Microorganisms Live And Die For You
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.

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.

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.

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