[Chap. XLlll BACTERIA OF THE SOIL 537 



elements, resulting in a nitrogenous compound, perhaps an amino com- 

 pound. The formation of these complex compounds is a reduction proc- 

 ess and requires the energy made available through the oxidation of 

 suc^ars made by the leguminous plant. The bacteria multiply, mature, 

 and die within the nodules, and the nitrogen compounds are used or 

 accumulate within the legume. At the death of any part of the roots of 

 the legume these compounds enter the soil. 



Still another group of bacteria fix nitrogen without being associated 

 with higher green plants. These organisms are saprophytes and make 

 possible a series of processes sometimes called non-symbiotic nitrogen- 

 fixation, in contrast to the nitrogen-fixation in legumes, which is called 

 si/mbiotic. They live especially in humous soils and derive their energy 

 by the oxidation of such organic compounds as carbohydrates, organic 

 acids, and other products of fermentation. In addition to energy, the 

 fixation of nitrogen by the bacteria is affected by many other conditions 

 in the soil. Dark prairie soils are ideal conditions for the development 

 of such bacteria. 



Non-symbiotic nitrogen-fixing bacteria may be classified into two 

 groups: those using free oxygen (aerobic), largely species of the genus 

 Azotobacter; and anaerobic species belonging to the genus Clostridium. 

 The species of Azotobacter are coccus forms and non-motile; those of 

 Clostridium are rod-shaped and spore-fonning. 



Summary: the nitrogen cvcle. We have now discussed the origin of 

 nitrogenous compounds in soils through the agency of bacteria. Neither 

 nitrification nor nitrogen-fixation in soils occurs in the absence of certain 

 microorganisms, chiefly bacteria. We may now summarize the complete 

 cycle of events in the origin and transformation of nitrogen compounds. 

 In the green plants, amino acids are made from carbohydrates and nitro- 

 gen salts and are used by both plants and animals in the synthesis of 

 proteins. The death of plants and animals results in a residue of sub- 

 stances, some of which are proteins. Through the agency of bacteria the 

 proteins are digested to simpler compounds such as amino acids. Then 

 through further chemical change ammonia is foraied from water and 

 the NH7 ions. Ammonification is the name applied to this series of 

 processes. 



Ammonia is then oxidized to nitrites and to nitrates through the 

 agencv of nitrifving bacteria. These processes are nitrification. The 

 nitrates mav be used by green plants and by most non-green plants in 

 the production of proteins, and we are back where we started. Certain 



