Chapter 9. Microorganisms Live and Die for You

Someone has said, and not too inaccurately, that soil management is nothing more nor less than the care and feeding of bacteria. Kill them off and what is left is no longer a true soil but an inert mass of rock debris contaminated with remains of dead plants and animals— wastes that without organic breakdown must remain permanently fixed in their sterile grave. Although they have an almost split-second life span, microorganisms such as bacteria, fungi (including mycor-rhizae), actinomyces, rotifers, and protozoa are all vital to the reduc­tion of organic and mineral wastes into plant nutrients, thus recy­cling the elements of existence from one generation to the next.

Fortunately for mankind as well as for all living things, these organisms are doggedly invasive and wonderfully capable of finding their way to places where they are needed. Few spots on earth are unfit for them to do their vital work. (By microorganisms is meant all living organisms, whether of plant or animal origin, that are too small to be seen with the naked eye.)

When we speak of the microorganisms of the soil, we are speaking of a living, throbbing community of well-nigh infinite numbers. Is it any wonder, then, that air, moisture, food and growing conditions are so vital to the production of Gardener's Loam? These trillions upon trillions of cells need to be fed, watered, warmed and protected so they can carry on their many functions in safety. Perhaps the most important single service performed by soil organisms (particularly bacteria) is to supply nitrogen in a form that can be used by higher plants and eventually by man. They do so in two ways: {1) by direct fixing of nitrogen from the air, and {2) by releasing nitrogen locked up in organic matter.

Direct Nitrogen Fixation

There is no general agreement among microbiologists as to the extent of direct fixation of nitrogen by soil bacteria. Most authorities agree that Azotobacters (there are several species) carry on a proc­ess known as non-symbiotic fixation (to distinguish it from nitrogen fixed by bacteria on the roots of clovers and other legumes). Non-symbiotic fixation is aerobic; that is, Azotobacters work only in the presence of oxygen. Another direct-fixation type, Clostridium pastorianum, is what is known as a facultative anaerobe; that is, it can work either in the presence or absence of oxygen. This last form is thought to take its nitrogen from ammonia in soil gases, thus pre­venting the ammonia from escaping into the air.

These direct-fixation bacteria work best when they have access to plenty of calcium, carbon dioxide and glucose. At a pH of 5.5 or below, nitrogen fixation practically comes to a standstill. Potassium chloride (muriate of potash) is particularly harmful and stops all action.

The amount of nitrogen these direct-fixing forms are capable of capturing is not definitely known. At Cornell, a series of accumula­tion tests gave figures of approximately 40 pounds per acre per year. Other authorities have given figures as high as 1,000 pounds per year per acre, but these seem unrealistic.

We might say that direct fixation is important, certainly, to native soils, woodlands, pastures and unplowed fields. It cannot possibly satisfy the needs of flowers and vegetables in home gardens, or the needs of crops like corn and grains which can blot up a couple of hundred pounds of nitrogen per acre per year.

Attempts have been made to isolate and improve superior strains of both Azotobacters and Clostridium so that pure cultures could be added to composts and soils. So far these efforts have been disap­pointing. Part of the difficulty lies in supplying the cultures with calcium, carbon dioxide and glucose without building up competing populations of more aggressive soil organisms. The use of potassium phosphate, suggested as a source of potash to help preserve nitrogen-fixing bacteria, has not resulted in significant improvement.

Nitrogen Release by Nodule Bacteria

Baptisias, lupines, sweet peas, clovers, alfalfas, lespedezas and other legumes—plants belonging to the Leguminosae or pea family —produce nodules or tubercles on their roots when attacked by certain bacteria. These bacteria are able to fix free nitrogen from the air, or at least capture ammonia gas that might otherwise escape into the atmosphere. Originally called Rhizobium radicicola, nowadays these are usually classed according to the plant they inoculate, such as R. trifoli for the bacteria that live on clover roots. For simplicity's sake, let's call them nodule bacteria.

The various nodule bacteria are quite specific and will not inoc­ulate all leguminous plants. One that inoculates garden beans will not invade the roots of peas. Lupines have a specific strain peculiar to them alone, and so on. To get around this the Nitragen Company, which probably sells 95 per cent of all the garden inoculants used in the United States, has developed a mixture of all the strains needed by various legume plants.

Some members of the Leguminosae do not harbor nodule bacteria (the redbud or Judas tree, Cercis canadensis, is one), but all lupines, baptisias and other herbaceous species within the family do.

As green manure or sheet composting crops, vetch, clover and others are just as valuable for their nitrogen contribution today as they were centuries ago when farmers grew them and plowed them under without knowing why.

Does Inoculation Pay?

