INFLUENCE OF GREEN MANURES 



207 



The degree of the effects thus exerted is related to a great 

 variety of factors, among which may be included the soil structure 

 and its reaction, moisture, temperature, and supply of nutrient 

 salts. Of major importance also are the composition and the 

 amount of the plant material which is added to the soil, as shown in 

 Table 49. The development of microbes upon the organic mate- 

 rials added to the soils follows a changing course (Fig. 77). The 

 initial growth of the bacteria and fungi upon the readily available 

 plant constituents is apparent from the rapid formation of carbon 



90 40 300 



45 60 75 



Time (Days) 



Fig. 77. — The influence of organic matter on biological activities in soil. 



Changes following the addition of 2 grams of ground dry alfalfa meal to 



1 kilogram of soil (after Waksman and Starkey). 



dioxide and from the increase in numbers of bacteria and fungi 

 during the period immediately following the addition of the 

 organic matter. This period of rapid development is followed by a 

 stage of reduced speed of decomposition of the unattacked por- 

 tions of the organic substance and of the incompletely decomposed 

 products. During this stage, there is a decrease in the abundance 

 and activities of the microorganisms, but the drop is slower than 

 the initial rise. The available food material continuously decreases 

 in amount, and the biological conditions slowly approach the more 

 stabilized state which existed previous to the treatment of the soil. 



