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

as being in a state of trembling activity, tending to place itself in full equilibrium with the surrounding contending forces which, themselves, constantly change. Were it not that the water film held closely around the soil particles is possessed of extreme mobility, it would not be possible to meet the demands of the plants upon the water at comparatively great distances. Even as it is, it frequently happens that when crops are planted too thickly on dry-farms, the soil-moisture cannot move quickly enough to the absorbing roots to maintain plant growth, and crop failure results. Incidentally, this points to planting that shall be proportional to the moisture contained by the soil. See Chapter XI. As the temperature rises in spring, with a decrease in the relative humidity, and an increase in direct sunshine, evaporation from the soil surface increases greatly. However, as the topsoil becomes drier, that is, as the water fihn becomes thinner, there is an attempt at readjustment, and water moves upward to take the place of that lost by evaporation. As this continues throughout the season, the moisture stored eight or ten feet or more below the surface is gradually brought to the top and evaporated, and thus lost to plant use. The effect of rapid top drying of soils As the water held by soils diminishes, and the water film around the soil grains becomes thinner, the capillary movement of the soil-water is retarded. This is easily understood by recalling that the soil particles have an attraction for water, which is of definite value, and may be measured by the thickest film that may be held against gravity. When the film is thinned, it does not diminish the attraction of the soil for water; it simply results in a stronger pull upon the water and a firmer holding of the film against the surfaces of the soil grains. To move soil-water under such conditions requires the expenditure of more energy than is necessary for moving water in a saturated or nearly saturated soil. Under like conditions, therefore, the thinner the soil-water film the more difficult will be the upward movement of the soil-water and the slower the evaporation from the topsoil. As drying goes on, a point is reached at which the capillary movement of the water wholly ceases. This is probably when little more than the hygroscopic moisture remains. In fact, very dry soil and water repel each other. This is shown in the common experience of driving along a road in summer, immediately after a light shower. The masses of dust are wetted only on the outside, and as the wheels pass through them the dry dust is revealed. It is an important fact that very dry soil furnishes a very effective protection against the capillary movement of water. In accordance with the principle above established if the surface soil could be dried to the point where capillarity is very slow, the evaporation would be diminished or almost wholly stopped. More than a quarter of a century ago, Eser showed experimentally that soil-water may be saved by drying the surface soil rapidly. Under dry-farm conditions it frequently occurs that the draft upon the water of the soil is so great that nearly all the water is quickly and so completely abstracted from the upper few inches of soil that they are left as an effective protection against further evaporation. For instance, in localities where hot dry winds are of common occurrence, the upper layer of soil is sometimes completely dried before the water in the lower layers can by slow capillary movement reach the top. The dry soil layer then prevents further loss of water, and the wind because of its intensity has helped to conserve the soil-moisture. Similarly in localities where the relative humidity is low, the sunshine abundant, and the temperature high, evaporation may go on so rapidly that the lower soil layers cannot supply the demands made, and the topsoil then dries out so completely as to form a protective covering against further evaporation. It is on this principle that the native desert soils of the United States, untouched by the plow, and the surfaces of which are sun-baked, are often found to possess large percentages of water at lower depths. Whitney recorded this observation with considerable surprise, many years ago, and other observers have found the same conditions at nearly all points of the arid region. This matter has been subjected to further study by Buckingham, who placed a variety of soils under artificially arid and humid conditions. It was found in every case that, the initial evaporation was greater under arid conditions, but as the process went on and the topsoil of the arid soil became dry, more water was lost under humid conditions. For the whole experimental period, also, more water was lost under humid conditions. It was notable that the dry protective layer was formed more slowly on alkali soils, which would point to the inadvisability of using alkali