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 wiU 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 3J 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 



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