Chapter 8 The Nature and Composition of Soil Organic Matter

There are wide variations in the characteristics of soil organic matter depending on the vegetation, climate, and biological conditions under which the material accumulates. This statement applies particularly to the total organic matter, which includes a large amount of plant and animal residues that have not yet been thoroughly humified; as decay proceeds, the humic substances formed tend to become more and more alike regardless of the nature of the original humus sources. Climatic conditions under which decay occurs are important in determining the nature of the final product. There are three types of reactions that yield humus or humus-like products. First, humus-like substances may be formed by the action of mineral acids upon hexose sugars, as has long been known. Second, humus-like substances may also be formed from phenolic compounds and quinones by oxidation. A third type of reaction leading to humus-like substances involves the reaction of amino-acids and other amino compounds with carbohydrates or aldehydes. The nitrogen in the final products is tenaciously retained as a part of the complex.