4 



Decomposition of Plant 



and Animal Residues 

 in Soils and in Composts 



For dust thou art, and unto dust shalt thou return. Genesis 111:19 



Nature of Plant and Animal Residues 



With the exception of autotrophic bacteria, the green or chloro- 

 phyll-bearing plants are the only living forms on this planet capable 

 of synthesizing organic matter out of inorganic elements and simple 

 compounds. These essential nutrients are obtained partly from the 

 atmosphere and partly from the soil. By utilizing the photosynthetic 

 energy of sunlight, plants are able to produce, from carbon dioxide 

 and water, sugar and stai'ch, which serve as the starting point for 

 the synthesis of numerous other carbohydrates, fats, proteins, and 

 various other compounds. The soluble forms of nitrogen and the 

 minerals required by the plant for synthetic purposes are obtained 

 from the soil; certain few plants, the legumes, are able, in association 

 with root-inhabiting bacteria, to obtain their nitrogen from the ele- 

 mentary form in the atmosphere. 



Plant materials are partly used for animal feeding and are partly 

 returned to the soil in the stubble and other plant residues. The 

 animals and their excretion products also find their way, sooner or 

 later, into the soil. These materials are subject to decomposition by 

 numerous groups of microorganisms and thereby contribute to the 

 soil organic matter. The various organic residues which undergo 

 decomposition in soils and in composts can be classified as follows: 



1. Plant and animal remains decomposing on the surface of the 

 soil; here belong the leaves, needles, branches, and twigs of all plant 

 life. 



95 



