Nematodes are simple multicellular soil animals that resemble tiny worms but are non-segmented. They tend to live in the water films around soil aggregates. Some types of nematodes feed on plant roots and are well-known plant pests. Fungi such as Pythium and Fusarium, which may enter nematode-feeding wounds on the root, sometimes cause greater disease severity and more damage than the nematode itself. A number of plant-parasitic nematodes vector important and damaging plant viruses of various crops. However, there are many beneficial nematodes that help in the breakdown of organic residues and feed on fungi, bacteria, and protozoa as secondary consumers. In fact, as with the protozoa, nematodes feeding on fungi and bacteria help convert nitrogen into forms for plants to use. As much as 50% or more of mineralized nitrogen comes from nematode feeding. A number of nematodes alone or with special bacteria parasitize and kill insects such as the larvae of the cabbage looper and the grubs of the Japanese beetle. Finally, several nematodes infect animals and humans, causing serious diseases such as river blindness and heartworm.
Earthworms are every bit as important as Charles Darwin believed they were more than a century ago. They are keepers and restorers of soil fertility. Different types of earthworms, including the night crawler, field (garden) worm, and manure (red) worm, have different feeding habits. Some feed on plant residues that remain on the soil surface, while other types tend to feed on organic matter that is already mixed with the soil.
The surface-feeding night crawlers fragment and mix fresh residues with soil mineral particles, bacteria, and enzymes in their digestive system. The resulting material is given off as worm casts. Worm casts are generally higher in available plant nutrients, such as nitrogen, calcium, magnesium, and phosphorus, than the surrounding soil and, therefore, contribute to the nutrient needs of plants. They also bring food down into their burrows, thereby mixing organic matter deep into the soil. Earthworms feeding on debris that is already below the surface continue to decompose organic materials and mix them with the soil minerals.
A number of types of earthworms, including the surface-feeding night crawler, make burrows that allow rainfall to easily infiltrate the soil. These worms usually burrow to 3 feet or more, unless the soil is saturated or very hard. Even those types of worms that don’t normally produce channels to the surface help loosen the soil, creating channels and cracks below the surface that help aeration and root growth. The number of earthworms in the soil ranges from close to zero to over a million per acre. Just imagine, if you create the proper conditions for earthworms, you could have 800,000 small channels per acre that conduct water into your soil during downpours.
Earthworms do some unbelievable work. They move a lot of soil from below up to the surface—from about 1 to 100 tons per acre each year. One acre of soil 6 inches deep weighs about 2 million pounds, or 1,000 tons. So 1 to 100 tons is the equivalent of about .006 of an inch to about half an inch of soil. A healthy earthworm population may function as nature’s plow and help replace the need for tillage by making channels and bringing up subsoil and mixing it with organic residues.
Earthworms do best in well-aerated soils that are supplied with plentiful amounts of organic matter. A study in Georgia showed that soils with higher amounts of organic matter contained higher numbers of earthworms. Surface feeders, a type we would especially like to encourage, need residues left on the surface. They are harmed by plowing or disking, which disturbs their burrows and buries their food supplies. Worms are usually more plentiful under no-till practices than under conventional tillage systems. Although many pesticides have little effect on worms, some insecticides are very harmful to earthworms.
Diseases or insects that overwinter on leaves of crops can sometimes be partially controlled by high earthworm populations. The apple scab fungus—a major pest of apples in humid regions—and some leaf miner insects can be partly controlled when worms eat the leaves and incorporate the residues deeper into the soil. Although the night crawler is certainly beneficial in farm fields, this European introduction has caused problems in some northern forests. As fishermen have discarded unused worms near forest lakes, night crawlers have become adapted to the forests. They have in some cases reduced the forest litter layer almost completely, accelerating nutrient cycling and changing species composition of the understory vegetation. So some forest managers view this organism, considered so positively by farmers, as a pest!
Insects and Other Small to Large Soil Animals
Insects are another group of animals that inhabit soils. Common types of soil insects include termites, springtails, ants, fly larvae, and beetles. Many insects are secondary and tertiary consumers. Springtails feed on fungi and animal remains, and in turn they themselves are food for predacious mites. Many beetles, in particular, eat other types of soil animals. Some beetles feed on weed seeds in the soil. Termites, well-known feeders of woody material, also consume decomposed organic residues in the soil.
Other medium-size to large soil animals include millipedes, centipedes, mites, slugs, snails, and spiders. Millipedes are primary consumers of plant residues, whereas centipedes tend to feed on other organisms. Mites may feed on food sources like fungi, other mites, and insect eggs, although some feed directly on residues. Spiders feed mainly on insects and keep insect pests from developing into large populations.