Evolution of CO^ from Soil Humus 157 



sition, measurements may also be made of the actual disapiDcarancc 

 of humus as a whole or of some of its sp'ecific chemical constituents. 

 This can be done by measuring the total caibon or nitrogen content 

 of the soil or by making a proximate analysis of the humus. The 

 latter is of particular advantage in the study of peats, forest soils, 

 and composts. 



Other methods can be utilized to measure humus decomposition, 

 as, for example, the change in calorific value of the humus in the 

 soil or the e\olution of heat accompanying processes of humus de- 

 composition. These methods amply demonstrate the fact that humus 

 is not stable, that it can disintegrate and disappear from the soil, 

 but that the rate of its disappearance is rather slow. Different farm 

 practices may result in the preservation and even accumulation of 

 humus or in its destruction. Which of these is more desirable de- 

 pends entirely upon the nature of the soil, the nature of the crop 

 grown, and general problems of soil utilization and soil conservation. 



As a result of extensive decomposition, humus may reach a 

 definite chemical equilibrium, as shown by its more or less constant 

 carbon-nitrogen ratio. This equilibrium is particularly characteristic 

 of humus in field and garden soils. In this condition, further decom- 

 position of humus results in a parallel liberation of carbon as COo 

 and of nitrogen as ammonia, rapidly oxidized to nitiate. In com- 

 posts, forest soils, and peats, the ratio of COo liberated to nitrate pro- 

 duced varies considerably, depending upon the nature of the ma- 

 terial and the state of its decomposition. 



A distinct parallelism was thus found to exist between the abun- 

 dance of microorganisms in the soil and the decomposition of the 

 soil humus; the more fertile a soil is, the greater will be the amount 

 of CO2 liberated in a given time. Wollny reported in 1880 that the 

 COo content of the soil rises and falls with the amount of organic 

 matter present. 



One of the early accurate studies on humus decomposition was 

 carried out by Deherain and Demoussy. They placed the soil under 

 examination in closed glass containers and kept them at different 

 constant temperatures. At the end of a definite period of incubation, 

 the gas was extracted from the soil and container and the COo pres- 

 ent determined. These workers were among the first to demonstrate 

 that the formation of COo was due almost entirely to the action of 

 microorganisms; it increased with an increase in temperature to 

 about 65°C, then decreased; at 90°C, another increase in COo for- 

 mation took place, which was due to the chemical oxidation of the 



