2l8 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Nature of the Material. Some substances are more easily 

 attacked than others by the organisms whose final products are 

 nitrates. While the importance of this fact is chiefly to be con- 

 sidered in reference to organic nitrogenous fertilizers, yet it is 

 necessary to bear in mind that the organic nitrogenous compounds 

 of the soil may vary decidedly in the resistance which they offer to 

 the nitrifying organisms. Probably in any soil, the less resistant 

 compounds are oxidized first, and the remainder at a decreasing 

 rate from year to year, so that the effect is a continual diminish- 

 ing of the nitrates produced for the use of plants, unless measures 

 are taken to introduce fresh nitrogenous material susceptible to 

 nitrification. 



Denitrification. The term denitrification is applied to the 

 destruction of nitrates. If an extract of stable manure is added 

 to a solution of potassium nitrate, and kept at a favorable tem- 

 perature, the nitrates in time will disappear entirely. 



Under certain circumstances the nitrates in the soil are de- 

 oxidized with the production of organic bodies, nitrites, ammonia, 

 or even free nitrogen. In the latter case there is a loss of nitro- 

 gen from the soil. We have already seen that the bacteria which 

 change organic nitrogen into ammonia require oxygen, otherwise 

 they will take oxygen from nitrates. The conditions favorable 

 for denitrification are as follows : 



(1) Insufficient Oxygen. In water-clogged soils, or soils which 

 are so compact that air cannot penetrate them, denitrification will 

 take place. 



(2) Presence of an Excess of Vegetable Matter. Cases are 

 known in which a heavy application of manure destroyed the 

 nitrates in the soil, and produced a smaller crop than if no manure 

 had been used. Some believe that the denitrifying organisms 

 introduced with the manure are the cause of the denitrification, 

 but as has been pointed out by Waririgton and others, farm manure 

 introduces into the soil another factor of importance, namely, 

 a large increase in oxidisable organic matter, and this may favor 

 denitrification both by lessening the gaseous oxygen and by tend- 

 ing to rob the nitrates of their oxygen. Undoubtedly the 



