Organic Matter

F. E. Broadbent.

Soil organic matter is a substance and a process. It is a mixture of materials plant and animal remains and products of decay processes that have been going on months or years.

Organic matter is produced in living organisms and is composed of a great many compounds of carbon. In soils it occurs in intimate admixture with inorganic soil constituents, which are derived from rocks and minerals.

Most of the benefits conferred on soils by organic matter are due to the never-ending decomposition of plant and animal residues, which are ultimately converted into simple inorganic compounds, such as carbon dioxide, water, and nitrate.

It is not very helpful to try to distinguish between relatively undecomposed residues on the one hand and material in a more advanced state of decay on the other, although the term "humus" often is applied to the latter.

Wherever plants grow, their roots, leaves, stems, and twigs are the raw material for the organic-matter process, and all the intermediate substances between them and the simple end products make up the organic fraction of soils. A small part of it is living the cells of such micro-organisms as bacteria, molds, and actinomycetes.

The changes the chemical activities of these living cells bring about are far more important than their proportion by weight might lead us to believe.

Their most important basic function is to destroy complex organic materials: If organic matter were an end product rather than an intermediate product in the biochemical factory of the soil, all the carbon dioxide in the atmosphere would be tied up sooner or later in the organic matter in the soil.

The organic fraction is not a mixture of inert substances accumulating by reason of their immunity to microbial attack. Whenever fresh residues are applied, the decomposition of "humified" materials already present is speeded up, so that complete turnover of the organic matter eventually occurs.

THE CHEMICAL COMPOSITION of plant residues that provide the starting material of most soil organic matter is fairly well understood. Their major constituents are in three groups.

The polysaccharides are a large class of natural carbohydrates, whose molecules are derived from the condensation of several or many molecules of simple sugars (monosaccharides). The polysaccharides include cellulose, the so-called hemicelluloses, starch, and pectic substances.

Lignins are complex materials that occur in the woody tissues of plants. Lignins possess a high degree of resistance to attack by most chemicals and micro-organisms. When lignin molecules are broken into tiny fragments by drastic chemical procedures, characteristic compounds having in common the benzene ring with a three-carbon side chain can be identified. Lignin is probably formed from condensation of many aromatic nuclei to form a large complex molecule with certain side groups attached.

Proteins, the principal nitrogen-containing constituents, are formed by linkage of many simpler units called amino acids. Proteins have high molecular weights (up to 10 million) and compose much of all living matter.

Besides these three groups, a variety of other, less important, substances occurs in plant residues.

We get some general ideas about the chemistry of soil organic matter when we consider the changes that microorganisms in the soil cause in the principal plant constituents.

The polysaccharides, which make up the bulk of mature plant tissues, are easily decomposed by many species of soil microbes. They are converted chiefly to carbon dioxide and water. New cells of molds, actinomycetes, and bacteria also are formed. They contain polysaccharides, which differ somewhat from those found in higher plants. The result is that soil organic matter contains very little polysaccharide of plant origin, but it does contain some polysaccharide of microbial origin.

Lignins, being less susceptible to microbial attack than polysaccharides, tend to accumulate as the decomposition process goes on, but they do not remain unaltered. Some of the side chains are split off, and methoxyl groups are cleaved. Phenolic hydroxyls are left in their place, and the number of acidic groupings, such as carboxyl, is increased. Perhaps internal condensation of the molecule also occurs as some of the linkages holding the aromatic rings together are broken.

These changes modify the properties of lignin. They make it more reactive in some ways (such as in retention of nutrient elements) but less susceptible to biochemical oxidation. Thus, although soil organic matter contains a substantial fraction of lignin-derived material, its chemical properties differ a good deal from unchanged plant lignin. Some soil molds contain a substance that resembles lignin in several respects; it also may accumulate.

Plant proteins are excellent food for soil micro-organisms. They contain the nitrogen essential for building microbial cells. Micro-organisms generally contain a higher percentage of nitrogen than do higher plants. Some bacteria contain as much as 90 percent of protein. Nitrogen therefore accumulates in soil organic matter, not because plant protein is resistant to microbial attack but because nitrogen is an important constituent of microbial cells and is used over and over again as old cells die and new ones are formed.

