156 



THE SOIL: ORGANIC MATTER 



hydrates, probably dextrose or maltose, and by the oxidation 

 of this material the bacteria fix nitrogen obtained from the 

 soil air through the nodules. They not only use some of the 

 resulting compounds for their own growth but apparently 

 pass on a large part of it for the use of the legumes. Fig. 36 

 shows the effect of these bacteria on clover growing in soil 

 containing no nitrates. 



«&*** 



**c&> 



I II 



Fig. 36. — Clover growing on soil containing no nitrates. /. No nitrogen 

 fixing bacteria. II. Supplied with bacteria. Soils Department, Wisconsin 

 Station. 



Both the bacteria and the legumes can utilize nitrates in 

 the soil, but apparently the symbiotic relationship is better 

 for both. The bacteria do not fix nitrogen when supplied 

 with nitrate nitrogen, nor do the legumes accumulate the 

 nodules to any extent when there are sufficient nitrates 

 in the soil. 



The bacteria are present in the soil to a considerable 

 extent, and although it is claimed they can, under suitable 

 conditions, fix nitrogen independently of legumes, they 

 apparently do not do it readily. When legume roots are 

 present in the soil these bacteria enter the root hairs, growing 

 into long gelatinous threads which penetrate the various 

 cells of the fine roots, and develop immense numbers of 

 bacteria. Their multiplication causes the peculiar nodule 



