can be
fully utilized in plant growth.
_Summary of characteristics.--_Arid soils differ from humid soils in
that they contain: less clay; more sand, but of fertile nature
because it is derived from rocks that in humid countries would
produce clay; less humus, but that of a kind which contains about
3-1/2 times more nitrogen than the humus of humid soils; more lime,
which helps in a variety of ways to improve the agricultural value
of soils; more of all the essential plant-foods, because the
leaching by downward drainage is very small in countries of limited
rainfall.
Further, arid soils show no real difference between soil and
subsoil; they are deeper and more permeable; they are more uniform
in structure; they have hardpans instead of clay subsoil, which,
however, disappear under the influence of cultivation; their
subsoils to a depth of ten feet or more are as fertile as the
topsoil, and the availability of the fertility is greater. The
failure to recognize these characteristic differences between arid
and humid soils has been the chief cause for many crop failures in
the more or less rainless regions of the world.
This brief review shows that, everything considered, arid soils are
superior to humid soils. In ease of handling, productivity,
certainty of crop-lasting quality, they far surpass the soils of the
countries in which scientific agriculture was founded. As Hilgard
has suggested, the historical datum that the majority of the most
populous and powerful historical peoples of the world have been
located on soils that thirst for water, may find its explanation in
the intrinsic value of arid soils. From Babylon to the United States
is a far cry; but it is one that shouts to the world the superlative
merits of the soil that begs for water. To learn how to use the
"desert" is to make it "blossom like the rose."
Soil divisions
The dry-farm territory of the United States may be divided roughly
into five great soil districts, each of which includes a great
variety of soil types, most of which are poorly known and mapped.
These districts are:--
1. Great Plains district.
2. Columbia River district
3. Great Basin district.
4. Colorado River district.
5. California district.
_Great Plains district.--_On the eastern slope of the Rocky
Mountains, extending eastward to the extreme boundary of the
dry-farm territory, are the soils of the High Plains and the Great
Plains. This vast soil district belongs to the drainage basin of the
Missouri, and includes North and South Dakota, Nebraska, Kansas,
Oklahoma, and parts of Montana, Wyoming, Colorado, New Mexico,
Texas, and Minnesota. The soils of this district are usually of high
fertility. They have good lasting power, though the effect of the
higher rainfall is evident in their composition. Many of the
distinct types of the plains soils have been determined with
considerable care by Snyder and Lyon, and may be found described in
Bailey's "Cyclopedia of American Agriculture," Vol. I.
_Columbia River district.--_The second great soil district of the
dry-farming territory is located in the drainage basin of the
Columbia River, and includes Idaho and the eastern two thirds of
Washington and Oregon. The high plains of this soil district are
often spoken of as the Palouse country. The soils of the western
part of this district are of basaltic origin; over the southern part
of Idaho the soils have been made from a somewhat recent lava flow
which in many places is only a few feet below the surface. The soils
of this district are generally of volcanic origin and very much
alike. They are characterized by the properties which normally
belong to volcanic soils; somewhat poor in lime, but rich in potash
and phosphoric acid. They last well under ordinary methods of
tillage.
_The Great Basin.--_The third great soil district is included in the
Great Basin, which covers nearly all of Nevada, half of Utah, and
takes small portions out of Idaho, Oregon, and southern California.
This basin has no outlet to the sea. Its rivers empty into great
saline inland lakes, the chief of which is the Great Salt Lake. The
sizes of these interior lakes are determined by the amounts of water
flowing into them and the rates of evaporation of the water into the
dry air of the region.
In recent geological times, the Great Basin was filled with water,
forming a vast fresh-water lake known as Lake Bonneville, which
drained into the Columbia River. During the existence of this lake,
soil materials were washed from the mountains into the lake and
deposited on the lake bottom. When at length, the lake disappeared,
the lake bottom was exposed and is now the farming lands of the
Great Basin district. The soils of this district are characterized
by great depth and uniformity, an abundance of lime, and all the
essential plant-foods with the exception of phosphoric acid, which,
while present in normal quantities, is not unusually abundant. The
Great Basin soils are among the most fertile on the American
Continent.
_Colorado River district.--_The fourth soil district lies in the
drainage basin of the Colorado River It includes much of the
southern part of Utah, the eastern part of Colorado, part of New
Mexico, nearly all of Arizona, and part of southern California. This
district, in its northern part, is often spoken of as the High
Plateaus. The soils are formed from the easily disintegrated rocks
of comparatively recent geological origin, which themselves are said
to have been formed from deposits in a shallow interior sea which
covered a large part of the West. The rivers running through this
district have cut immense canons with perpendicular walls which make
much of this country difficult to traverse. Some of the soils are of
an extremely fine nature, settling firmly and requiring considerable
tillage before they are brought to a proper condition of tilth. In
many places the soils are heavily charged with calcium sulfate, or
crystals of the ordinary land plaster. The fertility of the soils,
however, is high, and when they are properly cultivated, they yield
large and excellent crops.
_California district.--_The fifth soil district lies in California
in the basin of the Sacramento and San Joaquin rivers. The soils are
of the typical arid kind of high fertility and great lasting powers.
They represent some of the most valuable dry-farm districts of the
West. These soils have been studied in detail by Hilgard.
_Dry-farming in the five districts.--_It is interesting to note that
in all of these five great soil districts dry-farming has been tried
with great success. Even in the Great Basin and the Colorado River
districts, where extreme desert conditions often prevail and where
the rainfall is slight, it has been found possible to produce
profitable crops without irrigation. It is unfortunate that the
study of the dry-farming territory of the United States has not
progressed far enough to permit a comprehensive and correct mapping
of its soils. Our knowledge of this subject is, at the best,
fragmentary. We know, however, with certainty that the properties
which characterize arid soils, as described in this chapter' are
possessed by the soils of the dry-farming territory, including the
five great districts just enumerated. The characteristics of arid id
soils increase as the rainfall decreases and other conditions of
aridity increase. They are less marked as we go eastward or westward
toward the regions of more abundant rainfall; that is to say, the
most highly developed arid soils are found in the Great Basin and
Colorado River districts. The least developed are on the eastern
edge of the Great Plains.
The judging of soils
A chemical analysis of a soil, unless accompanied by a large amount
of other information, is of little value to the farmer. The main
points in judging a prospective dry-farm are: the depth of the soil,
the uniformity of the soil to a depth of at least 10 feet, the
native vegetation, the climatic conditions as relating to early and
late frosts, the total annual rainfall and its distribution, and the
kinds and yields of crops that have been grown in the neighborhood.
The depth of the soil is best determined by the use of an auger. A
simple soil auger is made from the ordinary carpenter's auger, 1-1/2
to 2 inches in diameter, by lengthening its shaft to 3 feet or more.
Where it is not desirable to carry sectional augers, it is often
advisable to have three augers made: one 3 feet, the other 6, and
the third 9 or 10 feet in length. The short auger is used first and
the others afterwards as the depth of the boring increases. The
boring should he made in a large number of average
places--preferably one boring or more on each acre if time and
circumstances permit--and the results entered on a map of the farm.
The uniformity of the soil is observed as the boring progresses. If
gravel layers exist, they will necessarily stop the progress of the
boring. Hardpans of any kind will also be revealed by such an
examination.
The climatic information must be gathered from the local weather
bureau and from older residents of the section.
The native vegetation is always an excellent index of dry-farm
possibilities. If a good stand of native grasses exists, there can
scarcely be any doubt about the ultimate success of dry-farming
under proper cultural methods. A healthy crop of sagebrush is an
almost absolutely certain indication that farming without irrigation
is feasible. The rabbit brush of the drier regions is also usually a
good indication, though it frequently indicates a soil not easily
handled. Greasewood, shadscale, and other related plants ordinarily
indicate heavy clay soils frequently charged with alkali. Such soils
should be the last choice for dry-farming purposes, though they
usually give good satisfaction under systems of irrigation. If the
native cedar or other native trees grow in profusion, it is another
indication of good dry-farm possibilities.
CHAPTER VI
THE ROOT SYSTEMS OF PLANTS
The great depth and high fertility of the soils of arid and semiarid
regions have made possible the profitable production of agricultural
plants under a rainfall very much lower than that of humid regions.
To make the principles of this system fully understood, it is
necessary to review briefly our knowledge of the root systems of
plants growing under arid conditions.
Functions of roots
The roots serve at least three distinct uses or purposes: First,
they give the plant a foothold in the earth; secondly, they enable
the plant to secure from the soil the large amount of water needed
in plant growth, and, thirdly, they enable the plant to secure the
indispensable mineral foods which can be obtained only from the
soil. So important is the proper supply of water and food in the
growth of a plant that, in a given soil, the crop yield is usually
in direct proportion to the development of the root system. Whenever
the roots are hindered in their development, the growth of the plant
above ground is likewise retarded, and crop failure may result. The
importance of roots is not fully appreciated because they are hidden
from direct view. Successful dry-farming consists, largely in the
adoption of practices that facilitate a full and free development-of
plant roots. Were it not that the nature of arid soils, as explained
in preceding chapters, is such that full root development is
comparatively easy, it would probably be useless to attempt to
establish a system of dry-farming.
Kinds of roots
The root is the part of the plant that is found underground. It has
numerous branches, twigs, and filaments. The root which first forms
when the seed bursts is known as the primary root. From this primary
root other roots develop, which are known as secondary roots. When
the primary root grows more rapidly than the secondary roots, the
so-called taproot, characteristic of lucerne, clover, and similar
plants, is formed. When, on the other hand, the taproot grows slowly
or ceases its growth, and the numerous secondary roots grow long, a
fibrous root system results, which is characteristic of the cereals,
grasses, corn, and other similar plants. With any type of root, the
tendency of growth is downward; though under conditions that are not
favorable for the downward penetration of the roots the lateral
extensions may be very large and near the surface
Extent of roots
A number of investigators have attempted to determine the weight of
the roots as compared with the weight of the plant above ground, hut
the subject, because of its great experimental difficulties, has not
been very accurately explained. Schumacher, experimenting about
1867, found that the roots of a well-established field of clover
weighed as much as the total weight of the stems and leaves of the
year's crop, and that the weight of roots of an oat crop was 43 per
cent of the total weight of seed and straw. Nobbe, a few years
later, found in one of his experiments that the roots of timothy
weighed 31 per cent of the weight of the hay. Hosaeus, investigating
the same subject about the same time, found that the weight of roots
of one of the brome grasses was as great as the weight of the part
above ground; of serradella, 77 per cent; of flax, 34 per cent; of
oats, 14 per cent; of barley, 13 per cent, and of peas, 9 per cent.
Sanborn, working at the Utah Station in 1893, found results very
much the same
Although these results are not concordant, they show that the weight
of the roots is considerable, in many cases far beyond the belief of
those who have given the subject little or no attention. It may be
noted that on the basis of the figures above obtained, it is very
probable that the roots in one acre of an average wheat crop would
weigh in the neighborhood of a thousand pounds--possibly
considerably more. It should be remembered that the investigations
which yielded the preceding results were all conducted in humid
climates and at a time when the methods for the study of the root
systems were poorly developed. The data obtained, therefore,
represent, in all probability, minimum results which would be
materially increased should the work be repeated now.
The relative weights of the roots and the stems and the leaves do
not alone show the large quantity of roots; the total lengths of the
roots are even more striking. The German investigator, Nobbe, in a
laborious experiment conducted about 1867, added the lengths of all
the fine roots from each of various plants. He found that the total
length of roots, that is, the sum of the lengths of all the roots,
of one wheat plant was about 268 feet, and that the total length of
the roots of one plant of rye was about 385 feet. King, of
Wisconsin, estimates that in one of his experiments, one corn plant
produced in the upper 3 feet of soil 1452 feet of roots. These
surprisingly large numbers indicate with emphasis the thoroughness
with which the roots invade the soil.
Depth of root penetration
The earlier root studies did not pretend to determine the depth to
which roots actually penetrate the earth. In recent years, however,
a number of carefully conducted experiments were made by the New
York, Wisconsin, Minnesota, Kansas, Colorado, and especially the
North Dakota stations to obtain accurate information concerning the
depth to which agricultural plants penetrate soils. It is somewhat
regrettable, for the purpose of dry-farming, that these states, with
the exception of Colorado, are all in the humid or sub-humid area of
the United States. Nevertheless, the conclusions drawn from the work
are such that they may be safely applied in the development of the
principles of dry-farming.
