DRY-FARMING: A SYSTEM OF AGRICULTURE FOR COUNTRIES UNDER A LOW RAINFALL TEXT

who by careful farming has stored his deep soil with an abundance of water. Storing the soil with water is, however, only the first step in making the rains of fall, winter, or the preceding year available for plant growth. As soon as warm growing weather comes, water-dissipating forces come into play, and water is lost by evaporation. The farmer must, therefore, use all precautions to keep the moisture in the soil until such time as the roots of the crop may draw it into the plants to be used in plant production. That is, as far as possible, direct evaporation of water from the soil must be prevented. Few farmers really realize the immense possible annual evaporation in the dry-farm territory. It is always much larger than the total annual rainfall. In fact, an arid region may be defined as one in which under natural conditions several times more water evaporates annually from a free water surface than falls as rain and snow. For that reason many students of aridity pay little attention to temperature, relative humidity, or winds, and simply measure the evaporation from a free water surface in the locality in question. In order to obtain a measure of the aridity, MacDougal has constructed the following table, showing the annual precipitation and the annual evaporation at several well-known localities in the dry-farm territory. True, the localities included in the following table are extreme, but they illustrate the large possible evaporation, ranging from about six to thirty-five times the precipitation. At the same time it must be borne in mind that while such rates of evaporation may occur from free water surfaces, the evaporation from agricultural soils under like conditions is very much smaller. Place Annual Precipitation Annual Evaporation Ratio (In Inches) (In Inches) El Paso, Texas 9.23 80 8.7 Fort Wingate, New Mexico 14.00 80 5.7 Fort Yuma, Arizona 2.84 100 35.2 Tucson, AZ 11.74 90 7.7 Mohave, CA 4.97 95 19.1 Hawthorne, Nevada 4.50 80 17.5 Winnemucca, Nevada 9.51 80 9.6 St. George, Utah 6.46 90 13.9 Fort Duchesne, Utah 6.49 75 11.6 Pineville, Oregon 9.01 70 7.8 Lost River, Idaho 8.47 70 8.3 Laramie, Wyoming 9.81 70 7.1 Torres, Mexico 16.97 100 6.0 To understand the methods employed for checking evaporation from the soil, it is necessary to review briefly the conditions that determine the evaporation of water into the air, and the manner in which water moves in the soil. The formation of water vapor Whenever water is left freely exposed to the air, it evaporates; that is, it passes into the gaseous state and mixes with the gases of the air. Even snow and ice give off water vapor, though in very small quantities. The quantity of water vapor which can enter a given volume of air is definitely limited. For instance, at the temperature of freezing water 2.126 grains of water vapor can enter one cubic foot of air, but no more. When air contains all the water possible, it is said to be saturated, and evaporation then ceases. The practical effect of this is the well-known experience that on the seashore, where the air is often very nearly fully saturated with water vapor, the drying of clothes goes on very slowly, whereas in the interior, like the dry-farming territory, away from the ocean, where the air is far from being saturated, drying goes on very rapidly. The amount of water necessary to saturate air varies greatly with the temperature. It is to be noted that as the temperature increases, the amount of water that may be held by the air also increases; and proportionately more rapidly than the increase in temperature. This is generally well understood in common experience, as in drying clothes rapidly by hanging them before a hot fire. At a temperature of 100 deg F., which is often reached in portions of the dry-farm territory during the growing season, a given volume of air can hold more than nine times as much water vapor as at the temperature of freezing water. This is an exceedingly important principle in dry-farm practices, for it explains the relatively easy possibility of storing water during the fall and winter when the temperature is low and the moisture usually abundant, and the greater difficulty of storing the rain that falls largely, as in the Great Plains area, in the summer when water-dissipating forces are very active. This law also emphasizes the truth that it is in times of warm weather that every precaution must be taken to prevent the evaporation of water from the soil surface. Temperature Grains of Water held in in Degrees F. One Cubic Foot of Air 32 2.126 40 2.862 50 4.089 60 5.756 70 7.992 80 10.949 90 14.810 100 19.790 It is of course well understood that the atmosphere as a whole is never saturated with water vapor. Such saturation is at the best only local, as, for instance, on the seashore during quiet days, when the layer of air over the water may be fully saturated, or in a field containing much water from which, on quiet warm days, enough water may evaporate to saturate the layer of air immediately upon the soil and around the plants. Whenever, in such cases, the air begins to move and the wind blows, the saturated air is mixed with the larger portion of unsaturated air, and evaporation is again increased. Meanwhile, it must be borne in mind that into a layer of saturated air resting upon a field of growing plants very little water evaporates, and that the chief water-dissipating power of winds lies in the removal of this saturated layer. Winds or air movements of any kind, therefore, become enemies of the farmer who depends upon a limited rainfall. The amount of water actually found in a given volume of air at a certain temperature, compared with the largest amount it can hold, is called the relative humidity of the air. As shown in Chapter IV, the relative humidity becomes smaller as the rainfall decreases. The lower the relative humidity is at a given temperature, the more rapidly will water evaporate into the air. There is no more striking confirmation of this law than the fact that at a temperature of 90 deg sunstrokes and similar ailments are reported in great number from New York, while the people of Salt Lake City are perfectly comfortable. In New York the relative humidity in summer is about 73 per cent; in Salt Lake City, about 35 per