CONTENT OF DRY-FARM SOILS 69 



in acids than arid soils, the difference being 84 to 

 69. Since the only plant-food in soils that may be 

 used for plant production is that which is soluble, 

 it follows that it is safe to assume that arid soils are 

 generally more fertile than humid soils. This is 

 borne out by a study of the constituents of the soil. 

 For instance, potash, one of the essential plant 

 foods ordinarily present in sufhcient amount, is found 

 in humid soils to the extent of 0.21 per cent, while in 

 arid soils the quantity present is 0.67 per cent, or over 

 three times as much. Phosphoric acid, another of 

 the very important plant-foods, is present in arid 

 soils in only slightly higher quantities than in humid 

 soils. This explains the somewhat well-known fact 

 that the first fertilizer ordinarily required by arid 

 soils is some form of phosphorus. 



The difference in the chemical composition of arid 

 and humid soils is perhaps shown nowhere better 

 than in the lime content. There is nearly eleven 

 times more lime in arid than in humid soils. Con- 

 ditions of aridity favor strongly the formation of 

 lime, and since there is very little leaching of the soil 

 by rainfall, the lime accumulates in the soil. 



The presence of large quantities of lime in arid 

 soils has a number of distinct advantages, among 

 which the following are most important: (1) It 

 prevents the sour condition frequently present in 

 humid climates, where much organic material is 

 incorporated with the soil. (2) When other con- 



