7o NITRIFICATION AND DENITRIFICATION 
and are usually absent from acid soils. Soils may become acid 
from various causes, one of the chief of which 1s the production of 
certain organic acids by bacterial activity in the soil. The value 
of liming such soils is evident. Lime neutralizes the acids and 
restores the alkaline condition necessary for the nitrifiers, so that 
they may resume the activity stopped by the acid. Too much 
lime, however, defeats its end by making the soil too alkaline. 
Humus.—That nitrification may take place, it is of course 
necessary that there be plenty of nitrogenous material to be nitri- 
fied. This must be in the form of an ammonium salt which, as we 
have seen, is the condition reached by the organic nitrogen at the 
end of 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 85°, becomes manifestly 
checked at rz0°, 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. 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. From this we 
learn the very practical lesson that cultivation of the soil stimu- 
lates nitrification. Experience and theory both tell that the 
loosening up of soil during,the growth of plants greatly stimulates 
plant growth, and one reason is evidently because this furnishes 