cent. At a high summer temperature evaporation from the skin goes on slowly in New York and rapidly in Salt Lake City, with the resulting discomfort or comfort. Similarly, evaporation from soils goes on rapidly under a low and slowly under a high percentage of relative humidity. Evaporation from water surfaces is hastened, therefore, by (1) an increase in the temperature, (2) an increase in the air movements or winds, and (3) a decrease in the relative humidity. The temperature is higher; the relative humidity lower, and the winds usually more abundant in arid than in humid regions. The dry-farmer must consequently use all possible precautions to prevent evaporation from the soil. Conditions of evaporation from from soils Evaporation does not alone occur from a surface of free water. All wet or moist substances lose by evaporation most of the water that they hold, providing the conditions of temperature and relative humidity are favorable. Thus, from a wet soil, evaporation is continually removing water. Yet, under ordinary conditions, it is impossible to remove all the water, for a small quantity is attracted so strongly by the soil particles that only a temperature above the boiling point of water will drive it out. This part of the soil is the hygroscopic moisture spoken of in the last chapter. Moreover, it must be kept in mind that evaporation does not occur as rapidly from wet soil as from a water surface, unless all the soil pores are so completely filled with water that the soil surface is practically a water surface. The reason for this reduced evaporation from a wet soil is almost self-evident. There is a comparatively strong attraction between soil and water, which enables the moisture to cling as a thin capillary film around the soil particles, against the force of gravity. Ordinarily, only capillary water is found in well-tilled soil, and the force causing evaporation must be strong enough to overcome this attraction besides changing the water into vapor. The less water there is in a soil, the thinner the water film, and the more firmly is the water held. Hence, the rate of evaporation decreases with the decrease in soil-moisture. This law is confirmed by actual field tests. For instance, as an average of 274 trials made at the Utah Station, it was found that three soils, otherwise alike, that contained, respectively, 22.63 per cent, 17.14 per cent, and 12.75 per cent of water lost in two weeks, to a depth of eight feet, respectively 21.0, 17. 1, and 10.0 pounds of water per square foot. Similar experiments conducted elsewhere also furnish proof of the correctness of this principle. From this point of view the dry-farmer does not want his soils to be unnecessarily moist. The dry-farmer can reduce the per cent of water in the soil without diminishing the total amount of water by so treating the soil that the water will distribute itself to considerable depths. This brings into prominence again the practices of fall plowing, deep plowing, subsoiling, and the choice of deep soils for dry-farming. Very much for the same reasons, evaporation goes on more slowly from water in which salt or other substances have been dissolved. The attraction between the water and the dissolved salt seems to be strong enough to resist partially the force causing evaporation. Soil-water always contains some of the soil ingredients in solution, and consequently under the given conditions evaporation occurs more slowly from soil-water than from pure water. Now, the more fertile a soil is, that is, the more soluble plant-food it contains, the more material will be dissolved in the soil-water, and as a result the more slowly will evaporation take place. Fallowing, cultivation, thorough plowing and manuring, which increase the store of soluble plant-food, all tend to diminish evaporation. While these conditions may have little value in the eyes of the farmer who is under an abundant rainfall, they are of great importance to the dry-farmer. It is only by utilizing every possibility of conserving water and fertility that dry-farming may be made a perfectly safe practice. Loss by evaporation chiefly at the surface Evaporation goes on from every wet substance. Water evaporates therefore from the wet soil grains under the surface as well as from those at the surface. In developing a system of practice which will reduce evaporation to a minimum it must be learned whether the water which evaporates from the soil particles far below the surface is carried in large quantities into the atmosphere and thus lost to plant use. Over forty years ago, Nessler subjected this question to experiment and found that the loss by evaporation occurs almost wholly at the soil surface, and that very little if any is lost directly by evaporation from the lower soil layers. Other experimenters have confirmed this conclusion, and very recently Buckingham, examining the same subject, found that while there is a very slow upward movement of the soil gases into the atmosphere, the total quantity of the water thus lost by direct evaporation from soil, a foot below the surface, amounted at most to one inch of rainfall in six years. This is insignificant even under semiarid and arid conditions. However, the rate of loss of water by direct evaporation from the lower soil layers increases with the porosity of the soil, that is, with the space not filled with soil particles or water. Fine-grained soils, therefore, lose the least water in this manner. Nevertheless, if coarse-grained soils are well filled with water, by deep fall plowing and by proper summer fallowing for the conservation of moisture, the loss of moisture by direct evaporation from the lower soil layers need not be larger than from finer grained soils Thus again are emphasized the principles previously laid down that, for the most successful dry-farming, the soil should always be kept well filled with moisture, even if it means that the land, after being broken, must lie fallow for one or two seasons, until a sufficient amount of moisture has accumulated. Further, the correlative principle is emphasized that the moisture in dry-farm lands should be stored deeply, away from the immediate action of the sun's rays upon the land surface. The necessity for deep soils is thus again brought out. The great loss of soil moisture due to an accumulation of