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 sufficient 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- 
