104 NATURE AND PROPERTIES OF SOILS 



mineral constituents is by no means an unimportant factor 

 in accounting for the complexity of the decaying organic mat- 

 ter. 



As the processes of fermentation and putrefaction go on 

 the complex intermediate compounds are gradually broken 

 down and certain simple products result. Such materials may 

 result from a progressive simplification of the partially de- 



H1GHER 

 PLANTS 



NUTRIENTS 



PLANT 

 TISSUE 



LOST FROM 

 THE 

 SOIL 



Fig. 21. — Diagram showing the transformations through which the con- 

 stituents of the plant tissue pass from the time the organic matter 

 enters the soil until it is in a condition to be used by succeeding 

 crops. The cycle is very largely biological. 



cayed matter or may be by-products or split-off compounds 

 from the more complex reactions. These simple materials are 

 partially solid and partially gaseous. Carbon dioxide is a 

 universal product of bacterial activity of all kinds and is 

 constantly being evolved. Other simple constituents arising 

 from organic decay are water, ammonia, nitrites, nitrates, free 

 nitrogen, and sulfur dioxide. Some of these are lost from 

 the soil, some lose their identity by reacting with the soil con- 

 stituents, while others may function as plant nutrients. When 



