106 SOILS AND FERTILIZERS 



from those associated with another, as lucern. The 

 amount of nitrogen which the various legumes return 

 to the soil is variable. Hellriegel's results are of the 

 greatest importance to agriculture, because they show 

 how the free nitrogen of the air can be utilized in- 

 directly as food by crops unable to appropriate it for 

 themselves. 



THE NITROGEN COMPOUNDS OF THE SOIL 



125. Origin of the Soil Nitrogen. The nitrogen of 

 the soil is derived chiefly from the accumulated re- 

 mains of animal and vegetable matter. The original 

 source of the soil nitrogen, that is the nitrogen which 

 furnished food to support the vegetation from which 

 our present stock of soil nitrogen 'is obtained, was 

 probably the free nitrogen of the air. All of the ways 

 in which the free nitrogen of the air has been made 

 available to plants of higher orders which require 

 combined nitrogen, are not known. It is supposed, 

 however, that this has been brought about by the 

 workings of lower forms of plant life, and by micro- 

 organisms. Whatever these agencies have been they 

 do not appear to be active in a soil under high culti- 

 vation, because the tendency of ordinary cropping is 

 to reduce the supply of soil nitrogen. 



126. Organic Nitrogen of the Soil. In ordinary 

 soils the nitrogen is present mainly in organic forms 

 combined with the carbon, hydrogen, and oxygen, 

 and to a less extent with the mineral elements, form- 

 ing nitrates. The organic forms of nitrogen, it is 

 generally considered, are incapable of supplying plants 



