THE SOIL AIR 479 



of the soil (see par. 268), the organic portion of which is 

 especially capable of condensing gases. Oxygen con- 

 densed on the surface of this organic matter would, in 

 the words of Johnson, 1 " spend itself in chemical action," 

 of which carbon dioxide would be the result. 



There is now no doubt, however, that biological pro- 

 cesses are largely responsible for the occurrence of the 

 large quantity of carbon dioxide in the soil air. There 

 are two distinct processes involved : (1) the physiological 

 action of bacteria by which they absorb oxygen and give 

 off carbon dioxide, and (2) the excretion of carbon dioxide 

 by plant roots. The extent to which carbon dioxide is 

 produced in normal soils in these two ways has been es- 

 timated by Stoklasa, 2 who has done much work on the 

 subject. He concludes that the microorganisms in 

 an acre of soil to a depth of four feet may produce between 

 sixty-five and seventy pounds of carbon dioxide a day 

 for two hundred days in the year, and that during the 

 growing period the roots of oats or wheat would give 

 off nearly as much to an acre. 



397. Production of carbon dioxide as affecting com- 

 position. — Although the formation of carbon dioxide 

 in the soil depends on the decomposition of organic 

 matter, it is not always proportional to the quantity 

 of organic matter present. The rate of decomposition 

 varies greatly, and where this is depressed, as is sometimes 

 seen in muck or forest soils, the content of carbon dioxide 

 is relatively low. A high percentage of organic matter 



1 Johnson, S. W. How Crops Feed, p. 218. New York, 

 1891. 



2 Stoklasa, J. Ueber den Ursprung die Menge und die 

 Bedeutung des Kohlendioxids im Boden. Centrlb. f. Bakt., II, 

 Band 14, Seite 723-736. 1905. 



