Living Organisms in the Soil

Francis E, Clark.

A teaspoonful of soil may contain billions of living organisms. On them crop growth, soil fertility, and even soil development depend in many ways.

Among the soil's inhabitants are specialists that rot organic matter, transform nitrogen, build soil filth, produce antibiotics, and otherwise affect plant welfare.

Bacteria are the smallest and the most numerous of the free-living organisms in the soil. About 25 thousand of them measure an inch. Despite their minute size, their total weight in the top foot of an acre of fertile soil may be as much as a thousand pounds, or 0.03 percent of the weight of the soil. Poor soils and some sandy soils may harbor few bacteria.

The bacteria are little more than tiny blobs of jellylike protoplasm enclosed in a cell membrane. Most of them subsist on waste organic materials.

Those that derive both their cell carbon and their energy from organic substances are called heterotrophic bacteria. They use energy previously stored by other microbes or by higher plants. Their metabolism or ability to carry on such life processes as oxidizing sugar to carbon dioxide and water is like that of the higher animals, all of which depend on the energy stored in carbohydrates, fats, and proteins.

A few bacteria possess pigments that enable them to trap the energy in light. They obtain their cell carbon directly from the carbon dioxide of the atmosphere. Green plants similarly possess this photosynthetic ability.

Still other bacteria, called autotrophic or chemosynthetic, draw upon the atmosphere for their carbon supply and obtain their energy by oxidizing relatively simple chemical materials. In this group are bacteria that oxidize carbon monoxide to carbon dioxide, sulfur to sulfates, hydrogen to water, ammonia to nitrous acid, and nitrous acid to nitric acid.

Most soil bacteria require nitrogen that previously has been combined either into mineral forms (such as ammonium and nitrate) or into organic nitrogen compounds (such as plant proteins and animal proteins).

Only a limited number of micro-organisms are able to make use of nitrogen gas as it commonly occurs in the air. Among the soil bacteria that can do so are the legume-nodule bacteria, or rhizobia, which use nitrogen from the air in partnership with leguminous host plants. The nitrogen taken from the atmosphere is available to both partners. Consequently legumes can be grown on soil that is poor in nitrogen but otherwise is favorable.

The amount of nitrogen fixed by nodulated legumes varies greatly, but the average is estimated to be 50 to 150 pounds of nitrogen an acre each year.

The right kind of legume bacteria must be present for each legume plant. If the legume is native to an area or has been grown for several years on the soil, the correct bacteria usually have become established. But for newly introduced legumes, for soils in which the correct rhizobia do not survive during the intervals between crops, and for soils in which there are only weak or parasitic strains of nodule bacteria, inoculation of the legume seed at planting time with nitrogen-fixing bacteria is desirable or necessary.

Packaged inoculants containing nitrogen-fixing bacteria are available at many seed stores. Because the inoculants are prepared for specific legumes or groups of legumes, the names of which are printed on the package, the purchaser needs only to specify to the dealer the legume seed he wishes to treat. Inoculant labeled as effective on one legume, such as soybeans, is not at all suitable for other legumes, such as clovers or garden peas.

Legume inoculant sufficient to treat a bushel of seed costs 15 to 55 cents, depending on the type of seed and the amount of inoculant purchased.

Directions on the package should be followed as closely as possible. The user should plant the inoculated seed within a very few hours after he treats them. Meanwhile the seeds should not be exposed to heat, drying, or sunlight. He should not purchase or use inoculant that is older than the expiration date stamped on the label.

Gardeners or farmers who have highly fertile soils or who apply liberal quantities of nitrogenous fertilizer can expect little or no additional benefit from legume inoculants. Commercial operators or farmers who grow large acreages of legumes commonly consider seed inoculation to be a desirable procedure and one that entails little extra cost.

The use of inoculant insures that the legume seedlings will be exposed to the right kind of nitrogen-fixing bacteria early in the growing season.

There also exist in the soil free-living, or nonsymbiotic, forms of bacteria, such as Azotobacter, that can use atmospheric nitrogen. These types occur in relatively small numbers. Their effect on fertility has been questioned. Increases of 50 pounds of nitrogen an acre a year have been attributed to Azotobacter. Some persons say nonleguminous crops and even compost piles should be inoculated with these bacteria, but we have little evidence that such inoculation is economically sound.

