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- 
