124 BACTERIA IN THE SOIL 



aeration that the supply of atmospheric oxygen shall not be in excess 

 relatively to the supply of organic matter; (4) the usual essential 

 conditions of bacterial growth. " Of these," he says, " the supply of 

 organic matter is by far the most important in determining the 

 extent to which denitrification will take place." The necessarily 

 somewhat unstable condition facilitates its being split up by means 

 of bacteria. The bacteria in their turn are ready to seize upon any 

 products of animal life which will serve as their food. Thus, by 

 reducing complex bodies to simple ones, these denitrifying organ- 

 isms act as the necessary link to connect again the excretions of 

 the animal body, or after death the animal body itself, with the 

 soil. 



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, My co- 

 derma urece, 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 (p. 120), that the 

 chief results of decomposition and denitrification are as follow : free 

 nitrogen, carbonic acid gas, and water, ammonia bodies, and some- 

 times 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 



