Like soil clays, colloidal particles of the organic fraction are negatively charged and attract positively charged ions to their surface. On a basis of unit weight, the retentive capacity of organic matter is greater than that of the most reactive clays, so the importance of this material in plant nutrition is far beyond its weight percentage.

Cation retention occurs in at least two ways. One is by the formation of salts of the carboxylic acids, which constitute a part of the organic fraction. Another is by forming molecular ring structures, in which the nutrient cation becomes part of a ring to form a metal-organic complex.

Nutrient elements, such as iron, which form highly insoluble inorganic compounds under conditions frequently encountered in soils, may not be present in soil solution in sufficient concentration to permit the best plant growth. The formation of iron-organic complexes, however, makes the iron more available. It has been known for many years that so-called iron-humate is a good source of iron for plants growing in solution culture. More recently, synthetic metal-organic complexes, having a chelate structure, have been used in remedying deficiencies of certain micronutrients, particularly iron. The greater solubility of these synthetic compounds accounts in part for their greater effectiveness as suppliers of certain elements.

THE FORMATION of metal-organic complexes sometimes stabilizes a soil nutrient that otherwise might not be retained in sufficient quantities for good plant growth. Adequate amounts of available boron usually exist in soils with high organic matter content an indication that organic matter can protect the boron against leaching without making it unavailable.

A characteristic result of microbial attack on plant residues is the formation of acidic groupings, usually carboxyl groups. These carboxylic acids ionize and influence the hydrogen-ion concentration, or degree of acidity of the soil solution. They can also form salts and complexes and so affect the equilibrium of bases in soil solution.

The principal effect of organic matter on soil reaction, or pH, is not in making a soil acid (although many organic soils are highly acid), but in increasing the buffer capacity of the soil. Stated another way: The presence of organic matter tends to lower the tendency of the soil pH to change when acid or alkaline materials are added. Soils high in organic matter require more lime to bring them to neutrality than soils low in lime if other factors are comparable. On the other hand, organic matter lessens the effect of acid-producing fertilizers.

Soil organic matter indirectly affects the availability of certain nutrient elements through its influence on physical and chemical properties of the soil environment.

The reducing conditions that accompany waterlogging are due partly to depletion of dissolved oxygen by the micro-organisms that decompose organic matter.

Reducing conditions often occur in the absence of excess water when oxygen in the soil atmosphere is depleted more rapidly than it can be replenished through diffusion of air through the soil pores. This rate of oxygen depletion depends on the level of microbial activity, which, in turn, is governed by the kind and amount of organic matter used for food by the soil population.

Vigorous microbial activity may therefore result in an increase of substances in the reduced state. For example, some of the iron in the soil will be reduced to the ferrous, or divalent, state, which has a much higher solubility than the trivalent, or ferric, form in which most of this element normally occurs in well-aerated soils. Availability of iron therefore is increased by vigorous microbial activity accompanying the breakdown of the organic matter.

A similar situation exists with regard to manganese, which can be reduced from tetravalent manganese found in soils as manganese dioxide to the more available divalent form through microbial activity.

THE BEST KNOWN functions of organic matter are those relating to physical properties.

An adequate level of organic matter improves soils that are too heavy and those that are too light. A heavy clay soil may be hard to work and slow to absorb water and have a tendency to puddle, so that rainfall or irrigation water has a tendency to run off rather than soak in. Such a soil is also likely to be poorly aerated.

Organic matter makes heavy soils more friable easier to work and promotes a crumbly structure. It stabilizes the soil crumbs so that they are held together under the slaking action of water. As a result, the soil absorbs water more rapidly. Runoff and erosion decline. A more porous structure permits better aeration in the root zone. Less crusting permits better emergence of seedlings.

On the other hand, the faults of light, sandy soils are that they will not hold much water penetration is too rapid. Aeration is good, but there is likely to be a deficiency of nutrient elements. Organic matter improves the water-holding ability of such soils and enhances the supply of nutrient elements. These elements are less susceptible to leaching when organic matter is present, because of its ability to adsorb them. Other nutrients are slowly made available as needed during decomposition of organic matter.

These desirable physical properties are not entirely dependent on a high organic matter level. As we have seen, soil organic matter is in equilibrium with its environment.

The level that can be maintained economically in a soil depends on its texture, the way it is managed, and climate.

LONG BEFORE the modern soil scientist appeared on the scene, farmers were well aware of the importance of soil organic matter. Perhaps they also realized that the manure they turned under did not last very long in the soil. For although it is true that there are few soil properties that are not affected directly or indirectly by organic matter, it is also true that much of its value results from its breakdown. The key to management of this valuable soil constituent is in control of the organic matter process so that the desired byproducts are obtained.