SOIL 615 



contain large numbers of microorganisms, and sand, which is notor- 

 iously infertile, contains relatively few. 



The normal bacterial flora of fertile soil consists essentially of at 

 least two distinct types of organisms; they may be classified accord- 

 ing to their chemical activity into those which effect a rapid deep- 

 seated decomposition of dead organic matter into simple combinations 

 of the elements which enter into its composition ammonia, carbon 

 dioxide, hydrogen sulphide, and so on and those which transform 

 these simple compounds, especially ammonium salts, into nitrites and 

 eventually into fully-oxidized (mineralized) nitrates. In the latter 



' FIG. 98. Bacillus subtilis showing spores. X 1000. 



form the nitrogen originally present in organic matter is available for 

 plant synthesis into protein through the action of sunlight upon the 

 chlorophyll of the vegetable kingdom, thus completing the cycle. 



The initial phase in the degradation of dead organic matter to 

 ammonium salts and simple compounds of the other elements which 

 comprise the protein molecule appears to be accomplished largely 

 through the activity of bacteria of the Subtilis-Mesentericus and Proteus 

 Groups. These organisms elaborate powerful active soluble proteo- 

 lytic enzymes which liquefy protein, and eventually the intracellular 

 digestion of the hydrolytic cleavage products of protein by these 

 microorganisms results in ammonia formation. 1 



The Proteus Group has been discussed elsewhere. 2 The cultural 

 characters of the Subtilis-Mesentericus Group are as follows: 



1 Kendall, Day and Walker, Jour. Am. Chem. Soc., 1913, xxxv, 1243; ibid., 1914, 

 xxxvi, 1966; Jour. Inf. Dis., November, 1915. 



2 Page 359. 