There is a general belief among farmers that the roots of all
cultivated crops are very near the surface and that few reach a
greater depth than one or two feet. The first striking result of the
American investigations was that every crop, without exception,
penetrates the soil deeper than was thought possible in earlier
days. For example, it was found that corn roots penetrated fully
four feet into the ground and that they fully occupied all of the
soil to that depth.
On deeper and somewhat drier soils, corn roots went down as far as
eight feet. The roots of the small grains,--wheat, oats,
barley,--penetrated the soil from four to eight or ten feet. Various
perennial grasses rooted to a depth of four feet the first year; the
next year, five and one half feet; no determinations were made of
the depth of the roots in later years, though it had undoubtedly
increased. Alfalfa was the deepest rooted of all the crops studied
by the American stations. Potato roots filled the soil fully to a
depth of three feet; sugar beets to a depth of nearly four feet.
Sugar Beet Roots
In every case, under conditions prevailing in the experiments, and
which did not have in mind the forcing of the roots down to
extraordinary depths, it seemed that the normal depth of the roots
of ordinary field crops was from three to eight feet. Sub-soiling
and deep plowing enable the roots to go deeper into the soil. This
work has been confirmed in ordinary experience until there can be
little question about the accuracy of the results.
Almost all of these results were obtained in humid climates on humid
soils, somewhat shallow, and underlain by a more or less infertile
subsoil. In fact, they were obtained under conditions really
unfavorable to plant growth. It has been explained in Chapter V that
soils formed under arid or semiarid conditions are uniformly deep
and porous and that the fertility of the subsoil is, in most cases,
practically as great as of the topsoil. There is, therefore, in arid
soils, an excellent opportunity for a comparatively easy penetration
of the roots to great depths and, because of the available
fertility, a chance throughout the whole of the subsoil for ample
root development. Moreover, the porous condition of the soil permits
the entrance of air, which helps to purify the soil atmosphere and
thereby to make the conditions more favorable for root development.
Consequently it is to be expected that, in arid regions, roots will
ordinarily go to a much greater depth than in humid regions.
It is further to be remembered that roots are in constant search of
food and water and are likely to develop in the directions where
there is the greatest abundance of these materials. Under systems of
dry-farming the soil water is stored more or less uniformly to
considerable depths--ten feet or more--and in most cases the
percentage of moisture in the spring and summer is as large or
larger some feet below the surface than in the upper two feet. The
tendency of the root is, then, to move downward to depths where
there is a larger supply of water. Especially is this tendency
increased by the available soil fertility found throughout the whole
depth of the soil mass.
It has been argued that in many of the irrigated sections the roots
do not penetrate the soil to great depths. This is true, because by
the present wasteful methods of irrigation the plant receives so
much water at such untimely seasons that the roots acquire the habit
of feeding very near the surface where the water is so lavishly
applied. This means not only that the plant suffers more greatly in
times of drouth, but that, since the feeding ground of the roots is
smaller, the crop is likely to be small.
These deductions as to the depth to which plant roots will penetrate
the soil in arid regions are fully corroborated by experiments and
general observation. The workers of the Utah Station have repeatedly
observed plant roots on dry-farms to a depth of ten feet. Lucerne
roots from thirty to fifty feet in length are frequently exposed in
the gullies formed by the mountain torrents. Roots of trees,
similarly, go down to great depths. Hilgard observes that he has
found roots of grapevines at a depth of twenty-two feet below the
surface, and quotes Aughey as having found roots of the native
Shepherdia in Nebraska to a depth of fifty feet. Hilgard further
declares that in California fibrous-rooted plants, such as wheat and
barley, may descend in sandy soils from four to seven feet. Orchard
trees in the arid West, grown properly, are similarly observed to
send their roots down to great depths. In fact, it has become a
custom in many arid regions where the soils are easily penetrable to
say that the root system of a tree corresponds in extent and
branching to the part of the tree above ground.
Now, it is to be observed that, generally, plants grown in dry
climates send their roots straight down into the soil; whereas in
humid climates, where the topsoil is quite moist and the subsoil is
hard, roots branch out laterally and fill the upper foot or two of
the soil. A great deal has been said and written about the danger of
deep cultivation, because it tends to injure the roots that feed
near the surface. However true this may be in humid countries, it is
not vital in the districts primarily interested in dry-farming; and
it is doubtful if the objection is as valid in humid countries as is
often declared. True, deep cultivation, especially when performed
near the plant or tree, destroys the surface-feeding roots, but this
only tends to compel the deeper lying roots to make better use of
the subsoil.
When, as in arid regions, the subsoil is fertile and furnishes a
sufficient amount of water, destroying the surface roots is no
handicap whatever. On the contrary, in times of drouth, the
deep-lying roots feed and drink at their leisure far from the hot
sun or withering winds, and the plants survive and arrive at rich
maturity, while the plants with shallow roots wither and die or are
so seriously injured as to produce an inferior crop. Therefore, in
the system of dry-farming as developed in this volume, it must be
understood that so far as the farmer has power, the roots must be
driven downward into the soil, and that no injury needs to be
apprehended from deep and vigorous cultivation.
One of the chief attempts of the dry-farmer must be to see to it
that the plants root deeply. This can be done only by preparing the
right kind of seed-bed and by having the soil in its lower depths
well-stored with moisture, so that the plants may be invited to
descend. For that reason, an excess of moisture in the upper soil
when the young plants are rooting is really an injury to them.
CHAPTER VII
STORING WATER IN THE SOIL
The large amount of water required for the production of plant
substance is taken from the soil by the roots. Leaves and stems do
not absorb appreciable quantities of water. The scanty rainfall of
dry-farm districts or the more abundant precipitation of humid
regions must, therefore, be made to enter the soil in such a manner
as to be readily available as soil-moisture to the roots at the
right periods of plant growth.
In humid countries, the rain that falls during the growing season is
looked upon, and very properly, as the really effective factor in
the production of large crops. The root systems of plants grown
under such humid conditions are near the surface, ready to absorb
immediately the rains that fall, even if they do not soak deeply
into the soil. As has been shown in Chapter IV, it is only over a
small portion of the dry-farm territory that the bulk of the scanty
precipitation occurs during the growing season. Over a large portion
of the arid and semiarid region the summers are almost rainless and
the bulk of the precipitation comes in the winter, late fall, or
early spring when plants are not growing. If the rains that fall
during the growing season are indispensable in crop production, the
possible area to be reclaimed by dry-farming will be greatly
limited. Even when much of the total precipitation comes in summer,
the amount in dry-farm districts is seldom sufficient for the proper
maturing of crops. In fact, successful dry-farming depends chiefly
upon the success with which the rains that fall during any season of
the year may be stored and kept in the soil until needed by plants
in their growth. The fundamental operations of dry-farming include a
soil treatment which enables the largest possible proportion of the
annual precipitation to be stored in the soil. For this purpose, the
deep, somewhat porous soils, characteristic of arid regions, are
unusually well adapted.
Alway's demonstration
An important and unique demonstration of the possibility of bringing
crops to maturity on the moisture stored in the soil at the time of
planting has been made by Alway. Cylinders of galvanized iron, 6
feet long, were filled with soil as nearly as possible in its
natural position and condition Water was added until seepage began,
after which the excess was allowed to drain away. When the seepage
had closed, the cylinders were entirely closed except at the
surface. Sprouted grains of spring wheat were placed in the moist
surface soil, and 1 inch of dry soil added to the surface to prevent
evaporation. No more water was added; the air of the greenhouse was
kept as dry as possible. The wheat developed normally. The first ear
was ripe in 132 days after planting and the last in 143 days. The
three cylinders of soil from semiarid western Nebraska produced 37.8
grams of straw and 29 ears, containing 415 kernels weighing 11.188
grams. The three cylinders of soil from humid eastern Nebraska
produced only 11.2 grams of straw and 13 ears containing 114
kernels, weighing 3 grams. This experiment shows conclusively that
rains are not needed during the growing season, if the soil is well
filled with moisture at seedtime, to bring crops to maturity.
What becomes of the rainfall?
The water that falls on the land is disposed of in three ways:
First, under ordinary conditions, a large portion runs off without
entering the soil; secondly, a portion enters the soil, but remains
near the surface, and is rapidly evaporated back into the air; and,
thirdly, a portion enters the lower soil layers, from which it is
removed at later periods by several distinct processes. The run-off
is usually large and is a serious loss, especially in dry-farming
regions, where the absence of luxuriant vegetation, the somewhat
hard, sun-baked soils, and the numerous drainage channels, formed by
successive torrents, combine to furnish the rains with an easy
escape into the torrential rivers. Persons familiar with arid
conditions know how quickly the narrow box canyons, which often
drain thousands of square miles, are filled with roaring water after
a comparatively light rainfall.
The run-off
The proper cultivation of the soil diminishes very greatly the loss
due to run-off, but even on such soils the proportion may often be
very great. Farrel observed at one of the Utah stations that during
a torrential rain--2.6 inches in 4 hours--the surface of the summer
fallowed plats was packed so solid that only one fourth inch, or
less than one tenth of the whole amount, soaked into the soil, while
on a neighboring stubble field, which offered greater hindrance to
the run-off, 1-1/2 inches or about 60 per cent were absorbed.
It is not possible under any condition to prevent the run-off
altogether, although it can usually be reduced exceedingly. It is a
common dry-farm custom to plow along the slopes of the farm instead
of plowing up and down them. When this is done, the water which runs
down the slopes is caught by the succession of furrows and in that
way the runoff is diminished. During the fallow season the disk and
smoothing harrows are run along the hillsides for the same purpose
and with results that are nearly always advantageous to the
dry-farmer. Of necessity, each man must study his own farm in order
to devise methods that will prevent the run-off.
The structure of soils
Before examining more closely the possibility of storing water in
soils a brief review of the structure of soils is desirable. As
previously explained, soil is essentially a mixture of disintegrated
rock and the decomposing remains of plants. The rock particles which
constitute the major portion of soils vary greatly in size. The
largest ones are often 500 times the sizes of the smallest. It would
take 50 of the coarsest sand particles, and 25,000 of the finest
silt particles, to form one lineal inch. The clay particles are
often smaller and of such a nature that they cannot be accurately
measured. The total number of soil particles in even a small
quantity of cultivated soil is far beyond the ordinary limits of
thought, ranging from 125,000 particles of coarse sand to
15,625,000,000,000 particles of the finest silt in one cubic inch.
In other words, if all the particles in one cubic inch of soil
consisting of fine silt were placed side by side, they would form a
continuous chain over a thousand miles long. The farmer, when he
tills the soil, deals with countless numbers of individual soil
grains, far surpassing the understanding of the human mind. It is
the immense number of constituent soil particles that gives to the
soil many of its most valuable properties.
It must be remembered that no natural soil is made up of particles
all of which are of the same size; all sizes, from the coarsest sand
to the finest clay, are usually present. These particles of all
sizes are not arranged in the soil in a regular, orderly way; they
are not placed side by side with geometrical regularity; they are
rather jumbled together in every possible way. The larger sand
grains touch and form comparatively large interstitial spaces into
which the finer silt and clay grains filter. Then, again, the clay
particles, which have cementing properties, bind, as it were, one
particle to another. A sand grain may have attached to it hundreds,
or it may be thousands, of the smaller silt grains; or a regiment of
smaller soil grains may themselves be clustered into one large grain
by cementing power of the clay. Further, in the presence of lime and
similar substances, these complex soil grains are grouped into yet
larger and more complex groups. The beneficial effect of lime is
usually due to this power of grouping untold numbers of soil
particles into larger groups. When by correct soil culture the
individual soil grains are thus grouped into large clusters, the
soil is said to be in good tilth. Anything that tends to destroy
these complex soil grains, as, for instance, plowing the soil when
it is too wet, weakens the crop-producing power of the soil. This
complexity of structure is one of the chief reasons for the
difficulty of understanding clearly the physical laws governing
soils.
Pore-space of soils
It follows from this description of soil structure that the soil
grains do not fill the whole of the soil space. The tendency is
rather to form clusters of soil grains which, though touching at
many points, leave comparatively large empty spaces. This pore space
in soils varies greatly, but with a maximum of about 55 per cent. In
soils formed under arid conditions the percentage of pore-space is
somewhere in the neighborhood of 50 per cent. There are some arid
soils, notably gypsum soils, the particles of which are so uniform
size that the pore-space is exceedingly small. Such soils are always
difficult to prepare for agricultural purposes.
