INFLUENCE OF ENVIRONMENTAL CONDITIONS 799 



soil). 121 It was suggested 122 that the lower nitrate formation in cropped 

 land may be due to the adverse effect of the crop upon bacterial activi- 

 ties or to some process of destruction of nitrates, at work in the cropped 

 soil which does not take place in the fallow soil. 123 The stimulative 

 effect of the growing crop upon the activities of heterotrophic organ- 

 isms (including decomposition of organic matter) and the injurious effect 

 upon nitrate accumulation explain one another since one process is 

 the cause of the other, the heterotrophic organisms, using the available 

 energy of the fresh organic material of a wide carbon-nitrogen ratio, 

 consume some of the nitrate. 



The growing crop brings about an increase in the carbon dioxide 

 content of the atmosphere, as indicated by the fact that the carbon 

 dioxide content in a cropped soil is much greater than in a corre- 

 sponding bare soil. 124 The excess of carbon dioxide was ascribed to the 

 respiratory activity of the plants rather than to the decomposition of 

 the root particles from the crop growing in the soil. This is in line 

 with the previous observations of Barakov 125 that plants produce 

 much greater quantities of carbon dioxide in the soil than do the bac- 

 teria; maximum carbon dioxide production was found to coincide with 

 the maximum life activity of the plant. Leather 126 also found greater 

 quantities of carbon dioxide in the neighborhood of roots of crops than 

 in fallow land. 



Neller 127 obtained quantitative measurements of the total carbon 

 dioxide liberated from oxidation processes taking place in the soil 

 during plant growth (fig. 76, no. 153, PI. XIX); much more rapid 



121 Voorhees, E. B., Lipman, J. G., and Brown, P. E. Some chemical and bac- 

 teriological effects of liming. N. J. Agr. Exp. Sta. Bui. 210. 1907. 



122 Russell, E. J. The nature and amount of fluctuations in nitrate contents 

 of arable soils. Jour. Agr. Sci., 6: 18-57. 1914; also Ibid., 7: L45. 1915. 



123 Burd, J. S. Water extracts of soils as criteria of their crop producing power. 

 Jour. Agr. Res., 12: 297-310 (304). 1918. 



124 Russell and Appleyard, 1915 (p. 721); Turpin, H. W. The carbon dioxide 

 of the soil air. Cornell Univ. Agr. Exp. Sta. Mem., 32: 319-362. 1920. 



126 Barakov, P. The carbon dioxide content of soils during different stages of 

 growth of plants. Zhur. Opit. Agron., 11: 321-342. 1910. 



126 Leather, J. W. Soil Gases. Mem. Dept. Agr. India, Chem. Series, 4: 

 85-132. 1915; see also Bizzell, J. A., and Lyon, T. L. The effect of certain fac- 

 tors on the carbon dioxide content of soil air. Jour. Amer. Soc. Agron., 10: 

 97-112. 1918. 



127 Neller, J. R. The influence of growing plants upon oxidation processes in 

 the soil. Soil Sci., 13: 139-159. 1922. 



