132 



Humus: Nature and Foniiation 



In the decomposition of plant and animal residues by microorgan- 

 isms in soils and composts, the materials are not attacked as a whole. 

 Some of the organic constituents are decomposed very readily, others 

 less quickly, and still others are fairly resistant and tend to disappear 

 only very slowly. Some of the compounds are decomposed com- 

 pletely; others are transformed into various products which are more 

 resistant. The sugars and starches are rapidly destroyed, followed 

 by some of the hemicelluloses, the proteins, and the celluloses. The 



1 1 \ \ \ r 



A = Humus production 



Bi = Humus destruction with plenty of air 



62= Humus destruction under water 



50 



Humus accumulation 

 in aerated soil 



Temperature, °C 



Fig. 57. Humus accumulation and humus decomposition in tropical soils (from 



Mohr). 



lignins and some of their derivatives, certain proteins and hemicellu- 

 loses, as well as the waxes, tannins, and other materials, are more 

 resistant to decomposition and therefore gradually accumulate. The 

 processes of decomposition are accompanied by the synthesis of mi- 

 crobial cell substance comprising fungus mycelium, bacterial bodies, 

 worms, and insects. 



When plant and animal residues are added to the soil or placed 

 in composts, rapid decomposition sets in at first. This is followed by 

 consumption of oxygen, evolution of heat, liberation of considerable 

 carbon dioxide and ammonia if the material is rich in nitrogen, and 

 darkening of the residual material. As the decomposition progresses 

 and as the more readily decomposable constituents disappear, the 

 process becomes slower until a certain level is reached, when the 

 residual mass has become brown to black. 



This mass of slowly decomposing and decomposed material, to- 



