234 



Plant Physiology 



plants, they accomplish this in the absence of light (chemo- 

 synthetically). 



In cultivated soils they are most abundant below the 

 surface mulch, and down to the limits of frequent culture. 

 The following table shows the distribution of nitrifying 

 bacteria and nitrates at different depths in a Dakota 

 soil l : 



129. Conditions favoring nitrification. - The general 

 conditions favorable for nitrification in soils are good 

 aeration (oxygen and carbon dioxid), a medium water- 

 content, a soil temperature not to exceed 40 C., the 

 presence of a basic compound such as calcium carbonate, 

 and the absence of much soluble organic matter and free 

 ammonia. In general these conditions are those of good 

 sanitation and tilth, a judicious application of lime, and 

 such a rotation of crops as will produce and maintain the 

 best of soil conditions. The advantage of paying the 



1 Ladd, E. F., North Dakota Agl. Exp. Sta., Bui. 47: pp. 685-704. 



