SOIL ORGANISMS 399 



possible to discuss in detail the transformations caused by 

 individual species of organisms or even by groups of related 

 species. Prom the standpoint of soil fertility such an at- 

 tempt is unnecessary, as a practical understanding of the 

 changes through which a given soil constituent passes as 

 it is prepared for plant nutrition, is much more important 

 than the possession of specific knowledge regarding the organ- 

 isms concerned. As a consequence it has become customary 

 to discuss the biological transformations of the more impor- 

 tant soil constituents, including as much regarding the speci- 

 fic organisms and groups of organisms involved as is con- 

 sistent with a clear fertility viewpoint.^ Four cycles are gen- 

 erally recognized, as follows: (1) the carbon cycle, (2) the 

 sulfur cycle, (3) the mineral cycle, and (4) the nitrogen 

 cycle. 



219. The carbon cycle. — Since all organic compounds 

 carry carbon, nitrogenous as well non-nitrogenous materials 

 are involved in the carbon cycle. Nevertheless attention will 

 be directed for the time being only toward the carbon and the 

 changes that it undergoes from the time it enters the soil 

 until it is removed either by aeration, leaching, or by plant 

 absorption. 



Most of the carbon compounds enter the soil as plant tissue, 

 although animal remains contribute appreciable amounts. 

 These carbonaceous materials are immediately attacked in the 

 soil by a host of different organisms capable of producing 

 fermentation. While such bacteria as Bacillus sultilis, Ba- 

 cillus mycoides, and the like have a great deal to do with the 

 decay processes, they are by no means the only agents. Most 

 of the microscopic fungi, as well as the larger fungi and algae, 



^ There are two general ways of studying the soil flora. A classification 

 of the organisms may be attempted. This requires the isolation and 

 study of individuals and has so far met with but little success. The 

 second approach is a biochemical one, in which the transformations oc- 

 curring in the soil are studied first, the specific organisms involved being 

 a secondary consideration. The determination of the capacity of the 

 soil to produce ammonia is an example of this method of study. 