It is the pore-space in soils that permits the storage of
soil-moisture; and it is always important for the farmer so to
maintain his soil that the pore-space is large enough to give him
the best results, not only for the storage of moisture, but for the
growth and development of roots, and for the entrance into the soil
of air, germ life, and other forces that aid in making the soil fit
for the habitation of plants. This can always be best accomplished,
as will be shown hereafter, by deep plowing, when the soil is not
too wet, the exposure of the plowed soil to the elements, the
frequent cultivation of the soil through the growing season, and the
admixture of organic matter. The natural soil structure at depths
not reached by the plow evidently cannot be vitally changed by the
farmer.
Hygroscopic soil-water
Under normal conditions, a certain amount of water is always found
in all things occurring naturally, soils included. Clinging to every
tree, stone, or animal tissue is a small quantity of moisture
varying with the temperature, the amount of water in the air, and
with other well-known factors. It is impossible to rid any natural
substance wholly of water without heating it to a high temperature.
This water which, apparently, belongs to all natural objects is
commonly called hygroscopic water. Hilgard states that the soils of
the arid regions contain, under a temperature of 15 deg C. and an
atmosphere saturated with water, approximately 5-1/2 per cent of
hygroscopic water. In fact, however, the air over the arid region is
far from being saturated with water and the temperature is even
higher than 15 deg C., and the hygroscopic moisture actually found
in the soils of the dry-farm territory is considerably smaller than
the average above given. Under the conditions prevailing in the
Great Basin the hygroscopic water of soils varies from .75 per cent
to 3-1/2 per cent; the average amount is not far from 12 per cent.
Whether or not the hygroscopic water of soils is of value in plant
growth is a disputed question. Hilgard believes that the hygroscopic
moisture can be of considerable help in carrying plants through
rainless summers, and further, that its presence prevents the
heating of the soil particles to a point dangerous to plant roots.
Other authorities maintain earnestly that the hygroscopic soil-water
is practically useless to plants. Considering the fact that wilting
occurs long before the hygroscopic water contained in the soil is
reached, it is very unlikely that water so held is of any real
benefit to plant growth.
Gravitational water
It often happens that a portion of the water in the soil is under
the immediate influence of gravitation. For instance, a stone which,
normally, is covered with hygroscopic water is dipped into water The
hydroscopic water is not thereby affected, but as the stone is drawn
out of the water a good part of the water runs off. This is
gravitational water That is, the gravitational water of soils is
that portion of the soil-water which filling the soil pores, flows
downward through the soil under the influence of gravity. When the
soil pores are completely filled, the maximum amount of
gravitational water is found there. In ordinary dry-farm soils this
total water capacity is between 35 and 40 per cent of the dry weight
of soil.
The gravitational soil-water cannot long remain in that condition;
for, necessarily, the pull of gravity moves it downward through the
soil pores and if conditions are favorable, it finally reaches the
standing water-table, whence it is carried to the great rivers, and
finally to the ocean. In humid soils, under a large precipitation,
gravitational water moves down to the standing water-table after
every rain. In dry-farm soils the gravitational water seldom reaches
the standing water-table; for, as it moves downward, it wets the
soil grains and remains in the capillary condition as a thin film
around the soil grains.
To the dry-farmer, the full water capacity is of importance only as
it pertains to the upper foot of soil. If, by proper plowing and
cultivation, the upper soil be loose and porous, the precipitation
is allowed to soak quickly into the soil, away from the action of
the wind and sun. From this temporary reservoir, the water, in
obedience to the pull of gravity, will move slowly downward to the
greater soil depths, where it will be stored permanently until
needed by plants. It is for this reason that dry-farmers find it
profitable to plow in the fall, as soon as possible after
harvesting. In fact, Campbell advocates that the harvester be
followed immediately by the disk, later to be followed by the plow
The essential thing is to keep the topsoil open and receptive to a
rain.
Capillary soil-water
The so-called capillary soil-water is of greatest importance to the
dry-farmer. This is the water that clings as a film around a marble
that has been dipped into water. There is a natural attraction
between water and nearly all known substances, as is witnessed by
the fact that nearly all things may be moistened. The water is held
around the marble because the attraction between the marble and the
water is greater than the pull of gravity upon the water. The
greater the attraction, the thicker the film; the smaller the
attraction, the thinner the film will be. The water that rises in a
capillary glass tube when placed in water does so by virtue of the
attraction between water and glass. Frequently, the force that makes
capillary water possible is called surface tension.
Whenever there is a sufficient amount of water available, a thin
film of water is found around every soil grain; and where the soil
grains touch, or where they are very near together, water is held
pretty much as in capillary tubes. Not only are the soil particles
enveloped by such a film, but the plant roots foraging in the soil
are likewise covered; that is, the whole system of soil grains and
roots is covered, under favorable conditions, with a thin film of
capillary water. It is the water in this form upon which plants draw
during their periods of growth. The hygroscopic water and the
gravitational water are of comparatively little value in plant
growth.
Field capacity of soils for capillary water
The tremendously large number of soil grains found in even a small
amount of soil makes it possible for the soil to hold very large
quantities of capillary water. To illustrate: In one cubic inch of
sand soil the total surface exposed by the soil grains varies from
42 square inches to 27 square feet; in one cubic inch of silt soil,
from 27 square feet to 72 square feet, and in one cubic inch of an
ordinary soil the total surface exposed by the soil grains is about
25 square feet. This means that the total surface of the soil grains
contained in a column of soil 1 square foot at the top and 10 feet
deep is approximately 10 acres. When even a thin film of water is
spread over such a large area, it is clear that the total amount of
water involved must be large It is to be noticed, therefore, that
the fineness of the soil particles previously discussed has a direct
bearing upon the amount of water that soils may retain for the use
of plant growth. As the fineness of the soil grains increases, the
total surface increases' and the water-holding capacity also
increases.
Naturally, the thickness of a water film held around the soil grains
is very minute. King has calculated that a film 275 millionths of an
inch thick, clinging around the soil particles, is equivalent to
14.24 per cent of water in a heavy clay; 7.2 per cent in a loam;
5.21 per cent in a sandy loam, and 1.41 per cent in a sandy soil.
It is important to know the largest amount of water that soils can
hold in a capillary condition, for upon it depend, in a measure, the
possibilities of crop production under dry-farming conditions. King
states that the largest amount of capillary water that can be held
in sandy loams varies from 17.65 per cent to 10.67 per cent; in clay
loams from 22.67 per cent to 18.16 per cent, and in humus soils
(which are practically unknown in dry-farm sections) from 44.72 per
cent to 21.29 per cent. These results were not obtained under
dry-farm conditions and must be confirmed by investigations of arid
soils.
The water that falls upon dry-farms is very seldom sufficient in
quantity to reach the standing water-table, and it is necessary,
therefore, to determine the largest percentage of water that a soil
can hold under the influence of gravity down to a depth of 8 or 10
feet--the depth to which the roots penetrate and in which root
action is distinctly felt. This is somewhat difficult to determine
because the many conflicting factors acting upon the soil-water are
seldom in equilibrium. Moreover, a considerable time must usually
elapse before the rain-water is thoroughly distributed throughout
the soil. For instance, in sandy soils, the downward descent of
water is very rapid; in clay soils, where the preponderance of fine
particles makes minute soil pores, there is considerable hindrance
to the descent of water, and it may take weeks or months for
equilibrium to be established. It is believed that in a dry-farm
district, where the major part of the precipitation comes during
winter, the early springtime, before the spring rains come, is the
best time for determining the maximum water capacity of a soil. At
that season the water-dissipating influences, such as sunshine and
high temperature, are at a minimum, and a sufficient time has
elapsed to permit the rains of fall and winter to distribute
themselves uniformly throughout the soil. In districts of high
summer precipitation, the late fall after a fallow season will
probably be the best time for the determination of the field-water
capacity.
Experiments on this subject have been conducted at the Utah Station.
As a result of several thousand trials it was found that, in the
spring, a uniform, sandy loam soil of true arid properties
contained, from year to year, an average of nearly 16-1/2 per cent
of water to a depth of 8 feet. This appeared to be practically the
maximum water capacity of that soil under field conditions, and it
may be called the field capacity of that soil for capillary water.
Other experiments on dry-farms showed the field capacity of a clay
soil to a depth of 8 feet to be 19 per cent; of a clay loam, to be
18 per cent; of a loam, 17 per cent; of another loam somewhat more
sandy, 16 per cent; of a sandy loam, 14-1/2 per cent; and of a very
sandy loam, 14 per cent. Leather found that in the calcareous arid
soil of India the upper 5 feet contained 18 per cent of water at the
close of the wet season.
It may be concluded, therefore, that the field-water capacities of
ordinary dry-farm soils are not very high, ranging from 15 to 20 per
cent, with an average for ordinary dry-farm soils in the
neighborhood of 16 or 17 per cent. Expressed in another way this
means that a layer of water from 2 to 3 inches deep can be stored in
the soil to a depth of 12 inches. Sandy soils will hold less water
than clayey ones. It must not be forgotten that in the dry-farm
region are numerous types of soils, among them some consisting
chiefly of very fine soil grains and which would; consequently,
possess field-water capacities above the average here stated. The
first endeavor of the dry-farmer should be to have the soil filled
to its full field-water capacity before a crop is planted.
Downward movement of soil-moisture
One of the chief considerations in a discussion of the storing of
water in soils is the depth to which water may move under ordinary
dry-farm conditions. In humid regions, where the water table is near
the surface and where the rainfall is very abundant, no question has
been raised concerning the possibility of the descent of water
through the soil to the standing water. Considerable objection,
however, has been offered to the doctrine that the rainfall of arid
districts penetrates the soil to any great extent. Numerous writers
on the subject intimate that the rainfall under dry-farm conditions
reaches at the best the upper 3 or 4 feet of soil. This cannot be
true, for the deep rich soils of the arid region, which never have
been disturbed by the husbandman, are moist to very great depths. In
the deserts of the Great Basin, where vegetation is very scanty,
soil borings made almost anywhere will reveal the fact that moisture
exists in considerable quantities to the full depth of the ordinary
soil auger, usually 10 feet. The same is true for practically every
district of the arid region.
Such water has not come from below, for in the majority of cases the
standing water is 50 to 500 feet below the surface. Whitney made
this observation many years ago and reported it as a striking
feature of agriculture in arid regions, worthy of serious
consideration. Investigations made at the Utah Station have shown
that undisturbed soils within the Great Basin frequently contain, to
a depth of 10 feet, an amount of water equivalent to 2 or 3 years of
the rainfall which normally occurs in that locality. These
quantities of water could not be found in such soils, unless, under
arid conditions, water has the power to move downward to
considerably greater depths than is usually believed by dry-farmers.
In a series of irrigation experiments conducted at the Utah Station
it was demonstrated that on a loam soil, within a few hours after an
irrigation, some of the water applied had reached the eighth foot,
or at least had increased the percentage of water in the eighth
foot. In soil that was already well filled with water, the addition
of water was felt distinctly to the full depth of 8 feet. Moreover,
it was observed in these experiments that even very small rains
caused moisture changes to considerable depths a few hours after the
rain was over. For instance, 0.14 of an inch of rainfall was felt to
a depth of 2 feet within 3 hours; 0.93 of an inch was felt to a
depth of 3 feet within the same period.
To determine whether or not the natural winter precipitation, upon
which the crops of a large portion of the dry-farm territory depend,
penetrates the soil to any great depth a series of tests were
undertaken. At the close of the harvest in August or September the
soil was carefully sampled to a depth of 8 feet, and in the
following spring similar samples were taken on the same soils to the
same depth. In every case, it was found that the winter
precipitation had caused moisture changes to the full depth reached
by the soil auger. Moreover, these changes were so great as to lead
the investigators to believe that moisture changes had occurred to
greater depths.
