158 



Decomposition of Soil Organic Matter 



humus at this high temperature. A certain amount of moisture was 

 required for the maximum production of COo by microorganisms. 

 The state of division of the soil and its aeration were found to affect 

 greatly the rate of decomposition of the humus. Sterile soils pro- 

 duced only small amounts of COo; when a soil infusion was added, 

 the process was increased twenty-five times. One of the most im- 

 portant points brought out in these investigations was the fact that 

 sterilized and inoculated soil gave two to five times as much COo 



125 



100 - A 



^ 75 



50 



25 



Fig. 69. Decomposition of various chemical constituents of alfalfa witliout addi- 

 tional nutrient salts (from Tenney and Waksman). 



as unsterilized and uninoculated soil. This indicates definitely that 

 the process of sterilization rendered the soil humus more susceptible 

 to decomposition. There is also an optimum moisture content for 

 the formation of COo; this is influenced by the state of division of 

 the soil and its aeration. 



A distinct parallelism was found to exist between the amount of 

 oxygen absorbed and the amount of COo produced from different 

 soils. A similar parallelism was also found between COo evolution 

 and nitrogen accumulation in the form of ammonia and nitrate. 

 Russell measured the amount of oxygen absorbed by the soil as an 

 index of soil oxidation instead of determining the COo produced. 

 The rate of absorption of oxygen was found to increase with tempera- 

 ture, the amount of water (up to a certain point), and the amount 

 of calcium carbonate present in the soil. Since these factors also 

 paralleled soil fertility, Russell suggested the use of soil oxidation as 



