536 



PRINCIPLES OF SOIL MICROBIOLOGY 



Toluol, in strength of 0.1 cc. per 100 grams of soil, and CS 2 , in strength 

 of less than 1 cc. per 100 grams of soil, do not exert any appreciable 

 effect upon nitrate formation. 71 Larger quantities (5 to 10 times) 

 exert a temporary retarding effect; as a matter of fact, when these 

 substances are used for partial sterilization of soil nitrate forming 

 bacteria are killed, and it takes a long time before the soil becomes 

 inoculated again. 



Nitrate formation in solution and in soil. Stevens and Withers 72 

 were the first to call attention to the fact that nitrate formation in 

 solution inoculated with a certain amount of soil is not the same as 



R, « 10 /* 13 St 26 to M 36 -S* 46 <* #» C8 



Cc. Water per 1 00am. Soil 



Fig. 36. Influence of moisture content upon nitrate production in the soil 

 (from Gainey). 



nitrate formation in the soil itself. Nitrates are formed in the soil 

 in the upper layers, 90 per cent of the process being carried out in the 

 upper 40 to 50 cm. 73 This is due to the need of oxygen for the activities 



71 Gainey, P. L. Effect of CS 2 and toluol upon nitrification. Centrbl. Bakt. 

 II, 39: 584-595. 1914. 



72 Stevens, F. L., and Withers, W. A. Studies in soil bacteriology. I. Nitri- 

 fication in soils and in solutions. Centrbl. Bakt. II, 23: 355-373. 1909; 34: 

 187-203. 1912. 



73 Koch, A. Versuche iiber die Salpeterbildung im Ackerboden. Jour. 

 Landw., 69: 293-315. 1911; MacBeth, I. G., and Smith, N. R. The influence of 

 irrigation and crop production on soil nitrification. Centrbl. Bakt. II, 40: 

 24-51. 1914. 



