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; (b) 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 



\ / 



x 



1 ...... _ - _.:,„._.-™A_. 



|, : FiQ. 1191— Different forms of alfalfa nodules. (U. S. D. A.) 



r'-' ■■ ' ■'/'■■ . => .'•'; I- -. 



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 

 Ipy nodule bacteria and by other nitrogen-fixing organisms. 



! Nitrogen fixation may also be accomplished artificially through 

 the use of electricity. 



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

 fixing free atmospheric nitrogen. .^■-'^----\ 



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

 important soil bacteria. ^W':---' " ♦^ 



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 



