Excluding the higher plants, ferns and mosses, and the more highly organised forms of animal life, the living organisms present in the soil can be divided into the following groups: –

INSECTS and similar animate phyla. With a few exceptions, e.g. centipedes and most of the fast running beetles, insects, in the soil are regarded as pests, because the majority feed on the roots and other underground parts of plants. Among the worst of these pests are millipedes and eelworms, but actively-feeding larvae, particularly of insects, are very injurious to plant growth, e.g. vine weevil, leather-jackets, carrot fly, wireworm and cutworms; also slugs, snails, springtails and woodlice. Many insects live in the soil during only a part of their life, often only as pupae, so these cannot be regarded as true soil pests. Apart from their damage to the underground parts of plants, insects do tend to increase soil fertility by pushing their way through the soil, thus forming air and water channels.

WORMS. By far the most common member of this class is the earthworm which is found in all fertile soils. Its effect on fertility is very considerable. The earthworm consumes soil for the organic matter it contains. This process breaks the soil down into a highly fertile state and eventually returns it in the form of worm casts. Worms also extract from the soil and break down decaying matter which might otherwise render the soil sour. In burrowing through the soil worms create definite drainage channels which have the beneficial effect of keeping it well aerated. What is more, they tend to turn over the soil, bringing earth from below to the surface, and vice versa.

FUNGI. Fungi are present in most soils, particularly in damp humus soils such as those found in woods. Feeding on the decaying organic matter in the soil, the saprophytic types of fungi actively decompose cellulose and all carbohydrate and nitrogenous matter. Thus the fertility of the soil is increased by the break-down of dead plant tissues into simple compounds available for the use of a new generation of plants.

ALGAE. These minute, chlorophyll-producing organisms occur in many damp soils. They cannot be said to increase fertility in any way, and they often compete with higher plants for supplies of nitrate, mineral salts, etc.

5. THE MICROSCOPIC SOIL FAUNA. In this group are the larger unicellular organisms most of which are marine, living only in swampy soils, and the microscopic bacteria. Both are saprophytic, and the latter play a most important part in soil fertility. A few are harmful, e.g. some are said to produce toxins, but most are very beneficial.

As plants die back to the soil level, countless bacteria break down the complex carbohydrate, nitrogenous and other plant materials to form simple compounds, which are liberated as gases or returned to the soil as water and simple foods available for the use of other plants. The process of nitrification is an excellent illustration of the work of bacteria. In the breakdown of protein matter in the soil, the action of one set of bacteria causes nitrogen and ammonia to be formed. The nitrogen escapes into the air; another set of bacteria act on the ammonia and convert it into nitrous acid. A third bacterium converts the nitrous acid into nitric acid, which reacts with the salts of calcium, potassium and sodium in the soil to give nitrates. These are of great value to the plants.

Bacteria which form nodules or swellings on the roots of leguminous plants take in free nitrogen from the air. This they convert into nitrogenous compounds which are passed on to the host plants and finally given up to the soil when the plants decay. Because of this, plants such as clovers, vetches and lupins are very valuable as green manures.

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