water in the upper twelve inches is well brought out in the experiments conducted by the Utah Station. The following is selected from the numerous data on the subject. Two soils, almost identical in character, contained respectively 17.57 per cent and 16.55 per cent of water on an average to a depth of eight feet; that is, the total amount of water held by the two soils was practically identical. Owing to varying cultural treatment, the distribution of the water in the soil was not uniform; one contained 23.22 per cent and the other 16.64 per cent of water in the first twelve inches. During the first seven days the soil that contained the highest percentage of water in the first foot lost 13.30 pounds of water, while the other lost only 8.48 pounds per square foot. This great difference was due no doubt to the fact that direct evaporation takes place in considerable quantity only in the upper twelve inches of soil, where the sun's heat has a full chance to act. Any practice which enables the rains to sink quickly to considerable depths should be adopted by the dry-farmer. This is perhaps one of the great reasons for advocating the expensive but usually effective subsoil plowing on dry-farms. It is a very common experience, in the arid region, that great, deep cracks form during hot weather. From the walls of these cracks evaporation goes on, as from the topsoil, and the passing winds renew the air so that the evaporation may go on rapidly. The dry-farmer must go over the land as often as needs be with some implement that will destroy and fill up the cracks that may have been formed. In a field of growing crops this is often difficult to do; but it is not impossible that hand hoeing, expensive as it is, would pay well in the saving of soil moisture and the consequent increase in crop yield. How soil water reaches the surface It may be accepted as an established truth that the direct evaporation of water from wet soils occurs almost wholly at the surface. Yet it is well known that evaporation from the soil surface may continue until the soil-moisture to a depth of eight or ten feet or more is depleted. This is shown by the following analyses of dry-farm soil in early spring and midsummer. No attempt was made to conserve the moisture in the soil:-- Per cent of water in Early spring Midsummer 1st foot 20.84 8.83 2nd foot 20.06 8.87 3rd foot 19.62 11.03 4th foot 18.28 9.59 5th foot 18.70 11.27 6th foot 14.29 11.03 7th foot 14.48 8.95 8th foot 13.83 9.47 Avg 17.51 9.88 In this case water had undoubtedly passed by capillary movement from the depth of eight feet to a point near the surface where direct evaporation could occur. As explained in the last chapter, water which is held as a film around the soil particles is called capillary water; and it is in the capillary form that water may be stored in dry-farm soils. Moreover, it is the capillary soil-moisture alone which is of real value in crop production. This capillary water tends to distribute itself uniformly throughout the soil, in accordance with the prevailing conditions and forces. If no water is removed from the soil, in course of time the distribution of the soil-water will be such that the thickness of the film at any point in the soil mass is a direct resultant of the various forces acting at that particular point. There will then be no appreciable movement of the soil-moisture. Such a condition is approximated in late winter or early spring before planting begins. During the greater part of the year, however, no such quiescent state can occur, for there are numerous disturbing elements that normally are active, among which the three most effective are (l) the addition of water to the soil by rains; (2) the evaporation of water from the topsoil, due to the more active meteorological factors during spring, summer, and fall; and (3) the abstraction of water from the soil by plant roots. Water, entering the soil, moves downward under the influence of gravity as gravitational water, until under the attractive influence of the soil it has been converted into capillary water and adheres to the soil particles as a film. If the soil were dry, and the film therefore thin, the rain water would move downward only a short distance as gravitational water; if the soil were wet, and the film therefore thick, the water would move down to a greater distance before being exhausted. If, as is often the case in humid districts, the soil is saturated, that is, the film is as thick as the particles can hold, the water would pass right through the soil and connect with the standing water below. This, of course, is seldom the case in dry-farm districts. In any soil, excepting one already saturated, the addition of water will produce a thickening of the soil-water film to the full descent of the water. This immediately destroys the conditions of equilibrium formerly existing, for the moisture is not now uniformly distributed. Consequently a process of redistribution begins which continues until the nearest approach to equilibrium is restored. In this process water will pass in every direction from the wet portion of the soil to the drier; it does not necessarily mean that water will actually pass from the wet portion to the drier portion; usually, at the driest point a little water is drawn from the adjoining point, which in turn draws from the next, and that from the next, until the redistribution is complete. The process is very much like stuffing wool into a sack which already is loosely filled. The new wool does not reach the bottom of the sack, yet there is more wool in the bottom than there was before. If a plant-root is actively feeding some distance under the soil surface, the reverse process occurs. At the feeding point the root continually abstracts water from the soil grains and thus makes the film thinner in that locality. This causes a movement of moisture similar to the one above described, from the wetter portions of the soil to the portion being dried out by the action of the plant-root. Soil many feet or even rods distant may assist in supplying such an active root with moisture. When the thousands of tiny roots sent out by each plant are recalled. it may well be understood what a confusion of pulls and counter-pulls upon the soil-moisture exists in any cultivated soil. In fact, the soil-water film may be viewed