The Chemistry of Soil Organic Matter

The progress made in the elucidation of the chemical nature of the organic matter of the soil is distressingly slow.

It is now generally accepted that the organic matter of the soil has its origin in the vegetation on the land. It consists only partly of residues of plant constituents that are less available or are modified by the soil microflora. A part originates in cell-substance synthesized by the micro-organisms, or residues therefrom. Indeed, this microbially derived fraction forms a substantial portion of the soil organic matter. Attempts have been made to distinguish between decomposing plant residues, the chemistry of which can easily be related to that of the parent plant material, and fully decomposed residues, or humus, the chemistry of which is obscure.

Studies of the changes undergone by plant materials in decomposition have indicated that lignin, a constituent of the mature cell walls of plants, is relatively resistant to attack, and tends to accumulate, although somewhat modified by loss of methoxyl groups. There is, therefore, a lignin-derived fraction of soil organic matter. Certain of the reactions of soil organic matter, such as its behavior with chlorine, are in accordance with this view.

The changes that the lignin undergoes in decomposition are such that some of the characteristic chemical reactions that normal lignin undergoes do not take place. S. Gottlieb and S. B. Hendricks at Beltsville have shown that these changes must be substantial. They subjected a muck soil and soil extracts that presumably contained a lignin-derived fraction to certain rather vigorous chemical treatments that when applied to native plant lignin gave easily recognizable products. It is a well-established procedure in organic chemistry to attempt to break up an unknown compound into smaller pieces that are simpler and can be identified. They did not obtain from the muck soil or the extracts the products that they expected to find had lignin been present, and they were forced to conclude therefore that "the material derived from plant lignin in the soil is drastically altered in the kind and position of the peripheral groupings on the aromatic rings."

There may be interactions between the inorganic and organic components of soil that result in modification of the properties of both. Various European and Russian soil scientists believe that extremely stable inorganic-organic complexes are formed, particularly in chernozem soils, and that it is combination with the inorganic colloid that confers on the organic fraction its peculiar stability. The extreme in this viewpoint is probably represented by F. Yu Gel'tser, who defines as humus only that fraction of the organic colloid that is capable of forming a stable complex with inorganic colloids. The stable organic-inorganic complexes are said to play an important role in determining soil structure. Furthermore, the organic fraction concerned is not believed to be of plant origin, but to be formed by and from the micro-organisms accomplishing decomposition of the plant residues.

German workers have attempted to distinguish both chemically and functionally between "nutrient humus" (Nahrhumus) and "stable humus" (Dauerhumus). Although the former is held to be the substrate upon which the soil microflora develops, and from which some plant nutrients are freed, the latter is regarded as being agriculturally the most vital fraction of the soil organic matter because of its influence on the physiochemical properties of the soil. Paradoxically enough, accumulation of "stable humus" is held to occur primarily under conditions of high biological activity and in the presence of calcium and montmorillonitic clays. Chemically the distinction is based on resistance to treatment with acetyl bromide or a mixture of acetic anhydride, acetic acid, and sulphuric acid.

These various theories indicate that there is much yet to be done to clarify the problems of the nature of soil organic matter. Their solution should permit the scientific management of soil organic matter to achieve the maintenance of fertility and conservation of soil.