Selected liihliography 165 



The inicrooigauisins are thus fotiiul to act as regulators of the CO2 

 tension in the atmosphere and of the an'iount of CO2 available to 

 plants. Were these or<j;anisins less aeti\e in the liberation of the 

 COo from the dead plants and animals or from the waste products of 

 their metabolism, the earth would soon become covered with unde- 

 composed plant and animal debris, while the limited supply of CO2, 

 ha\ing become exhausted, would gradually bring to an end all forms 

 of life. The actixities of microorganisms which bring about the 

 decomposition of this organic debris prevent its abundant accumula- 

 tion and make axailable to plants a constant stream of COo, as well 

 as of nitrogen and other nutrient mineral elements. On the other 

 hand, were these organisms more active in bringing about these 

 decomposition processes, the surface of the earth would soon become 

 covered with a layer of sand or clay free from all traces of organic 

 matter; this would soon prevent the normal growth of the majorit)^ 

 of economic plants by pro\ iding an unfavorable physical and chem- 

 ical medium for plant growth, and even more by preventing any 

 possible accumulation of nitrogen, which is also important for plant 

 growth. 



Selected Bibliography 



1. Appleman, C. O., Percentage of carbon dioxide in soil air. Soil Sci., 24:241— 

 245, 1927. 



2. Humfeld, H., A method for measuring carbon dioxide e\olution from soil. 

 Soil Sci., 30:1-9, 1930. 



3. Lundegardh, H., Carbon dioxide evolution of soil and crop growtli. Soil 

 Sci., 23:417-450, 1927. 



4. Marsh, F. W., A laboratory apparatus for the measurement of carbon dioxide 

 evolved from soils, Soil Sci., 25:253-260, 1928. 



5. Smith, F. B., and Brown, P. E., Soil respiration, /. Am. Soc. Agron., 23: 

 909-916, 1931; 24:577-583, 1932. 



6. Starkey, R. L., Some observations on the decomposition of organic matter 

 in soils. Soil Sci., 17:293-314, 1924. 



7. Waksman, S. A., and Starkey, R. L., Microbiological analysis of soil as an 

 index of soil fertility. VII. Carbon dioxide evolution, Soil Sci., 17:141-161, 

 1924. 



