Edited by Charles Aldarondo (aldarondo@yahoo.com).
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