In a book of this nature it has been deemed advisable not to enter into minute description of all the species of bacteria mentioned. Some of the chief are described more or less fully. We cannot, however, do more than name several of the chief organisms concerned in reducing and breaking down compounds. As we shall find in the bacteria of nitrification, so also here, the _entire_ process is rarely, if ever, performed by one species. There is indeed a remarkable division of labour, not only between decomposition bacteria and denitrification bacteria, but between different species of the same group. _Bacillus fluorescens non-liquefaciens_, _Mycoderma ureæ_, and some of the staphylococci break down nitrates (denitrification), and also decompose other compound bodies. Amongst the group of putrefactive bacteria found in soil may be named _B. coli_, _B. mycoides_, _B. mesentericus_, _B. liquidus_, _B. prodigiosus_, _B. ramosus_, _B. vermicularis_, _B. liquefaciens_, and many members in the great family of _Proteus_. Some perform their function in soil, others in water, and others, again, in dead animal bodies. Dr. Buchanan Young, to whose researches in soil we have referred, has pointed out that in the upper reaches of burial soil, where these bacteria are most largely present, there is as a result no excess of organic carbon and nitrogen. Even in the lower layers of such soil it is rapidly broken down.
It will be observed, from a glance at the table, that the chief results of decomposition and denitrification are as follows: free nitrogen, carbonic acid, gas and water, ammonia bodies, and sometimes nitrites. The nitrogen passes into the atmosphere, and is "lost"; the carbonic acid and water return to nature and are at once used by vegetation. The ammonia and nitrites await further changes. These further changes become necessary on account of the fact, already discussed, that plants require their nitrogen to be in the form of nitrates in order to use it. Nitrates obviously contain a considerable amount of oxygen, but ammonia contains no oxygen, and nitrites very much less than nitrates. Hence a process of oxidation is required to change the ammonia into nitrites and the nitrites into nitrates.