ATMOSPHERIC NITROGEN 105 



tained 4.60 per cent, nitrogen. The root itself con- 

 tained 2.21 per cent. 43 



The root nodules also contain definite and charac- 

 teristic micro-organisms, and it was the spores of these 

 organisms that were present in the soil extract in both 

 Hellriegel's and Wilfarth's experiments. In the ster- 

 ilized soils they were not present. These organisms 

 found in root nodules, are the essential agents which 

 aid in the fixation of the free nitrogen of the air, and 

 in its ultimate use as plant food. Experiments have 

 shown that these organisms are capable of being 

 propogated in nutritive media, separate from clover 

 roots. 87 



124. Nitrogen in the Root Nodules Returned to 

 the Soil. Ward has shown that when clover roots 

 decay, the organisms and nitrogen present in the 

 nodules are distributed within the soil. 38 Hence, 

 whenever 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 

 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 