lands for dry-farm purposes. All in all, however, it appears "that under very arid conditions a soil automatically protects itself from drying by the formation of a natural mulch on the surface." Naturally, dry-farm soils differ greatly in their power of forming such a mulch. A heavy clay or a light sandy soil appears to have less power of such automatic protection than a loamy soil. An admixture of limestone seems to favor the formation of such a natural protective mulch. Ordinarily, the farmer can further the formation of a dry topsoil layer by stirring the soil thoroughly. This assists the sunshine and the air to evaporate the water very quickly. Such cultivation is very desirable for other reasons also, as will soon be discussed. Meanwhile, the water-dissipating forces of the dry-farm section are not wholly objectionable, for whether the land be cultivated or not, they tend to hasten the formation of dry surface layers of soil which guard against excessive evaporation. It is in moist cloudy weather, when the drying process is slow, that evaporation causes the greatest losses of soil-moisture. The effect of shading Direct sunshine is, next to temperature, the most active cause of rapid evaporation from moist soil surfaces. Whenever, therefore, evaporation is not rapid enough to form a dry protective layer of topsoil, shading helps materially in reducing surface losses of soil-water. Under very arid conditions, however, it is questionable whether in all cases shading has a really beneficial effect, though under semiarid or sub-humid conditions the benefits derived from shading are increased largely. Ebermayer showed in 1873 that the shading due to the forest cover reduced evaporation 62 per cent, and many experiments since that day have confirmed this conclusion. At the Utah Station, under arid conditions, it was found that shading a pot of soil, which otherwise was subjected to water-dissipating influences, saved 29 per cent of the loss due to evaporation from a pot which was not shaded. This principle cannot be applied very greatly in practice, but it points to a somewhat thick planting, proportioned to the water held by the soil. It also shows a possible benefit to be derived from the high header straw which is allowed to stand for several weeks in dry-farm sections where the harvest comes early and the fall plowing is done late, as in the mountain states. The high header stubble shades the ground very thoroughly. Thus the stubble may be made to conserve the soil-moisture in dry-farm sections, where grain is harvested by the "header" method. A special case of shading is the mulching of land with straw or other barnyard litter, or with leaves, as in the forest. Such mulching reduces evaporation, but only in part, because of its shading action, since it acts also as a loose top layer of soil matter breaking communication with the lower soil layers. Whenever the soil is carefully stirred, as will be described, the value of shading as a means or checking evaporation disappears almost entirely. It is only with soils which are tolerably moist at the surface that shading acts beneficially. Alfalfa in cultivated rows. This practice is employed to make possible the growth of alfalfa and other perennial crops on arid lands without irrigation. The effect of tillage Capillary soil-moisture moves from particle to particle until the surface is reached. The closer the soil grains are packed together, the greater the number of points or contact, and the more easily will the movement of the soil-moisture proceed. If by any means a layer of the soil is so loosened as to reduce the number of points of contact, the movement of the soil-moisture is correspondingly hindered. The process is somewhat similar to the experience in large r airway stations. Just before train time a great crowd of people is gathered outside or the gates ready to show their tickets. If one gate is opened, a certain number of passengers can pass through each minute; if two are opened, nearly twice as many may be admitted in the same time; if more gates are opened, the passengers will be able to enter the train more rapidly. The water in the lower layers of the soil is ready to move upward whenever a call is made upon it. To reach the surface it must pass from soil grain to soil grain, and the larger the number of grains that touch, the more quickly and easily will the water reach the surface, for the points of contact of the soil particles may be likened to the gates of the railway station. Now if, by a thorough stirring and loosening of the topsoil, the number of points of contact between the top and subsoil is greatly reduced, the upward flow of water is thereby largely checked. Such a loosening of the topsoil for the purpose of reducing evaporation from the topsoil has come to be called cultivation, and includes plowing, harrowing, disking, hoeing, and other cultural operations by which the topsoil is stirred. The breaking of the points of contact