CONDITIONS OF LIFE OF NITRIFYING ORGANISMS. 63 



to the nitrifiers and, if it -becomes too abundant, will stop nitrification 

 until, either by vaporization or by denitrification, or otherwise, 

 it is reduced to an amount not deleterious to nitrification, when 

 the process begins. This checking of the action of nitrifiers by 

 organic matter or ammonia, certainly occurs in laboratory solutions 

 and concentrated compost heaps, but it does not appear to be of 

 much significance in ordinary soil. Nitrification takes place more 

 vigorously in soil than in solutions, and proper testing has shown 

 that the organic matter in ordinary soil does not prevent a vigorous 

 nitrification. 



Moisture. The nitrifiers require a moderate amount of moisture. 

 Too dry a soil will not allow of their growth. But, on the other hand, 

 too much moisture is equally detrimental. It has just been stated 

 that they do not grow so readily in laboratory solutions as in soil. 

 It is also a fact that in very wet soil, "water-logged," nitrification 

 is greatly reduced or lacking. 



Reaction. The nitrifiers cannot develop in an acid medium 

 and are usually absent from acid soils. Soils may become acid 

 from various causes, one of the chief of which is the production of 

 certain organic acids (lactic, succinic, acetic, butyric, etc.) , from the 

 bacterial decomposition of carbohydrate material. Large amounts 

 of sugar might give rise to an acid condition, but the more common 

 cause is the decomposition of cellulose and woody substances. In 

 forest land the decay of leaves and branches, as well as other vege- 

 table structures containing cellulose material, usually fills the soil 

 with these acids and, as a result, nitrification has practically ceased 

 in forest soils. The same is true in some open pastures and other 

 soils where such decay is extensive. 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 nitrified. 

 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 



