400 THE PRINCIPLES OF SOIL MANAGEMENT 



for their growth. A notable decrease in the moisture 

 content of the soil may temporarily decrease the 

 number of bacteria by limiting their development to 

 the films of moisture surrounding the particles. With 

 a decrease in the moisture content of any soil, there 

 occurs an increase in the oxygen in the interstitial 

 spaces. Those bacteria thriving in the presence of 

 oxygen are thereby favored, and the character of the 

 bacterial flora is correspondingly changed. When the 

 soil remains saturated, or nearly so, for any consider- 

 able period, the anaerobic forms assert themselves, 

 and the usually beneficial activities of the aerobic 

 bacteria are temporarily suspended. The most favor- 

 able moisture conditions for the activity of the most 

 desirable bacteria is that found in a well-drained soil. 

 271. Temperature. Soil bacteria, like other plants, 

 continue life and growth under a considerable range 

 of temperature. Freezing, while rendering bacteria 

 dormant, does not kill them, and growth begins slightly 

 above that point. Warrington has. shown that nitri- 

 fication goes on at temperatures as low as 37 to 39 

 Fahr. It is not, however, until the temperature is 

 considerably higher that the functions of any of the 

 soil bacteria are pronounced. From 70 to 110 Fahr. 

 their activity is greatest, and it diminishes perceptibly 

 below or above those points. The thermal death points 

 of most forms of bacteria is found at some point 

 between 110 and 160 Fahr., but the spore forms 

 even resist boiling. Only in some desert soils does the 

 natural temperature reach a point sufficiently high 

 to actually destroy bacteria, and there only in the 



