Denitrification. 191 



nt. If the wet weather continues for any length of time, 

 the crops, especially those that need large amounts of ni- 

 trates and can not use nitrogen in the form of ammonia at 

 all, begin to suffer from nitrogen hunger which is shown 

 by the yellow color of the leaves in place of the dark green 

 of the well nourished plant. As soon as the water leaches 

 from the soil and air is drawn into the same, the nitrify- 

 ing bacteria change the nitrites and ammonia back to 

 nitrates. In the lower parts of a field the conditions 

 favorable for denitrification may continue so long that 

 the crop is permanently injured. This property of re- 

 ducing nitrates is a very common one among the soil bac- 

 teria. 



In the process just described the nitrogen is not per- 

 manently lost. A more important process is one in 

 which, by the action of bacteria, the nitrogen of nitrates 

 is set free and passes into the air. The bacteria able to 

 do this are always present in the soil. They demand for 

 their action the absence of oxygen, and an abundance of 

 organic matter, as well as the presence of nitrates. They 

 are of small significance in the soil for the conditions 

 necessary for their action are not likely to obtain, /^They 

 are especially abundant in fresh manure, but much less 

 so in rotted manure. If a large amount of msanure is 

 added to the soil together with sodium nitrate, as may be 

 done in gardening or in green house work, the nitrate 

 may be destroyed by the action of this class of bacteria. 



It will be seen that the conditions favoring dentrifica- 

 tion are the opposite of those favorable to nitrification. 

 The farmer by frequent cultivation, by drainage, and by 

 maintaining the soil in good tilth, favors the beneficial 

 process and retards the harmful one. It is fortunate 

 that those conditions that are most favorable for the for- 



