722 PRINCIPLES OF SOIL MICROBIOLOGY 



that all these phenomena are related. A rise in bacterial numbers was 

 accompanied by a rise in the C0 2 in the soil air, and somewhat later by 

 a rise in the nitrate content. The rate of decomposition of organic 

 matter in the soil was, therefore, looked upon as a function of bacterial 

 activity. It was further demonstrated 24 that the principal factors 

 affecting carbon dioxide production are, in order of importance, tempera- 

 ture, moisture, dissolved oxygen and the growing crop. 



In all the investigations up to 1915, the air was either drawn through 

 the soil, thus greatly accelerating microbiological activities, or inter- 

 mittenly over the soil. Potter and Snyder 25 found that field results 

 can be most closely duplicated in laboratory studies when the air is 

 passed continuously over the soil. The amount of air passing over 

 the soil in the laboratory does not affect materially the amount of C0 2 

 evolved. When the soil is placed under optimum laboratory conditions, 

 the initial rise in carbon dioxide formation is soon followed by a drop, 

 which becomes nearly a straight line. Klein 26 also found that the 

 amount of CO2 given off by the soil when brought into the laboratory 

 and the proper amount of moisture added is considerable for a short 

 period. He attributed this phenomenon to the previous drying of the 

 soil which made the soil a better medium, both physically and chemi- 

 cally, for the growth of bacteria. This is no doubt correct, since when 

 normally moist soil is used no such rapid drop is observed. Previous 

 drying of the soil alters its colloidal condition to the extent of in- 

 creasing the rate of oxidation. Because of this, rainfall may increase 

 the carbon dioxide formation in field soils, due to the increase in 

 moisture content more favorable for biological activities rather than 

 for the reason of oxygen brought down by the rain, as suggested by 

 some investigators. It was found, by the use of this method, that 

 manure stimulated C0 2 production while sodium nitrate and ammonium 

 sulfate did not. CaC0 3 did not indicate at first any increase of CO2 as 

 a result of the decomposition of the organic matter (the abundant 

 formation of C0 2 when CaC() 3 is added to an acid soil is due to 

 chemical interaction between the carbonate and the buffering sub- 



24 Russell, E. J., and Appleyard, A. The influence of soil conditions on the 

 decomposition of organic matter in the soil. Jour. Agr. Sci., 8: 3S5-417. 1917. 



25 Potter, R. S., and Snyder, R. S. Carbon dioxide production in soils and 

 carbon and nitrogen changes in soils variously treated. Iowa Agr. Exp. Sta. 

 Res. Bui. 39. 1916. 



26 Klein, M. A. Studies in the drying of soils. Jour. Amer. Soc. Agron., 7: 

 49. 1915. 



