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 jDrevious 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- 



