140 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



tion, amount and kind of organic matter, temperature, moisture, 

 and concentration of inorganic substances. 



Influence of Ammonia upon Nitrate Formation. — Since 

 formation of nitrate is dependent upon the previous formation of 

 ammonia, all factors affecting the speed of liberation of ammonia 

 from organic materials will also exert pronounced effects upon 

 nitrate formation. As pointed out above, the addition of an 

 abundance of non-nitrogenous organic materials will temporarily 

 depress the formation of ammonia. The lower the nitrogen con- 

 tent of the organic matter added to the soil, the greater will be the 

 depression and the longer will be the period during which the 

 depression lasts. Nitrate formation follows the production of 

 ammonia, and takes place readily either when the organic matter 

 itself contains sufficient nitrogen (over 1.7 per cent) to permit 

 formation of ammonia or when inorganic nitrogenous compounds 

 are added. The lower the nitrogen content of the organic matter 

 the longer will be the period elapsing before its complete decom- 

 position; the greater the nitrogen content, the shorter will be the 

 period before nitrate is formed in abundance. 



In the presence of readily decomposable nitrogenous materials, 

 such as proteins or related compounds, ammonia accumulates 

 rapidly and nitrification follows at a slower rate. This indicates 

 that the process of nitrification in soils may be a slower process than 

 ammonia production, when readily decomposable nitrogenous 

 organic materials are present. However, the organic substances 

 generally occurring in soils in the humus complex are not rapidly 

 transformed to ammonia. The nitrifying organisms are able to 

 oxidize the ammonia more rapidly than it is produced from such 

 materials in most arable soils, and consequently nitrate and not 

 ammoniacal nitrogen appears in greater abundance. The speed 

 of the entire series of reactions is determined by the amount and 

 availability of the organic nitrogenous compounds which are 

 undergoing decomposition. The nitrification of the organic con- 

 stituents of stable manure illustrates the relationship. When 

 stable manure is added to soil, only a part of the nitrogen (about 

 one-haK) is readily converted to nitrate. About one-half of the 

 nitrogen of the manure is in the form of urea, which is rapidly 

 hydrolyzed to ammonia. This portion of the nitrogen undergoes 

 further rapid change to nitrate. The other nitrogen of the manure 

 occurs in compounds which are comparatively resistant to decom- 



