318 SOIL PHYSICS AND MANAGEMENT 



potatoes and possibly others, may use more or less of it in the form 

 of ammonium compounds. By far the larger part is taken up by 

 plants as nitrates. The soil nitrogen must be changed to this form. 

 The process of nitrification is the changing of the nitrogen of soil 

 organic matter into nitrates, and is accomplished through the ac- 

 tion of certain classes of bacteria. The steps in the process are as 

 follows : 



(1) Ammonification, in which the organic matter is decom- 

 posed by bacteria and the nitrogen changed into ammonia or com- 

 pounds of ammonia. 



(2) Nitrification proper, which consists of the formation of 

 nitrous acid or nitrites from the ammonia or ammonium com- 

 pounds and the subsequent change to nitric acid or nitrates. This 

 is essentially oxidation. The nitrous and nitric acids unite with a 

 base of the soil. As calcium is one of the most common and readily 

 available bases, calcium nitrate is usually formed. 



DISTRIBUTION AND CONDITIONS 



1. Distribution. The bacteria concerned in nitrification are 

 very widely distributed in all kinds of soil, with the possible excep- 

 tion of swamp or long flooded soils. They are much more abundant 

 in soils containing limestone than in strongly acid ones. The sym- 

 biotic bacteria for legumes are found practically everywhere, but 

 not the specific forms for all legumes. The bacteria for alfalfa are 

 very widely distributed over western regions of the United States, 

 but in the eastern region inoculation, which is the process of sup- 

 plying the proper bacteria, is necessary. The same is true of many 

 other legumes. Wild legumes sometimes carry the same bacteria 

 as our cultivated ones. 



The number of bacteria changes with the type of soil. On the 

 same kind of soil the number of bacteria varies with the degree of 

 fertility, the tilth, and the rotation practiced. 



In vertical distribution the bacteria increase in number from 

 the surface downward for four to six inches and then decrease rap- 

 idly with depth and are found several feet below the surface only 

 as they are carried downward by percolating water. The zone of 

 greatest number of bacteria is from five to six inches beneath the 

 surface, but it will vary somewhat with the soil type, being a little 

 deeper in well-aerated soils. The optimum conditions of tempera- 

 ture, moisture and aeration are found at this depth. The following 



