178 SOIL ORGANISMS IN RELATION TO SOIL FERTILITY 



nitrogen from the soil air and converting it into a form suitable 

 for the plant (Figs. 114-C, 116, 117, 118 and 119). 



There are three main differences between legumes and other 

 plants: (a) They are very rich in nitrogen; (6) they usually have 

 nodules on their roots, and (c) they may increase the nitrogen 

 supply of the soil through the action of the nodule bacteria. 



Nodule organisms are also called "symbiotic bacteria." They 

 have the power of independent existence, but when they enter the 

 roots of legumes both the bacteria and plants are benefited by 

 the close association. 



Nitrogen Fixation by Free Soil Bacteria. Aside from the 

 bacteria which cause the formation of nodules, there are bacteria 



Fia. 119. Different forms of alfalfa nodules. (U. S. D. A.) 



in the soil which have the power of fixing or gathering nitrogen 

 independently of any roots or plants. These are commonly called 

 the free nitrogen-fixing organisms, or non-symbiotic bacteria. 



From what has been said it can be concluded that nitrogen 

 fixation in soils is the fixing or gathering of atmospheric nitrogen 

 by nodule bacteria and by free nitrogen-fixing organisms. 



Nitrogen fixation may also be accomplished artificially through 

 the use of electricity. 



Certain molds and algae in soils also seem to have the power of 

 fixing free atmospheric nitrogen. 



Nitrogen-fixing bacteria are classed as plants, as are the other 

 important soil bacteria. 



Amount of Nitrogen Gathered. Under field conditions it 

 has been estimated that the free nitrogen-fixing bacteria gather 

 and add to the soil annually from fifteen to forty pounds of nitrogen 



