800 PRINCIPLES OF SOIL MICROBIOLOGY 



oxidation was found to occur in a soil in which plants were growing 

 than in the corresponding uncropped soil kept under the same condi- 

 tions of moisture, aeration, temperature, etc. The growing roots were 

 found to exert a direct influence upon the decomposition of organic 

 matter in the soil. This will, of course, also bring about a greater 

 liberation of available plant nutrients and thus stimulate further plant 

 growth. A symbiotic relationship between the growing plant and the 

 oxidizing organisms in the soil was, therefore, suggested. Further 

 information on the influence of nature of crop upon the numbers of 

 bacteria and evolution of C0 2 from soil is given in tables 91 192 - 128 93 and 94. m 



It is of interest to note here that the harmful effect of grass upon 

 the growth of trees so commonly observed has been found 129 to be due 

 to the fact that the surface roots of the trees are deprived of combined 

 nitrogen; by producing a soil atmosphere rich in CO2, the grass 

 causes the surface roots to grow down and thus suffer from lack of 

 oxygen; there was no evidence of the formation of a toxin by the grass. 



Berthelot 130 recorded less nitrogen fixed by non-symbiotic organisms 

 in cropped than in uncropped soil. Heinze, 131 however, found that 

 fallowing increased the nitrogen-fixing capacity of the soil. Definite 

 information on this problem is difficult to obtain since our methods are 

 not sensitive enough to detect a minute increase in the total nitrogen 

 of the soil. In the case of leguminous plants, which offer a favorable 

 substrate for the growth of the different froms of B. radicicola, the 

 death of the plant and the decomposition of the roots lead to an increase 

 in the numbers of the nodule organisms in the soil. 



128 Stoklasa, J. Die modernen Ziele der biochemischen Forschung des Bodens. 

 Chemie d. Zelle u. Gewebe, 12: 22-44. 1924. 



129 Howard, A. The effect of grass on trees. Proc. Roy. Soc, 97B: 234-321. 

 1925. 



1 30 Berthelot, 1S88 (p. 104). 



131 Heinze, B. Bodenbakteriologische Untersuchungen. Landw. Jahrb. 39, 

 Erganzungbd., 3: 314-343. 1910. 



