The Soil and They That Dwell Therein 271 



in the soil, it is converted into ammonia (NH^). Animal protein 

 can also be changed into ammonia during the decomposition of the 

 dead animals. 



Ammonia is rapidly oxidized in a well-ventilated soil to a sub- 

 stance called nitrous acid ( HNO2 ) through the action of two bac- 

 terial genera, the Nitrosomonas and the Nitrosococcus, but the 

 acid is soon converted into nitrites (NO., KNOo, or NaNOo). An- 

 other group of bacteria pick up the ball, so to speak, and oxidize 

 the nitrites into nitrates ( NO3 ) . This entire process of converting 

 ammonia into nitrates is teiTued nitrification; the organisms 

 involved are called nitrifijing bacteria. 



Plants are capable of assimilating nitrogen when it is supplied 

 in the form of nitrates, and this is indicated by labeling the next 

 step in the cycle plant protein. There is evidence, however, that 

 some plants may assimilate ammonia as a source of nitrogen. Un- 

 fortunately, when soil is not sufficiently aerated, anaerobes and 

 facultative anaerobes are capable of reducing the nitrates back 

 into the nitrite form— a highly undesirable process from the stand- 

 point of soil fertility. Still other bacteria are able to denitrify ni- 

 trates and liberate gaseous nitrogen (N2) which constitutes about 

 78% of the atmosphere and is largely unavailable to plants. This 

 DENiTRiFiCATiON rcsults in a loss of soil nitrogen, and hence the 

 process is to be avoided if possible. 



A few genera of bacteria have the capacity to bind atmospheric 

 gaseous nitrogen and to utilize it in their metabolism. These 

 organisms are called nitrogen fixing bacteria. Members of the 

 Rhizobium genus can live in symbiosis with the root systems of 

 legumes (clover, peas, beans, alfalfa, vetch, peanuts, etc.), causing 

 the formation of root nodules. These organisms furnish available 

 nitrogen to the plants, and the legumes reciprocate by supplying 

 necessary nutrients and proper living conditions for the rhizobia. 

 It has been calculated that this symbiotic relationship between 

 legumes and root nodule bacteria results in fixation of as much as 

 240 pounds of nitrogen per acre during a single growing season. 

 When growing independently of the legumes, however, these 

 bacteria are apparently incapable of fixing gaseous nitrogen. 