Some of the pigmented bacteria that are capable of photosynthesis also can use atmospheric nitrogen. They exist mostly in stagnant water and mud. They are believed to be of little importance in the nitrogen fertility of ordinary field soil.

Soil bacteria are not distributed uniformly through the soil. They commonly occur in clumps or colonies of few to thousands of individual cells.

Because bacteria depend largely on organic matter for their food, they occur most abundantly near organic residues. The upper layers of the soil profile are enriched almost continuously by plant wastes, and they contain many more bacteria than the deeper layers do. Even within the plow layer, islands of activity can be expected wherever food material exists.

One site of intensive microbial colonization is at the surface of plant roots. We usually think that plant roots are in contact with the soil solution or soil particles, but actually the roots and root hairs are almost fully coated by a film of micro-organisms as we can see through a microscope. The microbiological environment in the immediate vicinity of plant roots is known as the rhizosphere. Microbes are usually 10 to 50 times more numerous there than in soil away from the roots.

ACTINOMYCETES are microscopic organisms that in many respects resemble the bacteria. Their individual cells are of about the same size in cross-section, but (unlike the bacteria) the actinomycetes form long, threadlike, branched filaments. Therefore they sometimes are called the ray fungi. The actinomycetes in most soils are only about one-tenth to one-fifth as numerous as the bacteria.

Because their cells are much bigger, the total weight of actinomycetes in an acre-foot of soil roughly equals the weight of the bacteria. The actinomycetes usually constitute a greater fraction of the total microbial population in soils of low moisture content and in organic material that is in the later stages of rotting than they do in wet soils or in rapidly rotting residues.

Most actinomycetes live at the expense of organic residues on or in the soil. The slightly musty odor of newly plowed grass sods or of old grain and straw is due mainly to actinomycetes.

The actinomycetes as a group are important in the decomposition and the humification of organic residues. One species causes potato scab. Other species produce antibiotic substances, which have great value as medicines to mankind.

FUNGI that grow above the ground often can be seen. Their cottony, colorless, or variously colored growths on bread, jellies, shoes, and clothing are commonly spoken of as mold.

Their presence is less apparent in the soil. Many of the filamentous fungi living in the soil can be seen only with a hand lens or a microscope.

Numerically, the fungi are fewer in soil than the bacteria or the actinomycetes. They account for perhaps no more than i percent of the total census of the three groups, as determined by laboratory methods. In actual amount of cell substance, their total acre-weight roughly equals the combined acre-weight of the bacteria and the actinomycetes.

Many different fungi exist in soil. Some are microscopic yeasts and simple molds. Some are big and complex forms, like mushrooms and bracket fungi, whose sporulating bodies at the surface may be several inches wide.

Fungi have no green pigment chlorophyll and therefore must feed on organic materials. Many species are parasitic on plants and animals. Non-parasitic species attack a variety of substances in the soil, including such complex plant materials as cellulose and lignin.

Fungi are important in decay because they can initiate decomposition and because they grow vigorously once they have gained a foothold. They can attack organic residues on the surface of the ground, as well as stored agricultural products and household items, whose moisture contents are too low to permit bacterial invasion. When air circulation is good, fungi rapidly convert organic wastes into cell substance and to carbon dioxide and water.

The absence of oxygen sharply limits the growth of nearly all fungi. Fungi are inactive in waterlogged soil. They are present in acid soils and even contribute to their development.

Under many coniferous and some mixed hardwood-coniferous stands of trees, the profuse penetration of fresh forest litter by fungi and the formation of a strongly acid surface organic layer, called mor, influence the development of podzolized soils.

Conversely, the formation of peaty organic soils is due partly to the inability of fungi to grow in waterlogged conditions. Many plant constituents, such as cellulose and lignin, therefore escape decomposition, and the accumulation of organic matter sharply exceeds decomposition. The two processes again come into balance when the water table recedes or the accumulated organic matter is built up to the surface of the water.

Some fungi can colonize the surfaces of plant roots and also can penetrate them and form fungus-root associations, known as mycorhizae. Such associations occur most conspicuously in the forest. They seem to benefit the higher plant. The fungal association causes the development of many short roots in pines and some other trees. Part of the superior feeding ability of the fungus-bearing roots is believed due to their much greater absorptive area. Mycorhizal roots often present several hundred times more absorbing surface than nonmycorhizal roots.