^ 



IIO SOILS AND FERTILIZERS 



These experiments have been verified by many 

 other investigators until the fact has been established 

 that leguminous plants ma}', through the agency of 

 micro-organisms, make use of the free nitrogen of the 

 air. The work of Hellriegel was not accidental but 

 the result of careful and systematic investigation. As 

 early as 1863 he observed that clover would develop 

 alono- the roadway in sand in which there was scarcelv 

 a trace of combined nitrogen. 



120. Composition of Root Nodules. — The root 

 nodules referred to, are particularly rich in nitrogen. 

 In one experiment, the light-colored and active ones 

 contained 5.55 per cent, of nitrogen while the dark- 

 colored and older ones contained 3.21 per cent. The 

 entire nodules of the root both active and inactive 

 contained 4.60 per cent, nitrogen. The root itself 

 contained 2.21 per cent, nitrogen. 42 



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. 



121. Nitrogen in the Root Nodules Returned to 

 the Soil. — Ward has shown that when clover roots 

 decay, the organisms and nitrogen present in the nod- 



