NITRATE FORMATION 139 



forming organisms uninjured. This results in the accumulation 

 of ammonia in partially sterilized soils, since once the ammonia 

 is formed, it persists as such, because of the lack of organisms 

 capable of further transforming it to nitrate. 



In acid soils, such as raw-humus forest soils and certain acid 

 peats which have been drained and not limed liberally, the decom- 

 position of the nitrogenous organic substances is associated 

 with the formation and accumulation of nitrogen as ammonia, 

 without its being changed to nitrate. In the case of forest 

 soils this process is of greatest importance in modifying the very 

 nature of the forest vegetation. In the " raw-humus " types of 

 soil, only those trees develop which can utilize ammoniacal 

 nitrogen and are capable of forming mycorrhiza with special fungi, 

 which decompose the organic matter. However, in the case of 

 soils favorable for nitrate formation, the " mull " types are pro- 

 duced. These are considered richer soils; they bring about a more 

 rapid regeneration of the young forest, and the trees grow rapidly 

 without having to depend upon the formation of mycorrhiza. In 

 the case of the " raw-humus " soils, the organic matter is attacked 

 largely by fungi that allow an abundant accumulation of organic 

 matter which is characteristically brown; the nitrogen is liberated 

 as ammonia. In the case of the " mull " soils, decomposition is 

 more rapid and is brought about by a varied microbial population 

 including bacteria, actinomyces, and fungi, which give rise to a 

 black instead of a brown soil; nitrogen is liberated as nitrate. 



Undoubtedly some plants utihze nitrogen in forms other than 

 nitrate, but many plants use nitrate almost exclusively, and some 

 use other forms than nitrate principally during young stages of 

 development. The marked effect of the nitrification process on 

 plant growth is shown in Table 33. 



TABLE 33 



Influence of Nitrification on Growth of Barley (from Fred) 



Weight of 

 boil conditions , , 



plants, gm. 



Control — no nitrogen 1.5 



Nitrogen as (NH4)2S04 66.5 



Nitrogen as (NH4)2S04 with nitrifying bacteria 116.0 



Nitrate formation is controlled or affected by numerous factors, 

 chief among which are presence of ammonia, soil reaction, aera- 