In districts where the major part of the precipitation occurs during
the summer the same law is undoubtedly in operation; but, since
evaporation is most active in the summer, it is probable that a
smaller proportion reaches the greater soil depths. In the Great
Plains district, therefore, greater care will have to be exercised
during the summer in securing proper water storage than in the Great
Basin, for instance. The principle is, nevertheless, the same. Burr,
working under Great Plains conditions in Nebraska, has shown that
the spring and summer rains penetrate the soil to the depth of 6
feet, the average depth of the borings, and that it undoubtedly
affects the soil-moisture to the depth of 10 feet. In general, the
dry-farmer may safely accept the doctrine that the water that falls
upon his land penetrates the soil far beyond the immediate reach of
the sun, though not so far away that plant roots cannot make use of
it.
Importance of a moist subsoil
In the consideration of the downward movement of soil-water it is to
be noted that it is only when the soil is tolerably moist that the
natural precipitation moves rapidly and freely to the deeper soil
layers. When the soil is dry, the downward movement of the water is
much slower and the bulk of the water is then stored near the
surface where the loss of moisture goes on most rapidly. It has been
observed repeatedly in the investigations at the Utah Station that
when desert land is broken for dry-farm purposes and then properly
cultivated, the precipitation penetrates farther and farther into
the soil with every year of cultivation. For example, on a dry-farm,
the soil of which is clay loam, and which was plowed in the fall of
1904 and farmed annually thereafter, the eighth foot contained in
the spring of 1905, 6.59 per cent of moisture; in the spring of
1906, 13.11 per cent, and in the spring of 1907, 14.75 per cent of
moisture. On another farm, with a very sandy soil and also plowed in
the fall of 1904, there was found in the eighth foot in the spring
of 1905, 5.63 per cent of moisture, in the spring of 1906, 11.41 per
cent of moisture, and in the spring of 1907, 15.49 per cent of
moisture. In both of these typical cases it is evident that as the
topsoil was loosened, the full field water capacity of the soil was
more nearly approached to a greater depth. It would seem that, as
the lower soil layers are moistened, the water is enabled, so to
speak, to slide down more easily into the depths of the soil.
This is a very important principle for the dry farmer to understand.
It is always dangerous to permit the soil of a dry-farm to become
very dry, especially below the first foot. Dry-farms should be so
manipulated that even at the harvesting season a comparatively large
quantity of water remains in the soil to a depth of 8 feet or more.
The larger the quantity of water in the soil in the fall, the more
readily and quickly will the water that falls on the land during the
resting period of fall, winter, and early spring sink into the soil
and move away from the topsoil. The top or first foot will always
contain the largest percentage of water because it is the chief
receptacle of the water that falls as rain or snow but when the
subsoil is properly moist, the water will more completely leave the
topsoil. Further, crops planted on a soil saturated with water to a
depth of 8 feet are almost certain to mature and yield well.
If the field-water capacity has not been filled, there is always the
danger that an unusually dry season or a series of hot winds or
other like circumstances may either seriously injure the crop or
cause a complete failure. The dry-farmer should keep a surplus of
moisture in the soil to be carried over from year to year, just as
the wise business man maintains a sufficient working capital for the
needs of his business. In fact, it is often safe to advise the
prospective dry-farmer to plow his newly cleared or broken land
carefully and then to grow no crop on it the first year, so that,
when crop production begins, the soil will have stored in it an
amount of water sufficient to carry a crop over periods of drouth.
Especially in districts of very low rainfall is this practice to be
recommended. In the Great Plains area, where the summer rains tempt
the farmer to give less attention to the soil-moisture problem than
in the dry districts with winter precipitation farther West, it is
important that a fallow season be occasionally given the land to
prevent the store of soil moisture from becoming dangerously low.
To what extent is the rainfall stored in soils?
What proportion of the actual amount of water falling upon the soil
can be stored in the soil and carried over from season to season?
This question naturally arises in view of the conclusion that water
penetrates the soil to considerable depths. There is comparatively
little available information with which to answer this question,
because the great majority of students of soil moisture have
concerned themselves wholly with the upper two, three, or four feet
of soil. The results of such investigations are practically useless
in answering this question. In humid regions it may be very
satisfactory to confine soil-moisture investigations to the upper
few feet; but in arid regions, where dry-farming is a living
question, such a method leads to erroneous or incomplete
conclusions.
Since the average field capacity of soils for water is about 2.5
inches per foot, it follows that it is possible to store 25 inches
of water in 10 feet of soil. This is from two to one and a half
times one year's rainfall over the better dry-farming sections.
Theoretically, therefore, there is no reason why the rainfall of one
season or more could not be stored in the soil. Careful
investigations have borne out this theory. Atkinson found, for
example, at the Montana Station, that soil, which to a depth of 9
feet contained 7.7 per cent of moisture in the fall contained 11.5
per cent in the spring and, after carrying it through the summer by
proper methods of cultivation, 11 per cent.
It may certainly be concluded from this experiment that it is
possible to carry over the soil moisture from season to season. The
elaborate investigations at the Utah Station have demonstrated that
the winter precipitation, that is, the precipitation that comes
during the wettest period of the year, may be retained in a large
measure in the soil. Naturally, the amount of the natural
precipitation accounted for in the upper eight feet will depend upon
the dryness of the soil at the time the investigation commenced. If
at the beginning of the wet season the upper eight feet of soil are
fairly well stored with moisture, the precipitation will move down
to even greater depths, beyond the reach of the soil auger. If, on
the other hand, the soil is comparatively dry at the beginning of
the season, the natural precipitation will distribute itself through
the upper few feet, and thus be readily measured by the soil auger.
In the Utah investigations it was found that of the water which fell
as rain and snow during the winter, as high as 95-1/2 per cent was
found stored in the first eight feet of soil at the beginning of the
growing season. Naturally, much smaller percentages were also found,
but on an average, in soils somewhat dry at the beginning of the dry
season, more than three fourths of the natural precipitation was
found stored in the soil in the spring. The results were all
obtained in a locality where the bulk of the precipitation comes in
the winter, yet similar results would undoubtedly be obtained where
the precipitation occurs mainly in the summer. The storage of water
in the soil cannot be a whit less important on the Great Plains than
in the Great Basin. In fact, Burr has clearly demonstrated for
western Nebraska that over 50 per cent of the rainfall of the spring
and summer may be stored in the soil to the depth of six feet.
Without question, some is stored also at greater depths.
All the evidence at hand shows that a large portion of the
precipitation falling upon properly prepared soil, whether it be
summer or winter, is stored in the soil until evaporation is allowed
to withdraw it Whether or not water so stored may be made to remain
in the soil throughout the season or the year will be discussed in
the next chapter. It must be said, however, that the possibility of
storing water in the soil, that is, making the water descend to
relatively great soil depths away from the immediate and direct
action of the sunshine and winds, is the most fundamental principle
in successful dry-farming.
The fallow
It may be safely concluded that a large portion of the water that
falls as rain or snow may be stored in the soil to considerable
depths (eight feet or more). However, the question remains, Is it
possible to store the rainfall of successive years in the soil for
the use of one crop? In short, Does the practice of clean fallowing
or resting the ground with proper cultivation for one season enable
the farmer to store in the soil the larger portion of the rainfall
of two years, to be used for one crop? It is unquestionably true, as
will be shown later, that clean fallowing or "summer tillage" is one
of the oldest and safest practices of dry-farming as practiced in
the West, but it is not generally understood why fallowing is
desirable.
Considerable doubt has recently been cast upon the doctrine that one
of the beneficial effects of fallowing in dry-farming is to store
the rainfall of successive seasons in the soil for the use of one
crop. Since it has been shown that a large proportion of the winter
precipitation can be stored in the soil during the wet season, it
merely becomes a question of the possibility of preventing the
evaporation of this water during the drier season. As will be shown
in the next chapter, this can well be effected by proper
cultivation.
There is no good reason, therefore, for believing that the
precipitation of successive seasons may not be added to water
already stored in the soil. King has shown that fallowing the soil
one year carried over per square foot, in the upper four feet, 9.38
pounds of water more than was found in a cropped soil in a parallel
experiment; and, moreover, the beneficial effect of this. water
advantage was felt for a whole succeeding season. King concludes,
therefore, that one of the advantages of fallowing is to increase
the moisture content of the soil. The Utah experiments show that the
tendency of fallowing is always to increase the soil-moisture
content. In dry-farming, water is the critical factor, and any
practice that helps to conserve water should be adopted. For that
reason, fallowing, which gathers soil-moisture, should be strongly
advocated. In Chapter IX another important value of the fallow will
be discussed.
In view of the discussion in this chapter it is easily understood
why students of soil-moisture have not found a material increase in
soil-moisture due to fallowing. Usually such investigations have
been made to shallow depths which already were fairly well filled
with moisture. Water falling upon such soils would sink beyond the
depth reached by the soil augers, and it became impossible to judge
accurately of the moisture-storing advantage of the fallow. A
critical analysis of the literature on this subject will reveal the
weakness of most experiments in this respect.
It may be mentioned here that the only fallow that should be
practiced by the dry-farmer is the clean fallow. Water storage is
manifestly impossible when crops are growing upon a soil. A healthy
crop of sagebrush, sunflowers, or other weeds consumes as much water
as a first-class stand of corn, wheat, or potatoes. Weeds should be
abhorred by the farmer. A weedy fallow is a sure forerunner of a
crop failure. How to maintain a good fallow is discussed in Chapter
VIII, under the head of Cultivation. Moreover, the practice of
fallowing should be varied with the climatic conditions. In
districts of low rainfall, 10-15 inches, the land should be clean
summer-fallowed every other year; under very low rainfall perhaps
even two out of three years; in districts of more abundant rainfall,
15-20 inches, perhaps one year out of every three or four is
sufficient. Where the precipitation comes during the growing season,
as in the Great Plains area, fallowing for the storage of water is
less important than where the major part of the rainfall comes
during the fall and winter. However, any system of dry-farming that
omits fallowing wholly from its practices is in danger of failure in
dry years.
Deep plowing for water storage
It has been attempted in this chapter to demonstrate that water
falling upon a soil may descend to great depths, and may be stored
in the soil from year to year, subject to the needs of the crop that
may be planted. By what cultural treatment may this downward descent
of the water be accelerated by the farmer? First and foremost, by
plowing at the right time and to the right depth. Plowing should be
done deeply and thoroughly so that the falling water may immediately
be drawn down to the full depth of the loose, spongy, plowed soil,
away from the action of the sunshine or winds. The moisture thus
caught will slowly work its way down into the lower layers of the
soil. Deep plowing is always to be recommended for successful
dry-farming.
In humid districts where there is a great difference between the
soil and the subsoil, it is often dangerous to turn up the lifeless
subsoil, but in arid districts where there is no real
differentiation between the soil and the subsoil, deep plowing may
safely be recommended. True, occasionally, soils are found in the
dry-farm territory which are underlaid near the surface by an inert
clay or infertile layer of lime or gypsum which forbids the farmer
putting the plow too deeply into the soil. Such soils, however' are
seldom worth while trying for dry-farm purposes. Deep plowing must
be practiced for the best dry-farming results.
It naturally follows that subsoiling should be a beneficial practice
on dry-farms. Whether or not the great cost of subsoiling is offset
by the resulting increased yields is an open question; it is, in
fact, quite doubtful. Deep plowing done at the right time and
frequently enough is possibly sufficient. By deep plowing is meant
stirring or turning the soil to a depth of six to ten inches below
the surface of the land.
Fall plowing far water storage
It is not alone sufficient to plow and to plow deeply; it is also
necessary that the plowing be done at the right time. In the very
great majority of cases over the whole dry-farm territory, plowing
should be done in the fall. There are three reasons for this: First,
after the crop is harvested, the soil should be stirred immediately,
so that it can be exposed to the full action of the weathering
agencies, whether the winters be open or closed. If for any reason
plowing cannot be done early it is often advantageous to follow the
harvester with a disk and to plow later when convenient. The
chemical effect on the soil resulting from the weathering, made
possible by fall plowing, as will be shown in Chapter IX, is of
itself so great as to warrant the teaching of the general practice
of fall plowing. Secondly, the early stirring of the soil prevents
evaporation of the moisture in the soil during late summer and the
fall. Thirdly, in the parts of the dry-farm territory where much
precipitation occurs in the fall, winter, or early spring, fall
plowing permits much of this precipitation to enter the soil and be
stored there until needed by plants.
