584 



PRINCIPLES OF SOIL MICROBIOLOGY 



to 15 per cent. 106 Soil with a higher content of organic matter will 

 have higher moisture optima. An excess of water may stop the action 

 of Azotobacter but may stimulate the action of the anaerobic bacteria. 

 Using a loam soil with a maximum moisture holding capacity of 27.4 

 per cent, Traaen 107 observed a fixation of 1.9 mgm. of nitrogen in 100 

 grams of soil with 5 to 10 per cent moisture, 13.2 mgm. with 17.5 per 

 cent moisture, 16.6 mgm. with 25 per cent moisture, and 15.5 mgm. with 

 30 per cent moisture. At a temperature of 13°C, the amounts of 

 nitrogen fixed were less, with a similar maximum. With the higher 

 moisture contents, the anaerobic organisms play an important part in 

 the fixation of nitrogen. Two maxima for nitrogen-fixation in relation 



5.8 5.8 6.0 6.2 6.4 6.6 6.8 



pH 



.0 7.2 7.t 



'.li 7.8 



Fig. 42. A correlation between soil reaction and the presence of Azotobacter: 

 positive test; abundant growth of Azotobacter (from Christensen). 



to the water content of the soil are frequently recorded, depending on 

 whether the conditions are favorable to the action of aerobic or anaerobic 

 bacteria. 108 



The optimum temperature for the activities of nitrogen-fixing bacteria 



106 Krainsky, 1908 (p. 566). 



107 Traaen, A. E. tlber den Einflusz der Feuchtigkeit auf die Stickstoffum- 

 setzungen im Erdboden. Centrbl. Bakt. II, 45: 119-135. 1916. 



108 Lipman, C. B., and Sharp, L. T. Effect of moisture content of a sandy 

 soil on its nitrogen-fixing powers. Bot. Gaz., 59: 402-406. 1915; Greaves, J. E., 

 and Carter, E. G. Influence of barnyard manure and water upon the bac- 

 terial activities of the soil. Jour. Agr. Res., 6: 889-926. 1916; also 9: 293-341. 

 1917. 



