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 
