THE FERTILITY OF THE DRY LANDS 287 
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 meth- 
ods of dry-farm cultivation and cropping which per- 