between the top and subsoil is undoubtedly the main reason for the efficiency of cultivation, but it is also to be remembered that such stirring helps to dry the top soil very thoroughly, and as has been explained a layer of dry soil of itself is a very effective check upon surface evaporation. That the stirring or cultivation of the topsoil really does diminish evaporation of water from the soil has been shown by numerous investigations. In 1868, Nessler found that during six weeks of an ordinary German summer a stirred soil lost 510 grams of water per square foot, while the adjoining compacted soil lost 1680 grams,--a saving due to cultivation of nearly 60 per cent. Wagner, testing the correctness of Nessler's work, found, in 1874, that cultivation reduced the evaporation a little more than 60 per cent; Johnson, in 1878, confirmed the truth of the principle on American soils, and Levi Stockbridge, working about the same time, also on American soils, found that cultivation diminished evaporation on a clay soil about 23 per cent, on a sandy loam 55 per cent, and on a heavy loam nearly 13 per cent. All the early work done on this subject was done under humid conditions, and it is only in recent years that confirmation of this important principle has been obtained for the soils of the dry-farm region. Fortier, working under California conditions, determined that cultivation reduced the evaporation from the soil surface over 55 per cent. At the Utah Station similar experiments have shown that the saving of soil-moisture by cultivation was 63 per cent for a clay soil, 34 per cent for a coarse sand, and 13 per cent for a clay loam. Further, practical experience has demonstrated time and time again that in cultivation the dry-farmer has a powerful means of preventing evaporation from agricultural soils. Closely connected with cultivation is the practice of scattering straw or other litter over the ground. Such artificial mulches are very effective in reducing evaporation. Ebermayer found that by spreading straw on the land, the evaporation was reduced 22 per cent; Wagner found under similar conditions a saving of 38 per cent, and these results have been confirmed by many other investigators. On the modern dry-farms, which are large in area, the artificial mulching of soils cannot become a very extensive practice, yet it is well to bear the principle in mind. The practice of harvesting dry-farm grain with the header and plowing under the high stubble in the fall is a phase of cultivation for water conservation that deserves special notice. The straw, thus incorporated into the soil, decomposes quite readily in spite of the popular notion to the contrary, and makes the soil more porous, and, therefore, more effectively worked for the prevention of evaporation. When this practice is continued for considerable periods, the topsoil becomes rich in organic matter, which assists in retarding evaporation, besides increasing the fertility of the land. When straw cannot be fed to advantage, as is yet the case on many of the western dry-farms, it would be better to scatter it over the land than to burn it, as is often done. Anything that covers the ground or loosens the topsoil prevents in a measure the evaporation of the water stored in lower soil depths for the use of crops. Depth of cultivation The all-important practice for the dry-farmer who is entering upon the growing season is cultivation. The soil must be covered continually with a deep layer of dry loose soil, which because of its looseness and dryness makes evaporation difficult. A leading question in connection with cultivation is the depth to which the soil should be stirred for the best results. Many of the early students of the subject found that a soil mulch only one half inch in depth was effective in retaining a large part of the soil-moisture which noncultivated soils would lose by evaporation. Soils differ greatly in the rate of evaporation from their surfaces. Some form a natural mulch when dried, which prevents further water loss. Others form only a thin hard crust, below which lies an active evaporating surface of wet soil. Soils which dry out readily and crumble on top into a natural mulch should be cultivated deeply, for a shallow cultivation does not extend beyond the naturally formed mulch. In fact, on certain calcareous soils, the surfaces of which dry out quickly and form a good protection against evaporation, shallow cultivations often cause a greater evaporation by disturbing the almost perfect natural mulch. Clay or sand soils, which do not so well form a natural mulch, will respond much better to shallow cultivations. In general, however, the deeper the cultivation, the more effective it is in reducing evaporation. Fortier, in the experiments in California to which allusion has already been made, showed the greater value of deep cultivation. During a period of fifteen days, beginning immediately after an irrigation, the soil which had not been mulched