NITROGEN COMPOUNDS OF THE SOIL III 



ules are distributed within the soil. 38 Hence when- 

 ever a leguminous crop is raised, nitrogen is added to 

 the soil, instead of being taken away, as in the case of 

 a grain crop. The amount of nitrogen per acre 

 returned to the soil by a leguminous crop as clover, 

 varies with the growth of the crop. In the roots of a 

 clover crop a year old there are present from 20 to 30 

 pounds of nitrogen per acre, while in the roots and 

 culms of a dense clover sod, two or three years old, 

 there may be present 75 pounds or more of nitrogen. 

 Peas, beans, lucern, cow peas, and all members of the 

 leguminous family possess the power of fixing the free 

 nitrogen of the air bv means of micro-organisms. 

 The micro-organisms associated with one species as 

 clover differ from these associated with another as 

 lucern. The amount of nitrogen which the various 

 legumes return to the soil is variable. Hellriegers re- 

 sults 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 appro^ 

 priate it for themselves. 



THE NITROGEN COMPOUNDS OF THE SOIL 



122. 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 



