THE SOIL AS A CULTURE MEDIUM 



639 



C0 2 is present in the soil solution. Diffusion of C0 2 into the atmos- 

 phere and of oxygen into the soil is very rapid at a depth of six inches, 56 

 as shown in table 69. 



The soil atmosphere shows much greater fluctuations in composition 

 than atmospheric air. On the average, the soil air was found 57 to 

 contain 0.25 per cent C0 2 and 20.6 per cent oxygen. From November 

 to May the curves for C0 2 follow closely those of the soil temperature; 

 from May to November they follow rainfall and to a less extent the 

 soil temperature curves. The favorable effect of rainfall is believed to 

 be due 58 to the dissolved oxygen brought down. 



While the decomposition of green manure in normal soils leads to 

 the formation of carbon dioxide, the gases formed under anaerobic 



TABLE 69 

 Composition of gas in variously treated soils 



Nitrogen 



Oxygen 



Carbon dioxide 



Hydrogen 



Methane 



Argon 



N 



A 



AVERAGES OF SEVERAL DETERMINATIONS, IN PER CENT 



Fallow land 



Before 

 rainfall 



78.05 

 20.40 



0.58 

 None 

 None 



0.977 



80.0 



After 

 rainfall 



78.83 

 19.26 



0.95 

 None 

 None 



0.955 



82.5 



Gases near 



roots 



of corn 



80.15 

 9.00 

 9.11 

 0.73 



None 

 1.010 



79.5 



Green 



manured 



land 



79.18 

 7.71 



12.03 

 0.07 



None 

 1.003 



78.8 



Swamp 

 rice land 



85.59 

 0.54 

 4.42 

 6.42 

 2.81 

 0.893 



95.7 



conditions, as in swamp rice soils, are largely methane, a small amount 

 of nitrogen, carbon dioxide, and hydrogen. 59 



An increase in atmospheric pressure brings about first an increase 



56 Leather, J. W. Soil gases. Mem. Dept. Agr. India, Chem. Ser., 4: 85- 

 134. 1915. 



57 Russell, E. J., and Appleyard, A. The atmosphere of the soil; its com- 

 position and the cause of variation. Jour. Agr. Sci., 7:1. 1915. 



88 See also 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. 



69 Harrison, W. H., and Aiyer, P. A. S. The gases of swamp rice soils, their 

 composition and relationship to the crop. Mem. Dept. Agr. India, Chem. Ser., 

 3: 65-106. 1913. 