lost by evaporation nearly one fourth of the total amount of water that had been added. A mulch 4 inches deep saved about 72 per cent of the evaporation; a mulch 8 inches deep saved about 88 per cent, and a mulch 10 inches deep stopped evaporation almost wholly. It is a most serious mistake for the dry-farmer, who attempts cultivation for soil-moisture conservation, to fail to get the best results simply to save a few cents per acre in added labor. When to cultivate or till It has already been shown that the rate of evaporation is greater from a wet than from a dry surface. It follows, therefore, that the critical time for preventing evaporation is when the soil is wettest. After the soil is tolerably dry, a very large portion of the soil-moisture has been lost, which possibly might have been saved by earlier cultivation. The truth of this statement is well shown by experiments conducted by the Utah Station. In one case on a soil well filled with water, during a three weeks' period, nearly one half of the total loss occurred the first, while only one fifth fell on the third week. Of the amount lost during the first week, over 60 per cent occurred during the first three days. Cultivation should, therefore, be practiced as soon as possible after conditions favorable for evaporation have been established. This means, first, that in early spring, just as soon as the land is dry enough to be worked without causing puddling, the soil should be deeply and thoroughly stirred. Spring plowing, done as early as possible, is an excellent practice for forming a mulch against evaporation. Even when the land has been fall-plowed, spring plowing is very beneficial, though on fall-plowed land the disk harrow is usually used in early spring, and if it is set at rather a sharp angle, and properly weighted, so that it cuts deeply into the ground, it is practically as effective as spring plowing. The chief danger to the dry-farmer is that he will permit the early spring days to slip by until, when at last he begins spring cultivation, a large portion of the stored soil-water has been evaporated. It may be said that deep fall plowing, by permitting the moisture to sink quickly into the lower layers of soil, makes it possible to get upon the ground earlier in the spring. In fact, unplowed land cannot be cultivated as early as that which has gone through the winter in a plowed condition If the land carries a fall-sown crop, early spring cultivation is doubly important. As soon as the plants are well up in spring the land should be gone over thoroughly several times if necessary, with an iron tooth harrow, the teeth of which are set to slant backward in order not to tear up the plants. The loose earth mulch thus formed is very effective in conserving moisture; and the few plants torn up are more than paid for by the increased water supply for the remaining plants. The wise dry-fanner cultivates his land, whether fallow or cropped, as early as possible in the spring. Following the first spring plowing, disking, or cultivation, must come more cultivation. Soon after the spring plowing, the land should be disked and. then harrowed. Every device should be used to secure the formation of a layer of loose drying soil over the land surface. The season's crop will depend largely upon the effectiveness of this spring treatment. As the season advances, three causes combine to permit the evaporation of soil-moisture. First, there is a natural tendency, under the somewhat moist conditions of spring, for the soil to settle compactly and thus to restore the numerous capillary connections with the lower soil layers through which water escapes. Careful watch should therefore be kept upon the soil surface, and whenever the mulch is not loose, the disk or harrow should be run over the land. Secondly, every rain of spring or summer tends to establish connections with the store of moisture in the soil. In fact, late spring and summer rains are often a disadvantage on dry-farms, which by cultural treatment have been made to contain a large store of moisture. It has been shown repeatedly that light rains draw moisture very quickly from soil layers many feet below the surface. The rainless summer is not feared by the dry-farmer whose soils are fertile and rich in moisture. It is imperative that at the very earliest moment after a spring or summer rain the topsoil be well stirred to prevent evaporation. It thus happens that in sections of frequent summer rains, as in the Great Plains area, the farmer has to harrow his land many times in succession, but the increased crop yields invariably justify the added expenditure of effort. Thirdly, on the summer-fallowed ground weeds start vigorously in the spring and draw upon the soil-moisture, if allowed to grow, fully as heavily as a crop of wheat or corn. The dry-farmer must not allow a weed upon his land. Cultivation must he so continuous as to make weeds an impossibility. The belief that the elements added to the