126 SOILS AND FERTILIZERS 



while in the roots and culms of a dense clover sod, 

 two or three years old, there may be 100 pounds or 

 more of nitrogen, not including that which has been 

 added to the soil by the accumulative action of the crop. 

 Peas, beans, lucern, cow peas, and all legumes possess 

 the power of fixing the free nitrogen of the air by means 

 of micro-organisms. The micro-organisms associated with 

 one species, as clover, differ 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 indirectly as food by crops unable 

 to appropriate it for themselves. 



THE NITROGEN COMPOUNDS OF THE SOIL 



136. Origin of the Soil Nitrogen. - - The nitrogen of 

 the soil is derived chiefly from the accumulated remains 

 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, 

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



