Evolution of CX)^ from Soil Humus 159 



a measure of the fertility of the soil. The amount of oxygen absorbed 

 was thus belie\ ed to measure the total aotion of soil mieroorganisms, 

 whieh are responsible for the decomposition processes in the soil. 



Stoklasa and Ernest placed 1-kg portions of sieved soil in glass 

 exlinders through which a current of air was passed at the rate of 

 10 liters in 24 hours. They observed that the evolution of COo by a 

 soil, under certain conditions of moisture and temperature, in a given 

 time, can furnish a reliable and accurate method for the determina- 

 tion of bacterial activities in the soil; the presence of organic matter 

 and the temperatures were found to be of greatest importance. The 

 e\olution of COo was shown to be greatest in neutral or slightly alka- 

 line soils abundantly supplied with readily assimilable plant nutrients 

 and well aerated. The production of CO^ from the soil was found to 

 be in direct proportion to the available organic matter in the soil 

 rather than to the total organic matter. The evolution of COo was 

 thus found to be an index of the availability of the soil humus, or of 

 the ease with whieh it decomposes (Table 30). 



Table 30. Ixflcence of Soil Depth and Soil Treatment upon the Decomposition 

 OF Humus, as Measured by the Rate of CO2 Evolution (from Stoklasa) 



80-100 2.1 2.7 2.3 



Van Suchtelen passed a current of air, usually 16 liters in 24 hours, 

 through 6 kg of soil placed on pure sand in a jar. The intensity of 

 CO2 production was much greater at the beginning of the experi- 

 ment and decreased rapidly after a short time. The amount of COo 

 produced was measured until it reached a uniformly low level; the 

 average amounts of CO2 produced per unit time from the different 

 soils served as a basis for comparison. The conclusion was reached 

 that the determination of COo formation from different soils fur- 

 nishes a better means for estimating the bacterial activities in the 

 soil than the numbers of bacteria. Cultivation, aeration, and nutri- 



