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.