BACTERIOLOGY OF SOIL 213 



The first products of the putrefaction of proteins by 

 Proteus organisms are further broken up by B. mycoides, 

 B. subtilis, B. mesentericus, B. putrificus, B. fluorescens 

 liquefaciens, B. tumescens, and other organisms, to produce 

 ammonia. (6) Nitrosation: The ammonium salts are 

 converted into nitrites by Nitrosomonas, a variety of 

 Pseudomonas (short, fat, motile organisms). These or- 

 ganisms differ from ordinary bacteria, but have definite 

 life-cycles, which, however, differ with the locality. They 

 appear to derive their supply of carbon from carbon 

 dioxide alone, (c) Nitratation: An apparently different 

 class of organism (Nitrobacter) from those concerned in 

 nitrosation converts the nitrites into nitrates. 



Denitrification. A few organisms reduce nitrates with 

 production of free nitrogen e.g., B. denitrificans and 

 Denitrobacterium thermophilum, the latter growing at 

 65 C. Others can only carry the action so far as the 

 formation of nitrites and ammonia e.g., B. butyricus, 

 B. mycoides, B. subtilis, and B. tumescens. 



Partial Sterilisation of Soil. By heating soil to 55 to 

 60 C., protozoa and nitrifying organisms are destroyed, 

 while denitrifying organisms and those that produce 

 ammonia remain alive, the result being enhanced ammonia 

 production. Effects vary very much, according to the 

 nature of the soil and the temperature employed. Speak- 

 ing generally, the tendency is for both organic and in- 

 organic matter to become more soluble. At high tem- 

 peratures, such as 250 C., calcium and magnesium become 

 less soluble. It appears that a toxin is produced which, 

 though inimical to plant growth at first, eventually 

 benefits it. Steam has proved most effective, and Russell 

 and Buddin consider that in vegetation experiments a 

 steamed soil should be included as a standard. 



Partial sterilisation may also be effected by the applica- 

 tion of antiseptics. Lime, formaldehyde, chloroform, 

 toluene, pyridine, calcium sulphide, carbon disulphide, 

 and cresol have been used for the purpose of destroying 

 protozoa and increasing ammonification. 



Examination of Soil. When the deeper layers are to 

 be examined, care must be taken to prevent contamination 

 with the other portions, particularly the upper layers. 

 Frfinkol devised an instrument for taking samples from 

 various depths. A borer contains at its lower end a small 



