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.