76 DECOMPOSITION OF ORGANIC SUBSTANCES IN SOIL 



Scarcely any organic compounds are unlikely to be introduced 

 into the soil. The numerous substances synthesized by the great 

 variety of higher plants are continuously added in the form of plant 

 roots, stubble, leaves, twigs and branches. Immature plants are 

 also abundantly incorporated with the soil as green manures. 

 Plant residues are added to the soil in the form of stable manures. 

 Various waste products reach the soil subsequent to threshing, 

 milling, and utilization of various cereal crops, as well as cotton- 

 seed and linseed. Partly digested plant products, which have 

 been used in animal feeding, are contained in the manures. Various 

 animal products are also introduced into the soil, other than the 

 partially digested excreta. Those most commonly added are 

 by-products of industry, such as dried blood, meat scraps, bone 

 meal, hair, hoofs, and feathers. There are also large numbers of 

 small invertebrate and vertebrate animals which find in the soil 

 a temporary or permanent habitat. Finally there is a continuous 

 addition of considerable quantities of microbial cell substance to the 

 soil. Consequently, we may feel secure in assuming that there 

 may be added to the soil any compound contained in higher plants, 

 animals, or microorganisms, as well as any substance which might 

 be formed from these compounds as they are attacked by the 

 microorganisms in the soil. 



Composition of Plant and Animal Substances. — The plant 

 and animal residues comprise a large number of chemical com- 

 pounds which can be classified conveniently into the following 

 groups : 



I. Carbohydrates. 



1. Monosaccharides. 



a. Hexoses (CeHi-iOe), such as glucose, fructose, mannose. 



b. Pentoses (CaHioOj), such as arabinose and xylose. 



2. Disaccharides (Ci2H2:Oii), the most important of which are 

 sucrose and maltose. 



3. Trisaccharides (CisHsaOie), rafSnose. 



4. Polysaccharides. 



a. Starch, glycogen, inulin, and dextrins. 



b. Cellulose. 



c. Hemicelluloses and polyuronides. 



(1) Hexosans, as mannans and galactans. 



(2) Pentosans, of the structure (C6H804)j. 



(3) Pectins and other uronic acid compounds. 



II. Lignins. 



