COMPOSITION OF DRY-FARM SOILS 71 
is the soil as a whole and the more readily can plants 
secure their nutriment from the soil. It will be 
observed from the table, as previously stated, that 
more humus is found in humid than in arid soils, 
though the difference is not so large as might be ex- 
pected. It should be recalled, however, that the 
nitrogen content of humus formed under rainless 
conditions is many times larger than that of humus 
formed in rainy countries, and that the smaller per 
cent of humus in dry-farming countries is thereby 
offset. 
All in all, the composition of arid soils is very 
much more favorable to plant growth than that of 
humid soils. As will be shown in Chapter IX, the 
greater fertility of arid soils is one of the chief reasons 
for dry-farming success. Depth of the soil alone 
does not suffice. There must be a large amount of 
high fertility available for plants in order that the 
small amount of water can be fully utilized in plant 
growth. 
Summary of characteristics.— Arid soils differ 
from humid soils in that they contain: less clay; 
more sand, but of fertile nature because it is derived 
from rocks that in humid countries would produce 
clay; less humus, but that of a kind which contains 
about 34 times more nitrogen than the humus of 
humid soils; more lime, which helps in a variety of 
ways to improve the agricultural value of soils; 
more of all the essential plant-foods, because the 
