64 NITRIFICATION AND DENITRIFICATION. 



its decomposition. The ordinary humus will therefore furnish 

 plenty, but soil deficient in humus will show but little nitrification. 



Temperature. Nitrification occurs in the soil under a very 

 wide range of temperature. It goes on at temperatures fully as low 

 as 37 F.; it is most vigorous at about 99, becomes manifestly 

 checked at 110, and almost ceases at 122. From these facts it will 

 be seen that it may continue in the fall until the appearance of 

 frosts and, in many localities where the winter is not too cold, will go 

 on all winter long. For this reason a cultivation of soil in the fall 

 is undesirable, since cultivation, by mixing air with soil, hastens 

 nitrification, and during the winter or late fall there is no growing 

 crop to utilize the nitrates as they are formed. These, therefore, 

 drain away from the soil during the spring and winter, leaving it 

 poorer in the spring than if the cultivation had not taken place. 

 Nitrification is the most vigorous in the summer months, during 

 which season the growing crops are in best condition for absorbing it. 

 This is one of the reasons why a wheat crop is so exhausting to the 

 soil. It grows during the fall and spring, but the ground lies idle 

 in the summer and hence during the season of greatest formation 

 of nitrate, there is no crop growing to prevent the loss by drainage. 



Air. Nitrification is a process of oxidation and therefore re- 

 quires oxygen. The more thoroughly the air is mixed with the soil 

 the more vigorous will be the nitrification. This process, therefore, 

 is more pronounced in sandy loams or mixtures of clay and sand 

 than it is in heavy clay soils. In heavy soils, where the earth 

 particles are very fine, the soils are too poorly aerated to enable the 

 nitrifiers to get a sufficiency of oxygen. From this we learn the 

 very practical lesson that cultivation of the soil stimulates nitrifica- 

 tion. Experience and theory both tell that the loosening up of soil 

 during the growth of plants greatly stimulates plant growth, and the 

 primary reason is evidently because this furnishes the necessary 

 oxygen for a vigorous nitrification, thus furnishing the crops with a 

 larger supply of the easily assimilated nitrates. In this fact, too, we 

 find an explanation of the fact that only the upper layers of the soil 

 are fertile since nitrification will go on only in the layers where 

 oxygen readily penetrates. About 65 per cent, of the total nitrifica- 



