Evolution of CO^ from Soil Humus 161 



be accompanied by an increase in the (X)j content of tlie soil air 

 and somewhat later by a rise in the amonnt of nitrate in the soil. 

 The rate of decomposition of hnmns in the soil may, therefore, be 

 looked upon as a fnnetion of microbiological activities. Cultivation, 

 aeration, and presence of nutrient salts exert a stimulating effect 

 upon these processes. Among the most important factors, however, 

 are moisture content and abundance and nature of humus. A direct 

 relation exists between temperature and humus decomposition; the 

 process of humus decomposition goes on at temperatures below 0°C, 

 but it is greatly hastened by a rise in temperature. Variations in the 

 production of COo with season of year are due largely to variations 

 in temperature and to the available organic matter. The amount of 

 CO2 evohed in 24 hours from 1 square meter of soil was shown to 

 range from 2 to 20 gm calculated as carbon; the actual amount of 

 COo liberated depends on the nature of the soil, treatment, season 

 of year, and various other factors. 



Since evolution of COo from soil is a measiu*e of the rate of 

 decomposition of the humus, one may conclude that the rate of 

 decomposition depends upon the abundance of humus in the soil, 

 the physical and chemical nature of the soil, its treatment, and the 

 crops grown (Table 32). In fertile soils, humus decomposes more 

 readily, thus resulting in the liberation of a greater amount of COo 

 and ammonia. Those soils in which humus decomposes very slowly 

 are called infertile. Frequently this condition can be corrected by 

 addition of lime, by culti\ation, by drainage, or by other special 

 treatments. An increase in the rate of decomposition of humus will 

 be accompanied by an increase in the fertility of the soil. 



The decomposition of humus is brought about by the activities of 

 a large number of bacteria, fungi, and other microorganisms inhab- 

 iting the soil. These activities are rather slow as compared with the 

 decomposition of fresh plant and animal residues. The amount of 

 humus decomposed in a season is usually between 2 and 5 per cent 

 of the humus content of the soil. Under special conditions, the de- 

 composition may be greater; under other conditions, it may be less. 



Another early student of humus decomposition in soil, Bous- 

 singault, recorded, in 1873, observations on a humus-rich soil. He 

 found that one-half of the total organic carbon in the soil became 

 changed to COo in 11 years; one-third of the nitrogen appeared as 

 nitrate in that period. The more extensive experiments at Rotham- 

 sted, however, showed a loss of only one-third of the nitrogen in 50 

 years, from soils free from crops but cultivated. In prairie soils, a 



