CHAPTER XXV. 



The most important of the organisms found in the soil The nitrifying bac- 

 teriaThe bacillus of tetanus The bacillus of malignant oedema The bacil- 

 lus of symptomatic anthrax. 



BY the employment of bacteriological methods in the 

 study of the soil much light has been shed upon the cause 

 and nature of the interesting and momentous biological 

 phenomena that are there constantly in progress. Of 

 these, the one that is of the greatest importance com- 

 prises those changes that accompany the widespread 

 process of disintegration and decomposition, to which 

 reference has already been made (see Chap. I.). This 

 resolution of dead, complex, organic compounds into sim- 

 pler structures that are assimilable as food for growing 

 vegetation, is dependent upon the activities of bacteria 

 located in the superficial layers of the ground. It is not 

 throughout a simple process, brought about by a single, 

 specific species of bacteria, but represents a series of 

 metabolic alterations, each definite step of which is most 

 probably the result of the activities of different species 

 or groups of species, acting singly or together (symbioti- 

 cally). Our knowledge upon the subject is not suffi- 

 cient to permit of our following in detail the manifold 

 alterations undergone by dead organic material in the 

 process of decomposition that results in its conversion 

 into inorganic compounds, with the formation of car- 

 bonic acid, ammonia, and water as conspicuous end-pro- 

 ducts. It suffices to say that, wherever dead organic 



