Closely related to the protozoa are the slime molds. Sometimes they are considered to be animals because during their feeding stage they so closely resemble the amoeboid protozoa. They flourish in damp earth and on decaying organic matter. They feed largely on bacteria. After their feeding stage, the individual cells flow together and form large jellylike masses, which become immobile and produce spores. The slime molds during their fruiting state more closely resemble sporulating fungi than protozoa.

NEMATODES are nonsegmented worms.

Most of them are microscopic or nearly so in size. A few forms do attain lengths of several inches or several feet. The best known types agriculturally are those that parasitize plant roots.

A far more numerous group both in numbers and species are those that feed on the soil microflora and on protozoa and other very small animals. Such types may number several millions in a cubic foot of soil. The total weight of all the nematodes in an acre-foot of fertile soil may be 50 pounds.

The direct participation of the non-parasitic nematodes in the processes of decay is negligible. Nor does either group have any important role in soil mixing. Both groups move mostly through the existing soil spaces without increasing pore size or making tunnels.

MITES AND INSECTS are present in field and forest soils. The mites and small, wingless insects, such as the springtails, are the most numerous, but the larval, grub, and adult stages of the higher insects most frequently present serious agricultural problems.

The insects of direct agronomic importance are the subject of a later chapter. It suffices here simply to point out the abundance of mites and insects in the soil and to mention their relationships to organic decomposition and the physical condition of soil.

Mites range from microscopic to barely visible in size. They occasionally reach populations of several billions to the acre. The group includes types that feed on other small animals, but the species that feed on plant wastes and on fungi are the most numerous. They and the springtails are the smallest animals that participate significantly in the breakdown of plant residues.

Insect life in the soil is represented by many thousands of species that are diverse in appearance, life cycles, and feeding habits. Various insects attack living plants above the ground. Others continue the maceration and digestion of plant litter at the soil surface. If leaf fall is heavy, as in a forest, insects work over the organic material so thoroughly that they are responsible for much of its decomposition and for mixing the rest with the surface soil.

Their activity leads to the development of a characteristic mineral-organic layer, which is designated as insect or arthropod mull. Insects also affect the porosity of the soil and its water intake rate by their numerous burrows and galleries. As many as 84 emergence holes of cicadas have been counted in a square foot of soil surface.

Insects also carry on extensive soil transport. An outstanding example is the old termite mounds in the Tropics, which are 6 to 10 feet in average height and up to 50 feet in diameter. As many as 30 mounds may occur in an acre. Their construction involved the transport of something like I million pounds of soil in an acre.

EARTHWORMS are perhaps the one group of soil organisms most familiar to the public. Farmers, fishermen, bird watchers, gardeners, and organic gardening enthusiasts know them well.

When conditions are favorable, earthworms become the dominant animal life in the soil. At such times their weight equals or exceeds that of all the other soil-dwelling animals combined.

Earthworms flourish best in well-drained soils that contain abundant organic matter and a continuous supply of available calcium. They are susceptible to drought, cold, waterlogging, and extremes of acidity or alkalinity. Earthworms number several million to an acre in favorable soils. Their total weight in an acre of fertile soil may be one-half ton.

Earthworms are important agents in mixing surface organic residues with the underlying soil. The earthworms in an acre can bring to the surface 20 tons of soil a year. Their burrowing activity is most intense in the surface 6 inches, although some tunnels extend to depths of 6 feet. Earthworm channeling improves soil aeration and increases movement of water into and through soils.

Earthworms contribute to the formation of soil structure by their cast-forming activities. The earthworm in feeding ingests soil along with organic wastes. The organic material is humified in the gut. The residue, together with calcium carbonate and mucus secreted from the gut wall, is ejected as a granular cast. Some species eject their casts in the soil. Others drop them only on the soil surface.

If the surface organic residues are heavy, the soil reaction is favorable, and the types of earthworms that form surface casts are dominant, there accumulates a characteristic granular soil known as mull or earthworm mull. Such a mull is found on many forest soils that have a deciduous cover, but it is usually absent in coniferous stands. Pine needles rich in resin are unacceptable to earthworms as food material.

Most species of earthworms feed on fresh plant residues or on fresh animal dung, but a few species feed only on humic substances in the soil or on the fungal mycelia.

Sharp differences of opinion exist concerning the relationship of earthworms to soil fertility. Some persons insist that earthworms almost single-handedly are responsible for good fertility. Such a viewpoint has little basis in fact. The organic residues ejected by earthworms are poorer in nutrients than the original residues ingested.

We have no good evidence that passage of soil particles through a worm's gut increases the availability of the nutrients they contain.

The beneficial effects of earthworms on plant growth are largely those associated with improved aeration and improved tilth.

Earthworms are to be considered an indication of a good soil fertility rather than its cause.

Do CHEMICAL FERTILIZERS, even when they are applied at customary rates, damage the nutritive value of crops?

Are they detrimental to the earthworms and to the soil microflora?

The answer in both cases is no.

The nutrients released to plants by decaying organic matter cannot be told from the nutrients applied in fertilizer materials. Ammonia nitrogen released during the decay of plant proteins or animal proteins is indistinguishable from that washed from the atmosphere during thundershowers and carried down in rainwater and also from that in any one of several ammonia-bearing fertilizers.

For soils with favorable calcium, moisture, and aeration regimes, fertilizer applications are beneficial to earthworms to the extent that they increase plant growth and the amount of residues for return to the soil.

The snails and slugs deserve brief mention. Although the gardener is usually aware of only those that feed on living plants, most snails and slugs feed on dead and dying vegetation. They are typically scavengers and, like the earthworms, are important in the decomposition of organic matter.

MAMMALS and birds sometimes seek shelter or nesting sites in the ground. Many of them scarcely affect the soil as a medium for plant growth.

A notable exception is the soil-dwelling rodents. Although their acre-weight is usually only about a few pounds even in the uncultivated land, their soil transporting activities sometimes are considerable. In building their tunnels and nests, they transport large quantities of subsoil to the surface. The open burrows in turn catch dry or waterborne surface debris and humus-rich surface soil. Water movement into the subsoil may be facilitated by the presence of rodent burrows. In some circumstances this is desirable; in others it is not. The farmer applying irrigation water does not always express himself in temperate language when he finds gopher holes.