CHAPTER VII 



THE LIVING ORGANISMS OF THE SOIL 



Formation of Nitrates in the Soil. Bacteria in the Soil which 

 decompose Organic Matter. Bacterial Loss of Nitrogen from 

 the Soil. Formation of Humus. Fixation of Nitrogen by 

 Bacteria associated with Leguminous Plants. Value of Clover 

 Crops in the Rotation. Inoculation of Soil. Other Bacteria- 

 fixing Nitrogen in the Soil. Accumulation of Nitrogen in 

 Virgin Soils. Dependence of Soil Fertility upon Bacteria. 



So far, we have considered the soil as though it were 

 a merely dead medium in which the plant can fix itself, 

 and from which it can obtain food by processes of solution 

 and diffusion, as though the whole behaviour of the soil 

 towards the plant were regulated by chemical and 

 physical actions of the kind that can be reproduced in 

 the laboratory. Yet certain facts might serve to suggest 

 that other changes have their seat in the soil ; for 

 example, we have ascertained by water-culture experi- 

 ments that plants can only take in their necessary 

 nitrogen in the forms of nitrates or ammonia, small 

 amounts of which are very general constituents of 

 cultivated soils. But though nitrate of soda and sulphate 

 of ammonia are employed as manures, their purpose is 

 equally and more commonly served by other compounds 

 of nitrogen, such as the animal and vegetable residues 

 which are found in ordinary dung. Both experiment 

 and farming experience show that the plant will obtain 

 nitrogen from whichever of its compounds is put into 



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