28 PLANT SUCCESSION AND CROP PRODUCTION 



factors and presents them in their true role of a source of the in- 

 fluences separate from the climatic or edaphic influences. The im- 

 portance of biotic factors is seen nowhere so well as in the forma- 

 tion of soils, which in the majority of cases are due to the action 

 of plants. 



Among the various biotic factors the bacteria are of prime im- 

 portance, especially the bacteria of the Nitrogen Cycle. 



The first group are saprophytic, that is, they live upon plant 

 and animal residues after the death of the individual. Their reac- 

 tions reduce the plant and animal proteins to ammonia, hence 

 the process is known as ammonification. The chief micro-organisms 

 are Bacillus subtilis and B. mycoides. Ammonification is followed 

 by rapid decay of the plant or animal residue. The ammonia remain- 

 ing in the soil is commonly ammonium carbonate (NHOiCOa. De- 

 cay is followed by nitrification, an oxidation process carried on by 

 the Nitrosococcus and Nitrosomonas bacteria. The ammonia or am- 

 monium salts are oxidized to nitrites of the composition (R) NO2. 

 Further oxidation carried on by the Nitrobacter changes the 

 nitrites, (R) NOa to nitrates, (R) NO3. 



The nitrates are useful for green plants and with the excep- 

 tion of some of the ammonium compounds, are the only fomi in 

 which nitrogen can be absorbed by most green plants. At this stage 

 denitrification by bacteria of the denitrifying group may liberate 

 the nitrogen of nitrates into the free nitrogen of the air. The pres- 

 ence of these organisms is unfavorable to the growth of green 

 plants therefore. 



The completion of the nitrogen cycle is found in one direction 

 by the building up of nitrates in the body of green plants into, first, 

 the amino-acids, then the plant proteins, and finally animals sub- 

 sisting on the plants build up the amino acids and proteins of plants 

 into their own protoplasm. The cycle begins again with the death 

 of animals and plants. 



Free nitrogen of the air may under some conditions be fixed 

 by groups of soil bacteria which derive their energy from carbon- 

 material found in the soil or by living somewhat parasitically on 

 green plants- Azotobacter and Clostridium sp. are the chief of 

 these organisms. The former requires the presence of abundant 

 oxygen and so works only close to the surface of the soil, while the 

 organisms of the Bacillus radicicola group upon living roots of leg- 

 umes, furnish an example of another kind of organism capable 



