NITROGEN, NITRIFICATION, NITROGENOUS MANURES 125 



134. 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, while those dark-colored and older con- 

 tained 3.21 per cent, and all the nodules of the root, 

 both active and inactive, contained 4.60 per cent nitro- 

 gen. The root itself contained 2.21 per cent. 43 



The root nodules also contain definite and character- 

 istic micro-organisms, and it was the spores of these or- 

 ganisms that were in the soil extract in both Hellriegel's 

 and Wilfarth's experiments. In the sterilized soils they 

 were not present. These organisms found in root nod- 

 ules are the essential agents which aid in the fixation of 

 the free nitrogen of the air, and in its ultimate use as 

 plant food. The nitrogen assimilated by the micro- 

 organisms in the nodules of the legumes is in part ap- 

 propriated by the crop, which unaided is incapable of 

 making use of the free nitrogen of the air. 



135. 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 legu- 

 minous 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 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 from 20 to 30 pounds of nitrogen per acre, 



