216 MODIFICATION OF THE SOIL POPULATION 



sideration of a single constituent of the manure, namely, the urea. 

 Half of the nitrogen in the manure is in this form, while the other 

 half is in the form of proteins and protein derivatives. The urea is 

 immediately attacked by a number of specific groups of bacteria 

 and is changed to ammonium carbonate. The latter is soon acted 

 upon by the nitrite- and nitrate-forming bacteria. The nitrate 

 may be finally reduced by denitrifying bacteria and changed to 

 nitrogen gas and oxides of nitrogen, or is assimilated by fungi and 

 various heterotrophic bacteria (which use the constituents of the 

 straw in the manure as sources of energy) and changed to microbial 

 proteins. Thus the addition of one compound to the soil leads 

 to a number of different reactions which are accompanied by 

 numerous changes in the microbial population. 



Artificial Manures. — Because of the ever-decreasing quan- 

 tities of stable manure, methods have been developed for the com- 

 posting of straw and other farm residues with inorganic fertilizer, 

 yielding a product similar in its chemical composition and action 

 upon soils and crops, and referred to as artificial manure. The 

 principles upon which the process is based are the decomposition 

 of the cellulose and other carbohydrates in the plant residues by 

 microorganisms; accompanying this process is the synthesis of 

 large quantities of microbial cell substance for which the inorganic 

 nitrogen is required. As a result of this action, there is a narrow- 

 ing in the carbon/nitrogen ratio from 80 : 1, in the straw, to 20 : 1, 

 in the compost. Although this compost contains an abundance 

 of various microorganisms, its beneficial effect upon the soil and 

 the crop is due largely to the nature of the organic and inorganic 

 complexes introduced. 



Influence of Soil Cultivation. — The frequent cultivation 

 of the soil considerably modifies the numbers and activities of soil 

 microbes in various ways. The processes of plowing, harrowing, 

 and cultivating the soil accomphsh several distinct purposes: 

 (1) They mix the plant residues with the soil itself, thus leading to 

 better conditions for decomposition of the various residues by 

 microorganisms. (2) They favor aeration of the soil, thus accel- 

 erating the exchange of carbon dioxide of the soil air for the oxygen 

 of the atmosphere; the oxygen is required for the various oxidation 

 processes in the soil, especially nitrate formation, the decomposi- 

 tion of organic compounds, and the oxidation of the reduced inor- 

 ganic substances. (3) Finally, soil cultivation brings about a more 



