Since organic matter makes up less than 5% of the volume of most soils, it is not conspicuous in wash tests, yet in comparison with the pure mineral components, it is far more important. Actually, no mixture of clay, sand, and silt can be called a true soil unless it contains some organic matter.
In soils, organic matter has two important roles. The first of these is mechanical - as an absorbent and adsorbent agent to soak up solutions of plant nutrients such as nitrogen, phosphorus, and potash. It also serves as a reservoir for moisture, freeing the passageways between soil grains so air can enter freely.
By separating soil particles, organic matter helps form the desirable "crumb" structure that makes soil friable and easy to work. It also has an electrical attraction for the microscopically fine particles of clay, attracting them like flies to honey. This effect helps loosen heavy clay loam so it is more workable and able to contain the air vital to plant growth.
The second role of organic matter is to provide a home for the billions of microorganisms that exist in any true soil. Pick up a handful of loam from a garden and you will hold in your hand more living organisms than there are human beings on earth. Until the gardener understands that soil is alive, he cannot appreciate why he must add organic matter, why these tiny bits of matter are vital to proper feeding of plants, and why without them, the addition of fertilizer to soils would be all but wasted.
The most important function of these microorganisms is to attack the remains of plants such as roots and buried trash, as well as added organic substances, and break them down into simpler forms that living plants can use for food. Plants are no more capable of dining off the dead residues of last year's crop than they are of eating a hamburger. Without teeth or a digestive system similar to that of animals, they can only absorb nutrients in the form of a soup - a solution of nutrients that have been predigested for them by bacteria.
This process of predigestion may take place in stages, with one species of soil organism doing the preliminary breaking down of plant protein, for example, into amino acids, with a third reducing them to ammonia. Since only acid soil plants are able to use ammonia directly, this must be attacked by a fourth group which produces nitrate nitrogen, a compound most plants can absorb directly through their roots.
There is another way in which bacteria and other soil organisms enter into the nutritional aspect of growing plants. Most gardeners feel that when they add fertilizers to soil, these materials directly enter plant roots and are consumed completely. This might be true if root hairs filled practically every minute chink in the soil, but such is not the case. The roots of many plants are in contact with a small fraction of 1% of the volume of soil through which they penetrate. If there is any free water between the grains of soil, fertilizers dissolved in this water are pulled downward by the force of gravity and are lost in drainage channels.
Bacteria, fungi, and other soil organisms, however, are plants that need the same food elements as do higher plants with green leaves. Instead of allowing excess fertilizer to escape, they absorb it and use it for their own growth. Since these soil organisms do not live long - at the most a few days - when they die the food they absorbed is released to feed plants that have already used up the fertilizer the gardener applied days before.
A gardener - faced with the problem of soil unsuited to growing most plants - has two alternatives. Both will call for the use of organic matter. The first solution is the more simple - adapt the plants to the soil by choosing those that can survive under existing conditions. For example, he may want to garden at a summer home where sandy soil makes certain plants impossible to grow. Yet there are hundreds of species that thrive near salt water, both in tropical and temperate climates. Away from the spray there are shrubs, trees, and vines that can grow in sandy soil. His one problem will be the constant addition of organic matter which disappears rapidly in sandy soils. Even these tolerant species of plants must somehow be provided with the elements it provides.
A second alternative is to adapt the soil to the plants that he really wants to grow; in short - produce topsoil. A great deal of nonsense is written about the aeons needed to build an inch or two of black loam. True, in nature this is a slow process in which decaying leaves and plant wastes must work downward, with a slow accumulation of microorganisms, all modified by weather and other phenomena.
When, however, a gardener must work over a relatively small area, he can modify a heavy clay loam into an acceptable garden soil in a single year. Again, the solution is the incorporation of that magic ingredient of all good soils - organic matter. What can he use?
Provided it contains some fibrous material that has relative permanence in contact with soil, any form of organic matter will produce results. A substance such as dried blood, valuable as it is as the best organic fertilizer available, is worthless as a producer of the type of fibrous residue needed to build soil. Some substance such as lignin or cellulose is necessary to provide the long lasting, porous remains that soil needs for aeration.
When the material applied is already partially decayed, then not only is time saved, but the chance of plant injury is reduced. When fresh plant wastes and green manures are turned under in the soil or added to the compost pile, there is a period when active fermentation takes place, releasing more carbon dioxide than roots can use if growing at the time, or than bacteria can tolerate in the compost pile. This is an important reason for composting, of which more later.
Even when it has gone through a preliminary decomposition, an application of organic matter can be improved by fertilizer additions in twice the amount recommended for the crops to be grown. Bacteria, working on plant wastes in soil, demand so much nitrogen, for example, that they often cause plants to turn yellow because of the lack of this vital growth element. Fertilizing prevents this.
A compost pile is little more than a huge culture of microorganisms. They begin the process by which organic wastes are converted to a long lasting form that will improve soil for periods as long as half a century. Sine these organisms are plants, they require the same foods as plants with green leaves. In addition, they must have an outside source of starches and sugars for energy, since they cannot make their own. Instead, they extract these from plant wastes.