chapter iv 



rOle of microbes in the decomposition of 

 organic substances in the soil 



Principles of Decomposition of Organic jMatter by 

 Microbes. — Undoubtedly the most important function of micro- 

 organisms, from the point of view of man's economy, is their ability 

 to bring about the decomposition of organic plant and animal re- 

 mains not only in soils but in all natural environments. The sim- 

 plification of the chemical structure of the organic materials, or 

 their transformation from complex to simpler forms, leading finally 

 to the liberation of completely oxidized inorganic substances in 

 forms available to higher plants, is brought about by most of the 

 microorganisms in the soil. The end products of the complete 

 decomposition of non-nitrogenous materials are carbon dioxide, 

 water, and synthesized microbial cells. The nitrogenous sub- 

 stances give also, in addition to these, ammonia, and other simple 

 compounds of nitrogen, as well as compounds of sulfur and phos- 

 phorus. 



The attack of any complex compound by microorganisms sel- 

 dom results directly in complete decomposition even under aerobic 

 conditions; various intermediate products are usually formed. 

 The incompletely decomposed residues left from the action of cer- 

 tain organisms are further attacked by other members of the soil 

 population which have the ability to decompose them. Sooner 

 or later the initial material completely loses its identity and is 

 transformed principally to the completely oxidized end products, 

 and partly to a variety of synthesized substances in the form of 

 microbial cells. The various soil organisms differ from one an- 

 other greatly in their physiological characteristics; what may be a 

 readily available food for one species would not be available to 

 others. The great diversity of nutrition exhibited by the many 

 soil microbes makes possible the rapid digestion of whatever mate- 

 rial may be added or formed in the soil. 



75 



