The great depth of arid soils further permits the storage of rain and snow water, as has been explained in previous chapters, to depths of from ten to fifteen feet.  As the growing season proceeds, this water is gradually drawn towards the surface, and with it much of the plant-food dissolved by the water in the lower soil layers.  This process repeated year after year results in a concentration in the upper soil layers of fertility normally distributed in the soil to the full depth reach by the soil-moisture.  At certain seasons, especially in the fall, this concentration may be detected with greatest certainty.  In general, the same action occurs in virgin lands, but the methods of dry-farm cultivation and cropping which permit a deeper penetration of the natural precipitation and a freer movement of the soil-water result in a larger quantity of plant-food reaching the upper two or three feet from the lower soil depths.  Such concentration near the surface, when it is not excessive, favors the production of increased yields of crops.

The characteristic high fertility and great depth of arid soils are probably the two main factors explaining the apparent increase of the fertility of dry-farms under a system of agriculture which does not include the practice of manuring.  Yet, there are other conditions that contribute largely to the result.  For instance, every cultural method accepted in dry-farming, such as deep plowing, fallowing, and frequent cultivation, enables the weathering forces to act upon the soil particles.  Especially is it made easy for the air to enter the soil.  Under such conditions, the plant-food unavailable to plants because of its insoluble condition is liberated and made available.  The practice of dry-farming is of itself more conducive to such accumulation of available plant food than are the methods of humid agriculture.

Further, the annual yield of any crop under conditions of dry-farming is smaller than under conditions of high rainfall.  Less fertility is, therefore, removed by each crop and a given amount of available fertility is sufficient to produce a large number of crops without showing signs of deficiency.  The comparatively small annual yield of dry-farm crops is emphasized in view of the common practice of summer fallowing, which means that the land is cropped only every other year or possibly two years out of three.  Under such conditions the yield in any one year is cut in two to give an annual yield.

The use of the header wherever possible in harvesting dry-farm grain also aids materially in maintaining soil fertility.  By means of the header only the heads of the grain are clipped off:  the stalks are left standing.  In the fall, usually, this stubble is plowed under and gradually decays.  In the earlier dry-farm days farmers feared that under conditions of low rainfall, the stubble or straw plowed under would not decay, but would leave the soil in a loose dry condition unfavorable for the growth of