796 



PRINCIPLES OF SOIL MICROBIOLOGY 



later explained 116 by the fact that plant secretions and root residues 

 are sources of energy for the soil organisms. The nature of the crop 

 is of importance in this connection, due to the difference between the 

 absorptive power of the plant and the carbon-nitrogen ratio of the 



/T 



^ 



; t 



^"'■S'v , :l''...i" , '.^ i W 



r 



Fig. 76. Diagram representing a section through one of the units of the ap- 

 paratus used in determining the carbon dioxide evolved from the soil cultures 

 (from Neller). 



plant residues. Nitrate production was found 117 to be higher under a 

 cultivated crop, such as corn and potatoes, than under an uncultivated 



116 Lyon, Bizzell and Wilson, 1923 (p. 705). 



117 Jensen, C. A. Seasonal nitrification as influenced by crops and tillage. 

 U. S. Dept. Agr. Bur. PI. Ind. Bui. 173. 1910; Ladd, E. F. Humus and soil 

 nitrogen. N. D. Agr. Exp. Sta. Bui. 47, 685-721. 1901; Lyon, T. L., and Biz- 

 zell, J. A. The formation of nitrate in a soil following the growth of red clover 

 and timothy. Soil Sci., 9: 53-54. 1920. 