The proteins in soil organic matter do not behave the same as plant proteins or microbial proteins out of the soil.

Soil scientists have wondered why organic nitrogen in soils is converted so slowly to soluble inorganic forms that can be used by plants. Because much of this nitrogen is present as proteins, which normally are readily broken down by micro-organisms, we have several theories to explain why rapid decomposition of protein does not seem to occur in soils.

Part of the explanation is that clay minerals exert a protective action on protein molecules, trapping them within the lattice of the clay crystal in a space too small for bacteria to enter. But many micro-organisms can produce extracellular enzymes, substances much smaller than the bacterial cells, which can function some distance away from the organisms that produced them. But enzymes are proteins, too, and they can also be absorbed by clays, so that their activities are diminished in a soil environment.

Other theories are based upon the formation of organic complexes resulting from reactions between proteins and other constituents of soil organic matter, such as lignin. Such complexes are assumed to be quite resistant to microbial attack, exerting a protective action on proteins by reason of chemical composition.

Not all the organic nitrogen in the soil is present as protein. Some occurs in the form of chitin, a polymer of amino-sugar units. Chitin is a constituent of many soil fungi, but perhaps most of it comes from the remains of insects. Still another form of soil nitrogen may be one that, unlike protein, resists microbial attack and is probably the result of interaction between ammonium ion and lignin derivatives, by means of which the nitrogen atom is placed in an aromatic ring. When more is known about the nitrogen bound in this manner, we may be able to explain some of the puzzling things about nitrogen availability in soils.

Organic matter is not the same in all soils.

The type of vegetation, the nature of the soil population, drainage, rainfall, temperature, and management affect the kind and amount of the organic matter: Soil organic matter is a product of its environment.

A soil developed under deciduous forest in a cool, humid climate obtains most of its organic matter from leaf fall on the surface, and the material is concentrated in the upper few inches of the soil. A grass-prairie soil receives residues from a large mass of fibrous roots and has a fairly uniform distribution of organic matter through a considerable depth. Little organic matter is found in arid soils where vegetation is sparse, because the raw materials are lacking, but when they are brought into agricultural production through irrigation the organic matter level often increases.

Hans Jenny, at the Missouri Agricultural Experiment Station, showed that there is an inverse relationship between mean annual temperature and the level of organic matter in regions of comparable rainfall. Higher temperatures seem to stimulate microbial decomposition more than they stimulate the production of plant tissue.

This relationship has great practical importance as a guide in establishing feasible limits at which levels of organic matter can be maintained. For example, a farmer in Louisiana could not economically maintain the organic matter in his soil at the same level as does the farmer in Minnesota. The decay processes that break down organic matter are more rapid in the warmer climate and go on for a longer period during the year.

MATURE PLANT RESIDUES that provide the raw material for the soil organic matter process usually are about 50 percent carbon and less than 1 percent nitrogen.

Carbon in the residues is converted to carbon dioxide by enzymatic processes. Some of the energy released is used in the cellular processes of soil microbes. Some of the carbon and nitrogen from the residues is built into the cell constituents of the micro-organisms.

All the nitrogen in the plant protein ordinarily is needed for building microbial protein, but only a small part of the plant carbon is needed for synthesis of new cells. In other words, mature plant residues constitute an unbalanced ration for the soil population of microorganisms, with carbon in excess a way of saying that the carbon-nitrogen ratio is wide.

As decomposition proceeds, much carbon is lost as carbon dioxide and very little is used for building microbial cells, whereas essentially all the nitrogen available is conserved by incorporation into new protein molecules. The result is a narrowing of the carbon-nitrogen ratio. If additional nitrogen is supplied when the carbon-nitrogen ratio is wide, it also will be assimilated by the soil population up to a point where the population is as large as the food supply permits it to be.

Because decomposition is accompanied by narrowing of the carbon-nitrogen ratio, it does not follow that artificially lowering the ratio by addition of fertilizer nitrogen will stop or greatly lower the amount of carbon lost.