Soil Bacteria and the Nitrogen Cycle 



397 



F. Lohnis, U.S.D.A. 



Fig. 244. Nodules containing nitrogen-fixing bacteria on the roots of legumes: A, red 

 clover ; B, sweet pea ; C, soy bean. 



only in rich, well-drained soil. They are of great importance in 

 agriculture because nitrogen is the most expensive of all the ele- 

 ments that are bought for fertilizers. Their relation to the 

 humus IS very different from that of the nitrifying bacteria just 

 described. To fix nitrogen (N2) — that is, break up the mole- 

 cules — requires much energy. The nitrogen-fixing bacteria 

 secure this energy by oxidizing the carbon compounds (especially 

 carbohydrates) found in the humus. It is estimated that 100 

 pounds of humus must be oxidized for every pound of nitrate 

 formed in the soil. The bacteria that carry on this process are 

 true saprophytes. 



Bacteria and legumes. Clover, alfalfa, beans, soy beans, and 

 peas belong to a family of plants called legumes. They increase 

 the nitrogen in soils on which they are grown, and for many 

 years they have been used in crop rotations, following wheat or 