In soils already rich in nitrogen, little or no N-fixation occurs, even if seeds or plants are treated to be sure inoculating bacteria are present. Nevertheless, inoculation is worth while, since there is a chance it might add a little nitrogen to the soil. The cost of inocula­tion is so low that it is not worth considering. Results can be checked by direct observation of plants: if roots are covered with hard knots when pulled up in fall (barring a serious infestation of nematodes), chances are good that a gain in nitrogen was effected.

One other group of plants, the alders, can work like the legumes. A different organism is responsible, but it, too, fixes atmospheric nitrogen.

Mycorrhizae

The fungi called mycorrhizae are discussed at this point rather than later in the chapter with other fungi because they once were thought to be capable of fixing nitrogen from the air. This theory is not considered valid at this time, but so little is actually known about these fungi that scientists will not make definite statements as to their function. Mycorrhizae cover roots of certain plants like azaleas, rhododendrons, blueberries and beeches with a felt-like sheath of mycelium. Once this sheath covers the root system of a plant, few or no root hairs are produced—the sheath of mycelium seems to func­tion in their place. Apparently the fungus is able to predigest food from soil, to take ammonium compounds from the soil and feed them to the host plant. The fungus cannot, as far as I can learn, use ammonia gas directly.

Since most of the plants to which mycorrhizae seem to be impor­tant grow in acid soils (where full breakdown of ammonium com­pounds into nitrates is slow or non-existent), the fungus probably enables its host to survive where otherwise it might starve for lack of nitrogen.

Significantly, cultural practices which protect and stimulate my­corrhizae are best for the host plant as well. These include increasing organic content of soils, supplying constant moisture without satura­tion, aeration of the soil, and mulching to protect the soil from too much heat and from drying.

Nitrification

The conversion of complex proteins to usable nitrate compounds can be called nitrification (years ago it was called ammonification). The critical step in the nitrification process, in so far as the conserva­tion of soil fertility goes, is in the release of ammonium products. If the wrong kind of fermentation of the proteins takes place at this point, or if the right bacteria are not present to take up the gas, free ammonia may be developed and escape to the atmosphere. A good example of this is in a pile of fresh manure, where certain bacteria secrete an enzyme called urease, which transforms urea into ammo­nium carbonate. This is an unstable compound, and readily lets go of its contained ammonia, which produces the characteristic sharp smell of manure.

What science is working to develop (and what gardeners would introduce into the soil artificially if they were available) are large colonies of Nitrosomonas and Nitrococcus bacteria, which convert the ammonium products into nitrites. To carry on the process, these bacteria would then be supplemented by even larger numbers of Nitrobacter—the forms that convert nitrites into nitrates.

Temperature affects the nitrification process. Most of the bacteria involved do their most effective work in soil temperatures from about 70 to 85 degrees F. At temperatures below 50, they are quite in­active. Excessively dry or wet soil conditions also interfere with their effectiveness.

As always, these bacteria require energy foods to keep them alive —the starches and sugars that they can get only from decaying organic matter.

Denitrification

Unfortunately, the nitrification process is reversable, so if condi­tions are not right we have denitrification—nitrates produced as the end product of this long chain of organic breakdown revert to nitrites and ammonia. Several microorganisms exist which will do this. They are favored by anaerobic conditions (lack of oxygen) and by pres­ence of liberal amounts of fresh, decomposing organic matter. This is one reason for maintaining two or more compost piles instead of continually adding fresh material to one old pile.

Fungi

The role of fungi in the soil has not been as thoroughly studied as has the role of bacteria. Fungi, however, must also find their energy foods in organic matter. In breaking down organic material, fungi often do a better job of dissolving cellulose than do bacteria. They can often work where bacteria cannot. For example, fungi often invade a duff or mull on the surface of soil and begin working on the material before it can sift down far enough for bacteria to attack. Fungi need less soil moisture to survive and so continue to work after drought has checked bacterial decay action.

Fungi survive and remain active at pH readings much lower than those tolerated by bacteria. No fungus, however, with the possible exception of mycorrhizae, can fix nitrogen.

The exact role of antibiotics in soil is uncertain. Whether these substances (which are produced by a number of different fungi) are helpful or harmful to higher plants has not been fully explored, al­though at least one such product is used to control fire blight in pears and apples. We do know that these antibiotic substances are plant antagonists, not affecting the fungus that produces them but inhibit­ing or poisoning other fungi and bacteria.

Chapter Digest

The old saying "Dynamite comes in small packages" could have been coined for soil microorganisms. They are microscopic, but in their living and dying process they play an indispensable role: Without them the soil is "dead"; with them the soil is alive itself and is capable of giving life to plants. Although bacteria, fungi and all the others perform innumerable soil services, the crucial one is conversion of nitrogen into usable form for plants.

Bacteria that live on the roots of clovers and other legumes, for example, add significantly to a soil's supply of nitrogen. Fungi serve a different but highly important function in breaking down the soil's organic and mineral materials into plant nutrients.

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