46 



AGKONOMY 



have from 40 to 50 inches. Over most of the Gulf states the 

 rainfall is from 50 to 60 inches. About 215,000,000,000,000 

 cubic feet of water fall annually in the United States. This 

 would cover 5,000,000,000 acres a foot deep, or keep ten 

 Mississippi rivers constantly flowing. In figuring precipita- 

 tion, 10 inches of snow are considered equal to 1 inch of ram. 

 Dew may form within the soil as well as upon it, but the 

 amount of moisture added in this way is negligible. Dry soils 



Photograph by H. L. Hollister Laud Co. 



FIG. 21. Sagebrush on the Western plains 



The rainfall is insufficient for cultivated crops, but when irrigated the lan'd 

 yields abundant harvests 



are also able to absorb moisture from damp air, the moisture 

 condensing upon the soil particles. Sands absorb least and 

 humus most. Quicklime slakes when exposed to the air, by 

 absorbing moisture in this way. So strong is this tendency 

 of quicklime to absorb moisture that the chemist uses it to 

 dry the air used in various experiments. A fine soil will also 

 absorb moisture from a coarser one. Clay may sometimes take 

 water from sand, though the latter may contain less moisture 

 than the clay. 