A number of experiment stations have compared plowing done in the
early fall with plowing done late in the fall or in the spring, and
with almost no exception it has been found that early fall plowing
is water-conserving and in other ways advantageous. It was observed
on a Utah dry-farm that the fall-plowed land contained, to a depth
of 10 feet, 7.47 acre-inches more water than the adjoining
spring-plowed land--a saving of nearly one half of a year's
precipitation. The ground should be plowed in the early fall as soon
as possible after the crop is harvested. It should then be left in
the rough throughout the winter, so that it may be mellowed and
broken down by the elements. The rough lend further has a tendency
to catch and hold the snow that may be blown by the wind, thus
insuring a more even distribution of the water from the melting
snow.
A common objection to fall plowing is that the ground is so dry in
the fall that it does not plow up well, and that the great dry clods
of earth do much to injure the physical condition of the soil. It is
very doubtful if such an objection is generally valid, especially if
the soil is so cropped as to leave a fair margin of moisture in the
soil at harvest time. The atmospheric agencies will usually break
down the clods, and the physical result of the treatment will be
beneficial. Undoubtedly, the fall plowing of dry land is somewhat
difficult, but the good results more than pay the farmer for his
trouble. Late fall plowing, after the fall rains have softened the
land, is preferable to spring plowing. If for any reason the farmer
feels that he must practice spring plowing, he should do it as early
as possible in the spring. Of course, it is inadvisable to plow the
soil when it is so wet as to injure its tilth seriously, but as soon
as that danger period has passed, the plow should be placed in the
ground. The moisture in the soil will thereby be conserved, and
whatever water may fall during the spring months will be conserved
also. This is of especial importance in the Great Plains region and
in any district where the precipitation comes in the spring and
winter months.
Likewise, after fall plowing, the land must be well stirred in the
early spring with the disk harrow or a similar implement, to enable
the spring rains to enter the soil easily and to prevent the
evaporation of the water already stored. Where the rainfall is quite
abundant and the plowed land has been beaten down by the frequent
rains, the land should be plowed again in the spring. Where such
conditions do not exist, the treatment of the soil with the disk and
harrow in the spring is usually sufficient.
In recent dry-farm experience it has been fairly completely
demonstrated that, providing the soil is well stored with water,
crops will mature even if no rain falls during the growing season.
Naturally, under most circumstances, any rains that may fall on a
well-prepared soil during the season of crop growth will tend to
increase the crop yield, but some profitable yield is assured, in
spite of the season, if the soil is well stored with water at seed
time. This is an important principle in the system of dry-farming.
CHAPTER VIII
REGULATING THE EVAPORATION
The demonstration in the last chapter that the water which falls as
rain or snow may be stored in the soil for the use of plants is of
first importance in dry-farming, for it makes the farmer
independent, in a large measure, of the distribution of the
rainfall. The dry-farmer who goes into the summer with a soil well
stored with water cares little whether summer rains come or not, for
he knows that his crops will mature in spite of external drouth. In
fact, as will be shown later, in many dry-farm sections where the
summer rains are light they are a positive detriment to the farmer
who by careful farming has stored his deep soil with an abundance of
water. Storing the soil with water is, however, only the first step
in making the rains of fall, winter, or the preceding year available
for plant growth. As soon as warm growing weather comes,
water-dissipating forces come into play, and water is lost by
evaporation. The farmer must, therefore, use all precautions to keep
the moisture in the soil until such time as the roots of the crop
may draw it into the plants to be used in plant production. That is,
as far as possible, direct evaporation of water from the soil must
be prevented.
Few farmers really realize the immense possible annual evaporation
in the dry-farm territory. It is always much larger than the total
annual rainfall. In fact, an arid region may be defined as one in
which under natural conditions several times more water evaporates
annually from a free water surface than falls as rain and snow. For
that reason many students of aridity pay little attention to
temperature, relative humidity, or winds, and simply measure the
evaporation from a free water surface in the locality in question.
In order to obtain a measure of the aridity, MacDougal has
constructed the following table, showing the annual precipitation
and the annual evaporation at several well-known localities in the
dry-farm territory.
True, the localities included in the following table are extreme,
but they illustrate the large possible evaporation, ranging from
about six to thirty-five times the precipitation. At the same time
it must be borne in mind that while such rates of evaporation may
occur from free water surfaces, the evaporation from agricultural
soils under like conditions is very much smaller.
Place Annual Precipitation Annual Evaporation Ratio
(In Inches) (In Inches)
El Paso, Texas 9.23 80 8.7
Fort Wingate,
New Mexico 14.00 80 5.7
Fort Yuma,
Arizona 2.84 100 35.2
Tucson, AZ 11.74 90 7.7
Mohave, CA 4.97 95 19.1
Hawthorne,
Nevada 4.50 80 17.5
Winnemucca,
Nevada 9.51 80 9.6
St. George, Utah 6.46 90 13.9
Fort Duchesne,
Utah 6.49 75 11.6
Pineville,
Oregon 9.01 70 7.8
Lost River,
Idaho 8.47 70 8.3
Laramie,
Wyoming 9.81 70 7.1
Torres, Mexico 16.97 100 6.0
To understand the methods employed for checking evaporation from the
soil, it is necessary to review briefly the conditions that
determine the evaporation of water into the air, and the manner in
which water moves in the soil.
The formation of water vapor
Whenever water is left freely exposed to the air, it evaporates;
that is, it passes into the gaseous state and mixes with the gases
of the air. Even snow and ice give off water vapor, though in very
small quantities. The quantity of water vapor which can enter a
given volume of air is definitely limited. For instance, at the
temperature of freezing water 2.126 grains of water vapor can enter
one cubic foot of air, but no more. When air contains all the water
possible, it is said to be saturated, and evaporation then ceases.
The practical effect of this is the well-known experience that on
the seashore, where the air is often very nearly fully saturated
with water vapor, the drying of clothes goes on very slowly, whereas
in the interior, like the dry-farming territory, away from the
ocean, where the air is far from being saturated, drying goes on
very rapidly.
The amount of water necessary to saturate air varies greatly with
the temperature. It is to be noted that as the temperature
increases, the amount of water that may be held by the air also
increases; and proportionately more rapidly than the increase in
temperature. This is generally well understood in common experience,
as in drying clothes rapidly by hanging them before a hot fire. At a
temperature of 100 deg F., which is often reached in portions of the
dry-farm territory during the growing season, a given volume of air
can hold more than nine times as much water vapor as at the
temperature of freezing water. This is an exceedingly important
principle in dry-farm practices, for it explains the relatively easy
possibility of storing water during the fall and winter when the
temperature is low and the moisture usually abundant, and the
greater difficulty of storing the rain that falls largely, as in the
Great Plains area, in the summer when water-dissipating forces are
very active. This law also emphasizes the truth that it is in times
of warm weather that every precaution must be taken to prevent the
evaporation of water from the soil surface.
Temperature Grains of Water held in
in Degrees F. One Cubic Foot of Air
32 2.126
40 2.862
50 4.089
60 5.756
70 7.992
80 10.949
90 14.810
100 19.790
It is of course well understood that the atmosphere as a whole is
never saturated with water vapor. Such saturation is at the best
only local, as, for instance, on the seashore during quiet days,
when the layer of air over the water may be fully saturated, or in a
field containing much water from which, on quiet warm days, enough
water may evaporate to saturate the layer of air immediately upon
the soil and around the plants. Whenever, in such cases, the air
begins to move and the wind blows, the saturated air is mixed with
the larger portion of unsaturated air, and evaporation is again
increased. Meanwhile, it must be borne in mind that into a layer of
saturated air resting upon a field of growing plants very little
water evaporates, and that the chief water-dissipating power of
winds lies in the removal of this saturated layer. Winds or air
movements of any kind, therefore, become enemies of the farmer who
depends upon a limited rainfall.
The amount of water actually found in a given volume of air at a
certain temperature, compared with the largest amount it can hold,
is called the relative humidity of the air. As shown in Chapter IV,
the relative humidity becomes smaller as the rainfall decreases. The
lower the relative humidity is at a given temperature, the more
rapidly will water evaporate into the air. There is no more striking
confirmation of this law than the fact that at a temperature of 90
deg sunstrokes and similar ailments are reported in great number
from New York, while the people of Salt Lake City are perfectly
comfortable. In New York the relative humidity in summer is about 73
per cent; in Salt Lake City, about 35 per cent. At a high summer
temperature evaporation from the skin goes on slowly in New York and
rapidly in Salt Lake City, with the resulting discomfort or comfort.
Similarly, evaporation from soils goes on rapidly under a low and
slowly under a high percentage of relative humidity.
Evaporation from water surfaces is hastened, therefore, by (1) an
increase in the temperature, (2) an increase in the air movements or
winds, and (3) a decrease in the relative humidity. The temperature
is higher; the relative humidity lower, and the winds usually more
abundant in arid than in humid regions. The dry-farmer must
consequently use all possible precautions to prevent evaporation
from the soil.
Conditions of evaporation from from soils
Evaporation does not alone occur from a surface of free water. All
wet or moist substances lose by evaporation most of the water that
they hold, providing the conditions of temperature and relative
humidity are favorable. Thus, from a wet soil, evaporation is
continually removing water. Yet, under ordinary conditions, it is
impossible to remove all the water, for a small quantity is
attracted so strongly by the soil particles that only a temperature
above the boiling point of water will drive it out. This part of the
soil is the hygroscopic moisture spoken of in the last chapter.
Moreover, it must be kept in mind that evaporation does not occur as
rapidly from wet soil as from a water surface, unless all the soil
pores are so completely filled with water that the soil surface is
practically a water surface. The reason for this reduced evaporation
from a wet soil is almost self-evident. There is a comparatively
strong attraction between soil and water, which enables the moisture
to cling as a thin capillary film around the soil particles, against
the force of gravity. Ordinarily, only capillary water is found in
well-tilled soil, and the force causing evaporation must be strong
enough to overcome this attraction besides changing the water into
vapor.
The less water there is in a soil, the thinner the water film, and
the more firmly is the water held. Hence, the rate of evaporation
decreases with the decrease in soil-moisture. This law is confirmed
by actual field tests. For instance, as an average of 274 trials
made at the Utah Station, it was found that three soils, otherwise
alike, that contained, respectively, 22.63 per cent, 17.14 per cent,
and 12.75 per cent of water lost in two weeks, to a depth of eight
feet, respectively 21.0, 17. 1, and 10.0 pounds of water per square
foot. Similar experiments conducted elsewhere also furnish proof of
the correctness of this principle. From this point of view the
dry-farmer does not want his soils to be unnecessarily moist. The
dry-farmer can reduce the per cent of water in the soil without
diminishing the total amount of water by so treating the soil that
the water will distribute itself to considerable depths. This brings
into prominence again the practices of fall plowing, deep plowing,
subsoiling, and the choice of deep soils for dry-farming.
Very much for the same reasons, evaporation goes on more slowly from
water in which salt or other substances have been dissolved. The
attraction between the water and the dissolved salt seems to be
strong enough to resist partially the force causing evaporation.
Soil-water always contains some of the soil ingredients in solution,
and consequently under the given conditions evaporation occurs more
slowly from soil-water than from pure water. Now, the more fertile a
soil is, that is, the more soluble plant-food it contains, the more
material will be dissolved in the soil-water, and as a result the
more slowly will evaporation take place. Fallowing, cultivation,
thorough plowing and manuring, which increase the store of soluble
plant-food, all tend to diminish evaporation. While these conditions
may have little value in the eyes of the farmer who is under an
abundant rainfall, they are of great importance to the dry-farmer.
It is only by utilizing every possibility of conserving water and
fertility that dry-farming may be made a perfectly safe practice.
