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.