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.