Loss by evaporation chiefly at the surface
Evaporation goes on from every wet substance. Water evaporates
therefore from the wet soil grains under the surface as well as from
those at the surface. In developing a system of practice which will
reduce evaporation to a minimum it must be learned whether the water
which evaporates from the soil particles far below the surface is
carried in large quantities into the atmosphere and thus lost to
plant use. Over forty years ago, Nessler subjected this question to
experiment and found that the loss by evaporation occurs almost
wholly at the soil surface, and that very little if any is lost
directly by evaporation from the lower soil layers. Other
experimenters have confirmed this conclusion, and very recently
Buckingham, examining the same subject, found that while there is a
very slow upward movement of the soil gases into the atmosphere, the
total quantity of the water thus lost by direct evaporation from
soil, a foot below the surface, amounted at most to one inch of
rainfall in six years. This is insignificant even under semiarid and
arid conditions. However, the rate of loss of water by direct
evaporation from the lower soil layers increases with the porosity
of the soil, that is, with the space not filled with soil particles
or water. Fine-grained soils, therefore, lose the least water in
this manner. Nevertheless, if coarse-grained soils are well filled
with water, by deep fall plowing and by proper summer fallowing for
the conservation of moisture, the loss of moisture by direct
evaporation from the lower soil layers need not be larger than from
finer grained soils
Thus again are emphasized the principles previously laid down that,
for the most successful dry-farming, the soil should always be kept
well filled with moisture, even if it means that the land, after
being broken, must lie fallow for one or two seasons, until a
sufficient amount of moisture has accumulated. Further, the
correlative principle is emphasized that the moisture in dry-farm
lands should be stored deeply, away from the immediate action of the
sun's rays upon the land surface. The necessity for deep soils is
thus again brought out.
The great loss of soil moisture due to an accumulation of water in
the upper twelve inches is well brought out in the experiments
conducted by the Utah Station. The following is selected from the
numerous data on the subject. Two soils, almost identical in
character, contained respectively 17.57 per cent and 16.55 per cent
of water on an average to a depth of eight feet; that is, the total
amount of water held by the two soils was practically identical.
Owing to varying cultural treatment, the distribution of the water
in the soil was not uniform; one contained 23.22 per cent and the
other 16.64 per cent of water in the first twelve inches. During the
first seven days the soil that contained the highest percentage of
water in the first foot lost 13.30 pounds of water, while the other
lost only 8.48 pounds per square foot. This great difference was due
no doubt to the fact that direct evaporation takes place in
considerable quantity only in the upper twelve inches of soil, where
the sun's heat has a full chance to act.
Any practice which enables the rains to sink quickly to considerable
depths should be adopted by the dry-farmer. This is perhaps one of
the great reasons for advocating the expensive but usually effective
subsoil plowing on dry-farms. It is a very common experience, in the
arid region, that great, deep cracks form during hot weather. From
the walls of these cracks evaporation goes on, as from the topsoil,
and the passing winds renew the air so that the evaporation may go
on rapidly. The dry-farmer must go over the land as often as needs
be with some implement that will destroy and fill up the cracks that
may have been formed. In a field of growing crops this is often
difficult to do; but it is not impossible that hand hoeing,
expensive as it is, would pay well in the saving of soil moisture
and the consequent increase in crop yield.
How soil water reaches the surface
It may be accepted as an established truth that the direct
evaporation of water from wet soils occurs almost wholly at the
surface. Yet it is well known that evaporation from the soil surface
may continue until the soil-moisture to a depth of eight or ten feet
or more is depleted. This is shown by the following analyses of
dry-farm soil in early spring and midsummer. No attempt was made to
conserve the moisture in the soil:--
Per cent of water in Early spring Midsummer
1st foot 20.84 8.83
2nd foot 20.06 8.87
3rd foot 19.62 11.03
4th foot 18.28 9.59
5th foot 18.70 11.27
6th foot 14.29 11.03
7th foot 14.48 8.95
8th foot 13.83 9.47
Avg 17.51 9.88
In this case water had undoubtedly passed by capillary movement from
the depth of eight feet to a point near the surface where direct
evaporation could occur. As explained in the last chapter, water
which is held as a film around the soil particles is called
capillary water; and it is in the capillary form that water may be
stored in dry-farm soils. Moreover, it is the capillary
soil-moisture alone which is of real value in crop production. This
capillary water tends to distribute itself uniformly throughout the
soil, in accordance with the prevailing conditions and forces. If no
water is removed from the soil, in course of time the distribution
of the soil-water will be such that the thickness of the film at any
point in the soil mass is a direct resultant of the various forces
acting at that particular point. There will then be no appreciable
movement of the soil-moisture. Such a condition is approximated in
late winter or early spring before planting begins. During the
greater part of the year, however, no such quiescent state can
occur, for there are numerous disturbing elements that normally are
active, among which the three most effective are (l) the addition of
water to the soil by rains; (2) the evaporation of water from the
topsoil, due to the more active meteorological factors during
spring, summer, and fall; and (3) the abstraction of water from the
soil by plant roots.
Water, entering the soil, moves downward under the influence of
gravity as gravitational water, until under the attractive influence
of the soil it has been converted into capillary water and adheres
to the soil particles as a film. If the soil were dry, and the film
therefore thin, the rain water would move downward only a short
distance as gravitational water; if the soil were wet, and the film
therefore thick, the water would move down to a greater distance
before being exhausted. If, as is often the case in humid districts,
the soil is saturated, that is, the film is as thick as the
particles can hold, the water would pass right through the soil and
connect with the standing water below. This, of course, is seldom
the case in dry-farm districts. In any soil, excepting one already
saturated, the addition of water will produce a thickening of the
soil-water film to the full descent of the water. This immediately
destroys the conditions of equilibrium formerly existing, for the
moisture is not now uniformly distributed. Consequently a process of
redistribution begins which continues until the nearest approach to
equilibrium is restored. In this process water will pass in every
direction from the wet portion of the soil to the drier; it does not
necessarily mean that water will actually pass from the wet portion
to the drier portion; usually, at the driest point a little water is
drawn from the adjoining point, which in turn draws from the next,
and that from the next, until the redistribution is complete. The
process is very much like stuffing wool into a sack which already is
loosely filled. The new wool does not reach the bottom of the sack,
yet there is more wool in the bottom than there was before.
If a plant-root is actively feeding some distance under the soil
surface, the reverse process occurs. At the feeding point the root
continually abstracts water from the soil grains and thus makes the
film thinner in that locality. This causes a movement of moisture
similar to the one above described, from the wetter portions of the
soil to the portion being dried out by the action of the plant-root.
Soil many feet or even rods distant may assist in supplying such an
active root with moisture. When the thousands of tiny roots sent out
by each plant are recalled. it may well be understood what a
confusion of pulls and counter-pulls upon the soil-moisture exists
in any cultivated soil. In fact, the soil-water film may be viewed
as being in a state of trembling activity, tending to place itself
in full equilibrium with the surrounding contending forces which,
themselves, constantly change. Were it not that the water film held
closely around the soil particles is possessed of extreme mobility,
it would not be possible to meet the demands of the plants upon the
water at comparatively great distances. Even as it is, it frequently
happens that when crops are planted too thickly on dry-farms, the
soil-moisture cannot move quickly enough to the absorbing roots to
maintain plant growth, and crop failure results. Incidentally, this
points to planting that shall be proportional to the moisture
contained by the soil. See Chapter XI.
As the temperature rises in spring, with a decrease in the relative
humidity, and an increase in direct sunshine, evaporation from the
soil surface increases greatly. However, as the topsoil becomes
drier, that is, as the water fihn becomes thinner, there is an
attempt at readjustment, and water moves upward to take the place of
that lost by evaporation. As this continues throughout the season,
the moisture stored eight or ten feet or more below the surface is
gradually brought to the top and evaporated, and thus lost to plant
use.
The effect of rapid top drying of soils
As the water held by soils diminishes, and the water film around the
soil grains becomes thinner, the capillary movement of the
soil-water is retarded. This is easily understood by recalling that
the soil particles have an attraction for water, which is of
definite value, and may be measured by the thickest film that may be
held against gravity. When the film is thinned, it does not diminish
the attraction of the soil for water; it simply results in a
stronger pull upon the water and a firmer holding of the film
against the surfaces of the soil grains. To move soil-water under
such conditions requires the expenditure of more energy than is
necessary for moving water in a saturated or nearly saturated soil.
Under like conditions, therefore, the thinner the soil-water film
the more difficult will be the upward movement of the soil-water and
the slower the evaporation from the topsoil.
As drying goes on, a point is reached at which the capillary
movement of the water wholly ceases. This is probably when little
more than the hygroscopic moisture remains. In fact, very dry soil
and water repel each other. This is shown in the common experience
of driving along a road in summer, immediately after a light shower.
The masses of dust are wetted only on the outside, and as the wheels
pass through them the dry dust is revealed. It is an important fact
that very dry soil furnishes a very effective protection against the
capillary movement of water.
In accordance with the principle above established if the surface
soil could be dried to the point where capillarity is very slow, the
evaporation would be diminished or almost wholly stopped. More than
a quarter of a century ago, Eser showed experimentally that
soil-water may be saved by drying the surface soil rapidly. Under
dry-farm conditions it frequently occurs that the draft upon the
water of the soil is so great that nearly all the water is quickly
and so completely abstracted from the upper few inches of soil that
they are left as an effective protection against further
evaporation. For instance, in localities where hot dry winds are of
common occurrence, the upper layer of soil is sometimes completely
dried before the water in the lower layers can by slow capillary
movement reach the top. The dry soil layer then prevents further
loss of water, and the wind because of its intensity has helped to
conserve the soil-moisture. Similarly in localities where the
relative humidity is low, the sunshine abundant, and the temperature
high, evaporation may go on so rapidly that the lower soil layers
cannot supply the demands made, and the topsoil then dries out so
completely as to form a protective covering against further
evaporation. It is on this principle that the native desert soils of
the United States, untouched by the plow, and the surfaces of which
are sun-baked, are often found to possess large percentages of water
at lower depths. Whitney recorded this observation with considerable
surprise, many years ago, and other observers have found the same
conditions at nearly all points of the arid region. This matter has
been subjected to further study by Buckingham, who placed a variety
of soils under artificially arid and humid conditions. It was found
in every case that, the initial evaporation was greater under arid
conditions, but as the process went on and the topsoil of the arid
soil became dry, more water was lost under humid conditions. For the
whole experimental period, also, more water was lost under humid
conditions. It was notable that the dry protective layer was formed
more slowly on alkali soils, which would point to the inadvisability
of using alkali lands for dry-farm purposes. All in all, however, it
appears "that under very arid conditions a soil automatically
protects itself from drying by the formation of a natural mulch on
the surface."
Naturally, dry-farm soils differ greatly in their power of forming
such a mulch. A heavy clay or a light sandy soil appears to have
less power of such automatic protection than a loamy soil. An
admixture of limestone seems to favor the formation of such a
natural protective mulch. Ordinarily, the farmer can further the
formation of a dry topsoil layer by stirring the soil thoroughly.
This assists the sunshine and the air to evaporate the water very
quickly. Such cultivation is very desirable for other reasons also,
as will soon be discussed. Meanwhile, the water-dissipating forces
of the dry-farm section are not wholly objectionable, for whether
the land be cultivated or not, they tend to hasten the formation of
dry surface layers of soil which guard against excessive
evaporation. It is in moist cloudy weather, when the drying process
is slow, that evaporation causes the greatest losses of
soil-moisture.
The effect of shading
Direct sunshine is, next to temperature, the most active cause of
rapid evaporation from moist soil surfaces. Whenever, therefore,
evaporation is not rapid enough to form a dry protective layer of
topsoil, shading helps materially in reducing surface losses of
soil-water. Under very arid conditions, however, it is questionable
whether in all cases shading has a really beneficial effect, though
under semiarid or sub-humid conditions the benefits derived from
shading are increased largely. Ebermayer showed in 1873 that the
shading due to the forest cover reduced evaporation 62 per cent, and
many experiments since that day have confirmed this conclusion. At
the Utah Station, under arid conditions, it was found that shading a
pot of soil, which otherwise was subjected to water-dissipating
influences, saved 29 per cent of the loss due to evaporation from a
pot which was not shaded. This principle cannot be applied very
greatly in practice, but it points to a somewhat thick planting,
proportioned to the water held by the soil. It also shows a possible
benefit to be derived from the high header straw which is allowed to
stand for several weeks in dry-farm sections where the harvest comes
early and the fall plowing is done late, as in the mountain states.
The high header stubble shades the ground very thoroughly. Thus the
stubble may be made to conserve the soil-moisture in dry-farm
sections, where grain is harvested by the "header" method.
A special case of shading is the mulching of land with straw or
other barnyard litter, or with leaves, as in the forest. Such
mulching reduces evaporation, but only in part, because of its
shading action, since it acts also as a loose top layer of soil
matter breaking communication with the lower soil layers.
Whenever the soil is carefully stirred, as will be described, the
value of shading as a means or checking evaporation disappears
almost entirely. It is only with soils which are tolerably moist at
the surface that shading acts beneficially.
