ORGANISMS IN THE SOIL 441 



butyrates, and the like, the final product being carbon 

 dioxide and water. Thus, step by step, the non-nitroge- 

 nous matter incorporated with the soil is carried by 

 one and another form of organism from the most com- 

 plex to the simplest combinations. , 



The final product of the decomposition of carbonaceous 

 matter being carbon dioxide, there is a return to the air 

 of the compound from which the carbon of the decompos- 

 ing substance was originally derived. In the plant, un- 

 less it is saprophytic,, the carbon of the tissues comes 

 largely from the carbon dioxide of the air, from which 

 more complex carbon-bearing compounds are produced and 

 utilized in its functions or in its tissues. A portion of the 

 carbon is returned to the air by the plant in the form of 

 carbon dioxide ; the remainder is retained by the plant, 

 and may be returned by the process of decay or may be 

 consumed by an animal, and, as the result of its physio- 

 logical processes, either exhaled as carbon dioxide or 

 deposited in the tissues to be later decomposed and con- 

 verted into carbon dioxide. The soil is thus the scene of 

 at least a part of the varied transformations through 

 which carbon is continually passing as it is utilized by 

 higher plants, animals, bacteria, and fungi. 



The non-nitrogenous organic substances in their various 

 stages furnish food for a large number of bacteria, among 

 which are those concerned in the decomposition of mineral 

 matter and in the processes of nitrification and nitrogen 

 fixation. There are, therefore, two ways in which these 

 substances are of great importance in soil fertility : (1) as 

 a source of carbon dioxide and of organic acids ; (2) as 

 a food supply for useful soil bacteria. 



364. Decomposition of nitrogenous organic matter.— 

 The decomposition of nitrogenous organic matter is ac- 



