78 DECOMPOSITION OF ORGANIC SUBSTANCES IN SOIL 



In the plant it forms absorption or chemical compounds with 

 cellulose. The young plant contains largely cellulose in its 

 structural tissues, but with the maturing of the plant, the cel- 

 lulose fibers absorb (or combine with) lignin, thus forming 

 the ligno-cellulose of the plant tissues. 



(5) Proteins, composed of various amino acids. These 

 are the most important nitrogen constituents of the plant. 



(6) Fats, oils, and waxes, which are esters of alcohols and 

 one or more higher fatty acids. 



(7) The ash or the mineral constituents of the plant. 



A typical series of analyses of various plant products is given in 

 Table 17. 



TABLE 17 



Proximate Composition of Natural Organic Matter (from 

 Pringsheim) 



The remaining substances not given in the above table include 

 the water-soluble constituents (sugars, organic acids), various 

 hemicelluloses, in addition to the pentosans, the pigments, and 

 other substances present in smaller fractions. 



Not only do various plants vary in their composition, but the 

 composition of the same plant varies at different stages of growth, 

 as seen in Table 18. 



Factors Affecting Decomposition. — The various chemical 

 complexes are decomposed in the soil with different degrees of 

 rapidity and frequently by different organisms. Since different 

 plants vary so greatly in the relative abundance of the various 

 constituents, the speeds of their changes and the types of the resi- 

 dues are very dissimilar even under the same environmental con- 