Alfalfa in cultivated rows. This practice is employed to make
possible the growth of alfalfa and other perennial crops on arid
lands without irrigation.
The effect of tillage
Capillary soil-moisture moves from particle to particle until the
surface is reached. The closer the soil grains are packed together,
the greater the number of points or contact, and the more easily
will the movement of the soil-moisture proceed. If by any means a
layer of the soil is so loosened as to reduce the number of points
of contact, the movement of the soil-moisture is correspondingly
hindered. The process is somewhat similar to the experience in large
r airway stations. Just before train time a great crowd of people is
gathered outside or the gates ready to show their tickets. If one
gate is opened, a certain number of passengers can pass through each
minute; if two are opened, nearly twice as many may be admitted in
the same time; if more gates are opened, the passengers will be able
to enter the train more rapidly. The water in the lower layers of
the soil is ready to move upward whenever a call is made upon it. To
reach the surface it must pass from soil grain to soil grain, and
the larger the number of grains that touch, the more quickly and
easily will the water reach the surface, for the points of contact
of the soil particles may be likened to the gates of the railway
station. Now if, by a thorough stirring and loosening of the
topsoil, the number of points of contact between the top and subsoil
is greatly reduced, the upward flow of water is thereby largely
checked. Such a loosening of the topsoil for the purpose of reducing
evaporation from the topsoil has come to be called cultivation, and
includes plowing, harrowing, disking, hoeing, and other cultural
operations by which the topsoil is stirred. The breaking of the
points of contact between the top and subsoil is undoubtedly the
main reason for the efficiency of cultivation, but it is also to be
remembered that such stirring helps to dry the top soil very
thoroughly, and as has been explained a layer of dry soil of itself
is a very effective check upon surface evaporation.
That the stirring or cultivation of the topsoil really does diminish
evaporation of water from the soil has been shown by numerous
investigations. In 1868, Nessler found that during six weeks of an
ordinary German summer a stirred soil lost 510 grams of water per
square foot, while the adjoining compacted soil lost 1680 grams,--a
saving due to cultivation of nearly 60 per cent. Wagner, testing the
correctness of Nessler's work, found, in 1874, that cultivation
reduced the evaporation a little more than 60 per cent; Johnson, in
1878, confirmed the truth of the principle on American soils, and
Levi Stockbridge, working about the same time, also on American
soils, found that cultivation diminished evaporation on a clay soil
about 23 per cent, on a sandy loam 55 per cent, and on a heavy loam
nearly 13 per cent. All the early work done on this subject was done
under humid conditions, and it is only in recent years that
confirmation of this important principle has been obtained for the
soils of the dry-farm region. Fortier, working under California
conditions, determined that cultivation reduced the evaporation from
the soil surface over 55 per cent. At the Utah Station similar
experiments have shown that the saving of soil-moisture by
cultivation was 63 per cent for a clay soil, 34 per cent for a
coarse sand, and 13 per cent for a clay loam. Further, practical
experience has demonstrated time and time again that in cultivation
the dry-farmer has a powerful means of preventing evaporation from
agricultural soils.
Closely connected with cultivation is the practice of scattering
straw or other litter over the ground. Such artificial mulches are
very effective in reducing evaporation. Ebermayer found that by
spreading straw on the land, the evaporation was reduced 22 per
cent; Wagner found under similar conditions a saving of 38 per cent,
and these results have been confirmed by many other investigators.
On the modern dry-farms, which are large in area, the artificial
mulching of soils cannot become a very extensive practice, yet it is
well to bear the principle in mind. The practice of harvesting
dry-farm grain with the header and plowing under the high stubble in
the fall is a phase of cultivation for water conservation that
deserves special notice. The straw, thus incorporated into the soil,
decomposes quite readily in spite of the popular notion to the
contrary, and makes the soil more porous, and, therefore, more
effectively worked for the prevention of evaporation. When this
practice is continued for considerable periods, the topsoil becomes
rich in organic matter, which assists in retarding evaporation,
besides increasing the fertility of the land. When straw cannot be
fed to advantage, as is yet the case on many of the western
dry-farms, it would be better to scatter it over the land than to
burn it, as is often done. Anything that covers the ground or
loosens the topsoil prevents in a measure the evaporation of the
water stored in lower soil depths for the use of crops.
Depth of cultivation
The all-important practice for the dry-farmer who is entering upon
the growing season is cultivation. The soil must be covered
continually with a deep layer of dry loose soil, which because of
its looseness and dryness makes evaporation difficult. A leading
question in connection with cultivation is the depth to which the
soil should be stirred for the best results. Many of the early
students of the subject found that a soil mulch only one half inch
in depth was effective in retaining a large part of the
soil-moisture which noncultivated soils would lose by evaporation.
Soils differ greatly in the rate of evaporation from their surfaces.
Some form a natural mulch when dried, which prevents further water
loss. Others form only a thin hard crust, below which lies an active
evaporating surface of wet soil. Soils which dry out readily and
crumble on top into a natural mulch should be cultivated deeply, for
a shallow cultivation does not extend beyond the naturally formed
mulch. In fact, on certain calcareous soils, the surfaces of which
dry out quickly and form a good protection against evaporation,
shallow cultivations often cause a greater evaporation by disturbing
the almost perfect natural mulch. Clay or sand soils, which do not
so well form a natural mulch, will respond much better to shallow
cultivations. In general, however, the deeper the cultivation, the
more effective it is in reducing evaporation. Fortier, in the
experiments in California to which allusion has already been made,
showed the greater value of deep cultivation. During a period of
fifteen days, beginning immediately after an irrigation, the soil
which had not been mulched lost by evaporation nearly one fourth of
the total amount of water that had been added. A mulch 4 inches deep
saved about 72 per cent of the evaporation; a mulch 8 inches deep
saved about 88 per cent, and a mulch 10 inches deep stopped
evaporation almost wholly. It is a most serious mistake for the
dry-farmer, who attempts cultivation for soil-moisture conservation,
to fail to get the best results simply to save a few cents per acre
in added labor.
When to cultivate or till
It has already been shown that the rate of evaporation is greater
from a wet than from a dry surface. It follows, therefore, that the
critical time for preventing evaporation is when the soil is
wettest. After the soil is tolerably dry, a very large portion of
the soil-moisture has been lost, which possibly might have been
saved by earlier cultivation. The truth of this statement is well
shown by experiments conducted by the Utah Station. In one case on a
soil well filled with water, during a three weeks' period, nearly
one half of the total loss occurred the first, while only one fifth
fell on the third week. Of the amount lost during the first week,
over 60 per cent occurred during the first three days. Cultivation
should, therefore, be practiced as soon as possible after conditions
favorable for evaporation have been established. This means, first,
that in early spring, just as soon as the land is dry enough to be
worked without causing puddling, the soil should be deeply and
thoroughly stirred. Spring plowing, done as early as possible, is an
excellent practice for forming a mulch against evaporation. Even
when the land has been fall-plowed, spring plowing is very
beneficial, though on fall-plowed land the disk harrow is usually
used in early spring, and if it is set at rather a sharp angle, and
properly weighted, so that it cuts deeply into the ground, it is
practically as effective as spring plowing. The chief danger to the
dry-farmer is that he will permit the early spring days to slip by
until, when at last he begins spring cultivation, a large portion of
the stored soil-water has been evaporated. It may be said that deep
fall plowing, by permitting the moisture to sink quickly into the
lower layers of soil, makes it possible to get upon the ground
earlier in the spring. In fact, unplowed land cannot be cultivated
as early as that which has gone through the winter in a plowed
condition
If the land carries a fall-sown crop, early spring cultivation is
doubly important. As soon as the plants are well up in spring the
land should be gone over thoroughly several times if necessary, with
an iron tooth harrow, the teeth of which are set to slant backward
in order not to tear up the plants. The loose earth mulch thus
formed is very effective in conserving moisture; and the few plants
torn up are more than paid for by the increased water supply for the
remaining plants. The wise dry-fanner cultivates his land, whether
fallow or cropped, as early as possible in the spring.
Following the first spring plowing, disking, or cultivation, must
come more cultivation. Soon after the spring plowing, the land
should be disked and. then harrowed. Every device should be used to
secure the formation of a layer of loose drying soil over the land
surface. The season's crop will depend largely upon the
effectiveness of this spring treatment.
As the season advances, three causes combine to permit the
evaporation of soil-moisture.
First, there is a natural tendency, under the somewhat moist
conditions of spring, for the soil to settle compactly and thus to
restore the numerous capillary connections with the lower soil
layers through which water escapes. Careful watch should therefore
be kept upon the soil surface, and whenever the mulch is not loose,
the disk or harrow should be run over the land.
Secondly, every rain of spring or summer tends to establish
connections with the store of moisture in the soil. In fact, late
spring and summer rains are often a disadvantage on dry-farms, which
by cultural treatment have been made to contain a large store of
moisture. It has been shown repeatedly that light rains draw
moisture very quickly from soil layers many feet below the surface.
The rainless summer is not feared by the dry-farmer whose soils are
fertile and rich in moisture. It is imperative that at the very
earliest moment after a spring or summer rain the topsoil be well
stirred to prevent evaporation. It thus happens that in sections of
frequent summer rains, as in the Great Plains area, the farmer has
to harrow his land many times in succession, but the increased crop
yields invariably justify the added expenditure of effort.
Thirdly, on the summer-fallowed ground weeds start vigorously in the
spring and draw upon the soil-moisture, if allowed to grow, fully as
heavily as a crop of wheat or corn. The dry-farmer must not allow a
weed upon his land. Cultivation must he so continuous as to make
weeds an impossibility. The belief that the elements added to the
soil by weeds offset the loss of soil-moisture is wholly erroneous.
The growth of weeds on a fallow dry-farm is more dangerous than the
packed uncared-for topsoil. Many implements have been devised for
the easy killing of weeds, but none appear to be better than the
plow and the disk which are found on every farm. (See Chapter XV.)
When crops are growing on the land, thorough summer cultivation is
somewhat more difficult, but must be practiced for the greatest
certainty of crop yields. Potatoes, corn, and similar crops may be
cultivated with comparative ease, by the use of ordinary
cultivators. With wheat and the other small grains, generally, the
damage done to the crop by harrowing late in the season is too
great, and reliance is therefore placed on the shading power of the
plants to prevent undue evaporation. However, until the wheat and
other grains are ten to twelve inches high, it is perfectly safe to
harrow them. The teeth should be set backward to diminish the
tearing up of the plants, and the implement weighted enough to break
the soil crust thoroughly. This practice has been fully tried out
over the larger part of the dry-farm territory and found
satisfactory.
So vitally important is a permanent soil mulch for the conservation
for plant use of the water stored in the soil that many attempts
have been made to devise means for the effective cultivation of land
on which small grains and grasses are growing. In many places plants
have been grown in rows so far apart that a man with a hoe could
pass between them. Scofield has described this method as practiced
successfully in Tunis. Campbell and others in America have proposed
that a drill hole be closed every three feet to form a path wide
enough for a horse to travel in and to pull a large spring tooth
cultivator' with teeth so spaced as to strike between the rows of
wheat. It is yet doubtful whether, under average conditions, such
careful cultivation, at least of grain crops, is justified by the
returns. Under conditions of high aridity, or where the store of
soil-moisture is low, such treatment frequently stands between crop
success and failure, and it is not unlikely that methods will be
devised which will permit of the cheap and rapid cultivation between
the rows of growing wheat. Meanwhile, the dry-farmer must always
remember that the margin under which he works is small, and that his
success depends upon the degree to which he prevents small wastes.
Dry-farm potatoes, Rosebud Co., Montana, 1909. Yield, 282 bushels
per acre.
The conservation of soil-moisture depends upon the vigorous,
unremitting, continuous stirring of the topsoil. Cultivation!
cultivation! and more cultivation! must be the war-cry of the
dry-farmer who battles against the water thieves of an arid climate.
CHAPTER IX
REGULATING THE TRANSPIRATION
Water that has entered the soil may be lost in three ways. First, it
may escape by downward seepage, whereby it passes beyond the reach
of plant roots and often reaches the standing water. In dry-farm
districts such loss is a rare occurrence, for the natural
precipitation is not sufficiently large to connect with the country
drainage, and it may, therefore, be eliminated from consideration.
