208 



MODIFICATION OF THE SOIL POPULATION 



The time elapsing before this stage is reached is determined by a 

 variety of factors. In most cases it is an extended period and 

 may consume many months. 



TABLE 49 



Influence of Different Amounts of Straw on Development of 

 Bacteria in Soil — Average Numbers over a Period of 71 Weeks 

 AFTER Treatment (from Murray) 



Green manure, whether it consists of leguminous plants, of 

 young cereal plants, or others, is comparatively rich in water- 

 soluble substances (sugars, amino acids, etc.), as shown in Table 18. 

 Such material serves as a readily available source of energy, and 

 may be quite completely changed within a period of a few weeks in 

 the soil. After a series of transformations, the nitrogen of the 

 green manure is liberated as ammonia, then soon oxidized to 

 nitrite, and further oxidized to nitrate. The amount of nitrogen 

 liberated from the decomposition of the green manure depends 

 largely upon the nature and type of plant. The younger the 

 plant, the more rapidly it decomposes, and the more rapidly its 

 nitrogen is liberated in an available form. Consequently, less of it 

 remains to increase the organic matter of the soil over any appre- 

 ciable period of time. 



The addition of green manure thus brings about a series of 

 changes in the microbial population of the soil, affecting prac- 

 tically every group of microorganisms, either directly or indirectly. 

 Organic acids may be formed in the initial stages of decomposition, 

 but they do not persist for long under aerobic conditions. 



