420 THE PRINCIPLES OF SOIL MANAGEMENT 



minimum in an arid region when irrigation is not prac- 

 ticed. Under such conditions, there is a return of ni- 

 trates to the upper soil, as capillary water moves 

 upward to replace evaporated water. In fact, wherever 

 evaporation takes place to any considerable extent, 

 there is some movement of this kind. The need for 

 catch crops to take up and preserve nitrogen is there- 

 fore greater in a humid region than in an arid or semi- 

 arid one. An arrangement of crops that allows the 

 land to stand idle for some time, or a crop that requires 

 intertillage, as does maize, fails to utilize all of the 

 nitrates produced, and promotes the loss of nitrogen 

 in drainage water. 



286. Denitrification. The nitrogen transforming 

 bacteria thus far studied have been those that cause 

 the oxidation of nitrogen as the result of their activi- 

 ties. We may now consider a number of forms of bac- 

 teria that accomplish a reverse action. The several 

 processes involved are commonly designated by the 

 term denitrification, and comprise the following: 



(1) Reduction of nitrates to nitrites and ammonia. 



(2) Reduction of nitrates to nitrites, and these to 

 elementary nitrogen. 



The number of organisms that possess the ability 

 to accomplish one or more of these processes is very 

 large, in fact greater than the number involved in 

 the oxidation processes, but, in spite of their numbers, 

 permanent loss of nitrogen in ordinary arable soils is 

 unimportant in amount, although in heaps of barnyard 

 manure it may be a very serious cause of loss. 



Some of the specific bacteria reported to bring about 