Second, soil-water may be lost by direct evaporation from the
surface soil. The conditions prevailing in arid districts favor
strongly this manner of loss of soil-moisture. It has been shown,
however, in the preceding chapter that the farmer, by proper and
persistent cultivation of the topsoil, has it in his power to reduce
this loss enough to be almost negligible in the farmer's
consideration. Third, soil-water may be lost by evaporation from the
plants themselves. While it is not generally understood, this source
of loss is, in districts where dry-farming is properly carried on,
very much larger than that resulting either from seepage or from
direct evaporation. While plants are growing, evaporation from
plants, ordinarily called transpiration, continues. Experiments
performed in various arid districts have shown that one and a half
to three times more water evaporates from the plant than directly
from well-tilled soil. To the present very little has been learned
concerning the most effective methods of checking or controlling
this continual loss of water. Transpiration, or the evaporation of
water from the plants themselves and the means of controlling this
loss, are subjects of the deepest importance to the dry-farmer.
Absorption
To understand the methods for reducing transpiration, as proposed in
this chapter, it is necessary to review briefly the manner in which
plants take water from the soil. The roots are the organs of water
absorption. Practically no water is taken into the plants by the
stems or leaves, even under conditions of heavy rainfall. Such small
quantities as may enter the plant through the stems and leaves are
of very little value in furthering the life and growth of the plant.
The roots alone are of real consequence in water absorption. All
parts of the roots do not possess equal power of taking up
soil-water. In the process of water absorption the younger roots are
most active and effective. Even of the young roots, however, only
certain parts are actively engaged in water absorption. At the very
tips of the young growing roots are numerous fine hairs. These
root-hairs, which cluster about the growing point of the young
roots, are the organs of the plant that absorb soil-water. They are
of value only for limited periods of time, for as they grow older,
they lose their power of water absorption. In fact, they are active
only when they are in actual process of growth. It follows,
therefore, that water absorption occurs near the tips of the growing
roots, and whenever a plant ceases to grow the water absorption
ceases also. The root-hairs are filled with a dilute solution of
various substances, as yet poorly understood, which plays an
important tent part in the ab sorption of water and plant-food from
the soil.
Owing to their minuteness, the root-hairs are in most cases immersed
in the water film that surrounds the soil particles, and the
soil-water is taken directly into the roots from the soil-water film
by the process known as osmosis. The explanation of this inward
movement is complicated and need not be discussed here. It is
sufficient to say that the concentration or strength of the solution
within the root-hair is of different degree from the soil-water
solution. The water tends, therefore, to move from the soil into the
root, in order to make the solutions inside and outside of the root
of the same concentration. If it should ever occur that the
soil-water and the water within the root-hair became the same
concentration, that is to say, contained the same substances in the
same proportional amounts, there would be no further inward movement
of water. Moreover, if it should happen that the soil-water is
stronger than the water within the root-hair, the water would tend
to pass from the plant into the soil. This is the condition that
prevails in many alkali lands of the West, and is the cause of the
death of plants growing on such lands.
It is clear that under these circumstances not only water enters the
root-hairs, but many of the substances found in solution in the
soil-water enter the plant also. Among these are the mineral
substances which are indispensable for the proper life and growth of
plants. These plant nutrients are so indispensable that if any one
of them is absent, it is absolutely impossible for the plant to
continue its life functions. The indispensable plant-foods gathered
from the soil by the root-hairs, in addition to water, are:
potassium, calcium, magnesium, iron, nitrogen, and phosphorus,--all
in their proper combinations. How the plant uses these substances is
yet poorly understood, but we are fairly certain that each one has
some particular function in the life of the plant. For instance,
nitrogen and phosphorus are probably necessary in the formation of
the protein or the flesh-forming portions of the plant, while potash
is especially valuable in the formation of starch.
There is a constant movement of the indispensable plant nutrients
after they have entered the root-hairs, through the stems and into
the leaves. This constant movement of the plant-foods depends upon
the fact that the plant consumes in its growth considerable
quantities of these substances, and as the plant juices are
diminished in their content of particular plant-foods, more enters
from the soil solution. The necessary plant-foods do not alone enter
the plant but whatever may be in solution in the soil-water enters
the plant in variable quantities. Nevertheless, since the plant uses
only a few definite substances and leaves the unnecessary ones in
solution, there is soon a cessation of the inward movement of the
unimportant constituents of the soil solution. This process is often
spoken of as selective absorption; that is, the plant, because of
its vital activity, appears to have the power of selecting from the
soil certain substances and rejecting others.
Movement of water through plant
The soil-water, holding in solution a great variety of plant
nutrients, passes from the root-hairs into the adjoining cells and
gradually moves from cell to cell throughout the whole plant. In
many plants this stream of water does not simply pass from cell to
cell, but moves through tubes that apparently have been formed for
the specific purpose of aiding the movement of water through the
plant. The rapidity of this current is often considerable.
Ordinarily, it varies from one foot to six feet per hour, though
observations are on record showing that the movement often reaches
the rate of eighteen feet per hour. It is evident, then, that in an
actively growing plant it does not take long for the water which is
in the soil to find its way to the uppermost parts of the plant.
The work of leaves
Whether water passes upward from cell to cell or through especially
provided tubes, it reaches at last the leaves, where evaporation
takes place. It is necessary to consider in greater detail what
takes place in leaves in order that we may more clearly understand
the loss due to transpiration. One half or more of every plant is
made up of the element carbon. The remainder of the plant consists
of the mineral substances taken from the soil (not more than two to
10 per cent of the dry plant) and water which has been combined with
the carbon and these mineral substances to form the characteristic
products of plant life. The carbon which forms over half of the
plant substance is gathered from the air by the leaves and it is
evident that the leaves are very active agents of plant growth. The
atmosphere consists chiefly of the gases oxygen and nitrogen in the
proportion of one to four, but associated with them are small
quantities of various other substances. Chief among the secondary
constituents of the atmosphere is the gas carbon dioxid, which is
formed when carbon burns, that is, when carbon unites with the
oxygen of the air. Whenever coal or wood or any carbonaceous
substance burns, carbon dioxid is formed. Leaves have the power of
absorbing the gas carbon dioxid from the air and separating the
carbon from the oxygen. The oxygen is returned to the atmosphere
while the carbon is retained to be used as the fundamental substance
in the construction by the plant of oils, fats, starches, sugars,
protein, and all the other products of plant growth.
This important process known as carbon assimilation is made possible
by the aid of countless small openings which exist chicfly on the
surfaces of leaves and known as "stomata." The stomata are
delicately balanced valves, exceedingly sensitive to external
influences. They are more numerous on the lower side than on the
upper side of plants. In fact, there is often five times more on the
under side than on the upper side of a leaf. It has been estimated
that 150,000 stomata or more are often found per square inch on the
under side of the leaves of ordinary cultivated plants. The stomata
or breathing-pores are so constructed that they may open and close
very readily. In wilted leaves they are practically closed; often
they also close immediately after a rain; but in strong sunlight
they are usually wide open. It is through the stomata that the gases
of the air enter the plant through which the discarded oxygen
returns to the atmosphere.
It is also through the stomata that the water which is drawn from
the soil by the roots through the stems is evaporated into the air.
There is some evaporation of water from the stems and branches of
plants, but it is seldom more than a thirtieth or a fortieth of the
total transpiration. The evaporation of water from the leaves
through the breathing-pores is the so-called transpiration, which is
the greatest cause of the loss of soil-water under dry-farm
conditions. It is to the prevention of this transpiration that much
investigation must be given by future students of dry-farming.
Transpiration
As water evaporates through the breathing-pores from the leaves it
necessarily follows that a demand is made upon the lower portions of
the plant for more water. The effect of the loss of water is felt
throughout the whole plant and is, undoubtedly, one of the chief
causes of the absorption of water from the soil. As evaporation is
diminished the amount of water that enters the plants is also
diminished. Yet transpiration appears to be a process wholly
necessary for plant life. The question is, simply, to what extent it
may be diminished without injuring plant growth. Many students
believe that the carbon assimilation of the plant, which is
fundamentally important in plant growth, cannot be continued unless
there is a steady stream of water passing through the plant and then
evaporating from the leaves.
Of one thing we are fairly sure: if the upward stream of water is
wholly stopped for even a few hours, the plant is likely to be so
severely injured as to be greatly handicapped in its future growth.
Botanical authorities agree that transpiration is of value to plant
growth, first, because it helps to distribute the mineral nutrients
necessary for plant growth uniformly throughout the plant; secondly,
because it permits an active assimilation of the carbon by the
leaves; thirdly, because it is not unlikely that the heat required
to evaporate water, in large part taken from the plant itself,
prevents the plant from being overheated. This last mentioned value
of transpiration is especially important in dry-farm districts,
where, during the summer, the heat is often intense. Fourthly,
transpiration apparently influences plant growth and development in
a number of ways not yet clearly understood.
Conditions influencing transpiration
In general, the conditions that determine the evaporation of water
from the leaves are the same as those that favor the direct
evaporation of water from soils, although there seems to be
something in the life process of the plant, a physiological factor,
which permits or prevents the ordinary water-dissipating factors
from exercising their full powers. That the evaporation of water
from the soil or from a free water surface is not the same as that
from plant leaves may be shown in a general way from the fact that
the amount of water transpired from a given area of leaf surface may
be very much larger or very much smaller than that evaporated from
an equal surface of free water exposed to the same conditions. It is
further shown by the fact that whereas evaporation from a free water
surface goes on with little or no interruption throughout the
twenty-four hours of the day, transpiration is virtually at a
standstill at night even though the conditions for the rapid
evaporation from a free water surface are present.
Some of the conditions influencing the transpiration may be
enumerated as follows:--
First, transpiration is influenced by the relative humidity. In dry
air, under otherwise similar conditions, plants transpire more water
than in moist air though it is to be noted that even when the
atmosphere is fully saturated, so that no water evaporates from a
free water surface, the transpiration of plants still continues in a
small degree. This is explained by the observation that since the
life process of a plant produces a certain amount of heat, the plant
is always warmer than the surrounding air and that transpiration
into an atmosphere fully charged with water vapor is consequently
made possible. The fact that transpiration is greater under a low
relative humidity is of greatest importance to the dry-farmer who
has to contend with the dry atmosphere.
Second, transpiration increases with the increase in temperature;
that is, under conditions otherwise the same, transpiration is more
rapid on a warm day than on a cold one. The temperature increase of
itself, however, is not sufficient to cause transpiration.
Third, transpiration increases with the increase of air currents,
which is to say, that on a windy day transpiration is much more
rapid than on a quiet day.
Fourth, transpiration increases with the increase of direct
sunlight. It is an interesting observation that even with the same
relative humidity, temperature, and wind, transpiration is reduced
to a minimum during the night and increases manyfold during the day
when direct sunlight is available. This condition is again to be
noted by the dry-farmer, for the dry-farm districts are
characterized by an abundance of sunshine.
Fifth, transpiration is decreased by the presence in the soil-water
of large quantities of the substances which the plant needs for its
food material. This will be discussed more fully in the next
section.
Sixth, any mechanical vibration of the plant seems to have some
effect upon the transpiration. At times it is increased and at times
it is decreased by such mechanical disturbance.
Seventh, transpiration varies also with the age of the plant. In the
young plant it is comparatively small. Just before blooming it is
very much larger and in time of bloom it is the largest in the
history of the plant. As the plant grows older transpiration
diminishes, and finally at the ripening stage it almost ceases.
Eighth, transpiration varies greatly with the crop. Not all plants
take water from the soil at the same rate. Very little is as yet
known about the relative water requirements of crops on the basis of
transpiration. As an illustration, MacDougall has reported that
sagebrush uses about one fourth as much water as a tomato plant.
Even greater differences exist between other plants. This is one of
the interesting subjects yet to be investigated by those who are
engaged in the reclamation of dry-farm districts. Moreover, the same
crop grown under different conditions varies in its rate of
transpiration. For instance, plants grown for some time under arid
conditions greatly modify their rate of transpiration, as shown by
Spalding, who reports that a plant reared under humid conditions
gave off 3.7 times as much water as the same plant reared under arid
conditions. This very interesting observation tends to confirm the
view commonly held that plants grown under arid conditions will
gradually adapt themselves to the prevailing conditions, and i
Showing first 200,000 characters. Full text: gutenberg.org/ebooks/4924