256 PLANT BIOCHEMISTRY 



a greater oxygen supply at higher than at lower depths. Moisture 

 conditions shoukl ])e optimal for aeration and carbohydrate metab- 

 olism of the host plant. The optimal temperature for nitrogen fixa- 

 tion by the symbiotic organisms approximates 20°C., providing that 

 the pH of the environment is close to the neutral point. However, 

 nodules are formed under any soil condition in which the legumes can 

 grow. 



Non-symbiotic nitrogen fixation. In addition to the symbiotic ni- 

 trogen-fixing organisms, there are two other important groups of 

 nitrogen-fixing soil organisms. These function independently of higher 

 plants. One of these has been given the generic term of Azotohacter, 

 the best known species of which is Azotobactcr chroococus. These 

 organisms are aerobic and will fail to develop under slightly acid 

 conditions. 



Another group of non-symbiotic nitrogen-fixing bacteria is known 

 by the species name of Clostridiuyn pastorianum. These are anaerobic 

 organisms and appear to function effectively in acid as well as in 

 neutral soils and under conditions that are unfavorable to most species 

 of Azotobactcr. 



Ammonification 



The production of ammonia from organic nitrogenous materials of 

 the soils is called ammonification. This process is an intermediate 

 step in the process of nitrate formation by soil microorganisms. The 

 organisms concerned with ammonification are the same fungi, bac- 

 teria, and actinomycetes that decompose organic matter in soils. The 

 most important ammonifying organisms are aerobic and require a good 

 supply of oxygen and moisture. The most favorable pH range for their 

 action seems to be 6.0 to 8.0. 



Nitrification 



The conversion of ammonia nitrogen to nitrate nitrogen in soils is 

 accomplished in two steps by two classes of bacteria. The first step, 

 which is carried out by Nitrosomonas and Nitrosococcus organisms, 

 changes ammonia to nitrites. The second step is carried out by Nitro- 

 bactcr and oxidizes nitrites to nitrates. The final oxidation to nitrates 

 takes place more rapidly than the formation of ammonia or nitrites, 

 with the result that very little ammonia or nitrite nitrogen is present 

 at any one time. The presence of ammonia nitrogen in amounts more 

 than a few parts per million of soil is evidence that conditions are 

 unfavorable for nitrification. This is commonly the case in acid soils 

 rich in organic matter or in soils where the moisture content is high. 



