The intrinsic fertility of arid soils is not alone sufficient to explain the increase in plant-food which undoubtedly occurs in the upper foot or two of cultivated dry-farm lands. In seeking a suitable explanation of this phenomenon it must be recalled that the proportion of available plant-food in arid soils is very uniform to great depths, and that plants grown under proper dry-farm conditions are deep rooted and gather much nourishment from the lower soil layers. As a consequence, the drain of a heavy crop does not fall upon the upper few feet as is usually the case in humid soils. The dry-farmer has several farms, one upon the other, which permit even improper methods of farming to go on longer than would be the case on shallower soils. 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.