62 NITRIFICATION AND DENITRIFI CATION. 
need, but cannot grow in the presence of, organic matter. They do 
not, therefore, need any other living organisms to interpose between 
them and the mineral world, but may develop under conditions in 
which they are supplied with mineral substances alone. It is 
more surprising perhaps to find that they do not need light, but can 
utilize the mineral substances while growing in perfect darkness. 
This fact was at first conceived as quite contrary to our general 
ideas of the relation of life to physical energy. We have supposed 
that the only source of energy for living things is sunlight, and that 
this energy is stored up by green plants in the form of chemical 
compounds of high complexity. The animals and colorless plants 
use these stores as food, breaking them up and using the energy 
liberated for their own use. But here we have organisms which do 
not require organic material as a source of energy and are not able 
to utilize sunlight itself directly. Evidently they must obtain their 
energy from some other source than that which is commonly utilized 
by animals and plants. That they have a source of energy at com- 
mand is evident from the fact that they can assimilate CO 2 and 
build it into their own tissues, a process that requires energy. The 
present belief is that they obtain their energy from the oxidation 
of the ammonia compounds, a process that apparently can furnish 
them with all they need. - But whatever its source, these nitrifiers 
are able to live under conditions in which, other organisms cannot 
exist. 
Since the nitrifiers are injured by organic matter, it follows that 
nitrification cannot be expected in highly concentrated decompos- 
ing masses. Raw sewage contains so much high organic matter 
that nitrification does not take place in it, and if it is applied directly 
to the soil, in considerable quantity, it will effectually prevent the 
nitrification necessary to render the nitrogen available to plants. 
In the manure heap, too, nitrification cannot be expected so long as 
the quantity of organic matter is high. 
As the manure rots, however, the organic nitrogens are reduced, 
until finally nitrification can begin. We have seen that decomposi- 
tion gives rise to ammonia, usually combining with carbonic acid 
to form ammonium carbonate. This compound is also injurious 
