130 SOILS: PROPERTIES AND MANAGEMENT 



The character of the secondary products probably ex- 

 hibits a greater variation than does that of the original 

 plant tissue. In the process of decay these products 

 become black or brown in color, and are usually designated 

 as humous materials in the soil. Organic matter, then, 

 covers all the material of organic origin in the soil, and 

 may refer not only to the original plant tissue, but also 

 to that which has lost its identity in the secondary prod- 

 ucts. Humus refers specifically to the primary and the 

 secondary products of decay, and may be simple or com- 

 plex, according to conditions. 



As the process of decay goes on, certain end products 

 result. These are partially solid and partially gaseous. 

 Carbon dioxide is a universal product of bacterial activity 

 of all kinds, as is also water. Besides these, urea, am- 

 monia, nitrites, and nitrates may result from nitrogenous 

 decay. The three general classes of organic matter found 

 in soil may be illustrated by the following diagram : — 



PLANT TISSUE I HUMUS [END PRODUCTS 



UNDECOMPOSED MATTER | SECONDARY AND INTERMEDIATE | SIMPLE MATERIAL 



Fig. 21. — Diagram illustrating the three general classes of organic 

 matter found in soils. 



It is therefore possible to have present, besides the 

 original organic constituents which are mostly of plant 

 origin, not only their primary and secondary degradation 

 products, but also compounds either torn down or built 

 up from these. An attempt to enumerate even the 

 original compounds in the plant tissue, or even the simple 

 end products of complete decay, would result in a long 

 list of materials representing almost every known class 

 of organic compound. Such a procedure is possible, 

 but is unnecessary as the important ones have already 



