406 THE PRINCIPLES OF SOIL MANAGEMENT 



carbon dioxid and water. Thus, step by step, the non- 

 nitrogenous matter incorporated in the soil is carried 

 by one and another form of organisms from the most 

 complex to the simplest combinations. 



The final product of the decomposition of carbon- 

 aceous matter being carbon dioxid, there is a return 

 to the air of the compound from which the carbon of 

 the decomposing substance was originally derived. 

 In the plant, unless it is saprophytic, the carbon of 

 the tissues comes directly from the carbon dioxid 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 dioxid, 

 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 physiological 

 processes, either exhaled as carbon dioxid or deposited 

 in the tissues to be later decomposed and converted 

 into carbon dioxid. 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, there- 

 fore, two ways in which these substances are of great 

 importance in soil fertility: (1) As a source of organic 

 acids. (2) As a food-supply for useful soil bacteria. 



