FERTILITY AND HUMUS IN DRY AREAS 409 



slowly unlocked or made available by the processes 

 of cultivation. This is one reason why judicious culti- 

 vation is so important in dry areas. The difference be- 

 tween poor soils and soils that are rich, is mainly a 

 difference in the food elements that these soils contain 

 and chiefly in the inert form. It is fortunate that these 

 stores are made available chiefly through the processes 

 of cultivation. Were it otherwise, there would be a 

 great waste of fertility before it could be utilized. But 

 there are certain forms of bacteria in the soil that have 

 the power of gathering nitrogen from the air, and con- 

 verting it into forms suitable for the needs of plants. 

 According to Widtsoe these bacteria utilize for their 

 life processes the organic matter of the soil. To work 

 effectively they require a soil rich in lime, fairly dry and 

 warm and well aerated. Fortunately these conditions 

 are all met on the dry farms of the semi-arid west. 



The food carried up from the subsoil exercises an 

 important influence on fertility in semi-arid areas. The 

 soluble materials in arid areas go down to the lower 

 limit of moisture penetration. The soil and subsoil are 

 thus made of equal porosity. Because of the facility 

 with which air may penetrate the soil and subsoil mass, 

 the subsoils are in a sense weathered and made suit- 

 able for furnishing available plant nutrition to great 

 depths. In some instances in the semi-arid region, the 

 soil and subsoil are not very different from the surface 

 soil down to the distance of many feet, hence in these 

 deep plowing does not bring up unweathered soil as it 

 does in dry areas, and it enlarges at the same time the 

 reservoir for storing water. It has been claimed, and 

 probably correctly, that the relative fertility of different 

 areas in the semi-arid belt depends more on the nature 

 of the subsoil than of the surface soil. In semi-arid 

 regions the roots of plants go to stored water, and the 

 latter does not need to be brought to the surface. 



