90 DECOMPOSITION OF ORGANIC SUBSTANCES IN SOIL 



CH2-OOCCl7H33 



I 

 CHOOCC17H33 + 3H20= CHsOH-CHOH-CHoOH 



I Glycerol 



CH2 • OOC • C17H33 + 3C17H33COOH 



Triolein (a fat) Oleic acid 



The glycerol and fatty acids are further decomposed in the soil 

 by a number of various fungi and bacteria, giving lower acids, 

 carbon dioxide, and water. The decomposition of fats occurs 

 principally under aerobic conditions. 



Decomposition of Other Plant Constituents. — A detailed 

 discussion of the decomposition of proteins is given in the following 

 chapter, where other transformations of nitrogen are considered. 

 The subject is of great importance, since nitrogen forms one of 

 the most important elements in the protein molecule and since 

 nitrogen is apt to be the factor most limiting abundant crop 

 growth. The numerous organic substances present in plants in 

 quantities ranging from mere traces to one or more per cent, are 

 sooner or later decomposed by one or more groups of soil organisms, 

 and either destroyed completely or transformed into other organic 

 complexes. Even such substances as various aldehydes, vanillin, 

 toluene, phloroglucinol, phenol, cresol, naphthalene, and their 

 derivatives, are decomposed more or less rapidly in the soil by 

 specific or non-specific organisms. As a result, compounds which 

 might otherwise be toxic to plant growth become innocuous and 

 may even lead to the formation of substances beneficial to plant 

 growth. It is necessary, however, that the soil conditions, such 

 as proper aeration, reaction, and moisture, should be favorable to 

 the development of the particular organisms. 



Methane originates as an intermediate decomposition product 

 under anaerobic conditions, and may be attacked by a variety of 

 aerobic bacteria which oxidize it to carbon dioxide and water: 



CH4 + 2O2 = CO2 + 2H2O 



Some bacteria which are able to oxidize methane are also active in 

 decomposing benzene and paraffin. Numerous fungi and non- 

 spore-forming bacteria decompose paraffin, petroleum, vaseline, 

 benzene and kerosene under aerobic conditions. The disap- 

 pearance of these substances under anaerobic conditions is much 



