188 Agricultural Bacteriology. 



trous acid, the other the nitrous acid to nitric acid which 

 when combined with lime forms calcium nitrate. If the 

 second class of bacteria is not present in the soil, it can 

 not be fertile, since plants can not make use of nitrogen 

 in the form of nitrites. 



The absence or limited amount in the soil of any of the 

 elements necessary for plant growth may be the cause 

 of the low fertility of a soil. The condition most likely 

 to be associated with a reduced yield is lack of nitrogen. 

 The nitrates are very soluble in water and are easily 

 leached from the soil. Those formed during one season 

 are removed during the winter and spring, and as there 

 is no accumulation of available nitrogen in the soil the 

 supply is rarely much in excess of the needs of the 

 plant. It is thus necessary that enough nitrogen be ren- 

 dered available during the growing season to supply the 

 needs of a luxuriant crop. If the process of nitrification 

 is retarded, the crop will be a meager one. It is of the 

 greatest importance that the farmer be acquainted with 

 the conditions that favor the process of nitrification in the 

 soil, for unless the crop of nitrifying bacteria in his soil 

 is large, the yield of corn, oats, etc., can not be a profit- 

 able one. 



Conditions for nitrification. The nitrifying bacteria 

 are aerobic. They grow best in an open textured and 

 well aerated soil. In one that is close and dense, the 

 process of nitrification is retarded; in a water logged soil 

 it does not go at all. By draining a field the water is 

 removed, air is drawn into the soil and nitrification goes 

 on rapidly. Cultivation also allows the air to penetrate 

 the soil more thoroughly and favors nitrification. It will 

 be noted that the largest yields are in the case of the so- 

 called cultivated crops rather than with the grasses and 



