62 NITRIFICATION AND DENITRIFICATION. 



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 



