What Is the Impact of Soil Organisms on Soil Fertility?

Decomposition

• Most soil organisms, from bacteria to earthworms, help decompose organic matter in the soil. Fungi decompose pollutants in the soil and also break down carbon compounds, which improves soil quality. Earthworms benefit the soil by not only decomposing it, but also digesting it, which improves the nutrient content. The improved soil, now known as a worm cast, is then released back into the earth with higher availability, which means that plants can use it more easily as a food source.

Soil Structure

• Soils with a healthy structure are more fertile than poorly structured soils. Soil organisms increase soil fertility by increasing soil aggregation. Soil aggregates are small clumps of soil that increase its porosity and water retention. This in turn benefits plants by allowing them to retain water and nutrients at high rates. Earthworms also improve soil structure by burrowing into the ground and making tunnels. As they burrow, the earthworms mix the soil, increase pore space and form aggregates with their casts.

Predators and Mutualists

• Some soil organisms prey on other potentially damaging microbes and insects. For example, spiders, scorpions, pseudoscorpions, centipedes, predatory mites, ants, ladybugs, lacewings, syrphid flies and beetles are a few examples of predatory insects that feed on damaging pests. Protozoa help control populations of potentially harmful soil microbes. Other soil organisms, most notably the soil bacteria Rhizobia, improve soil fertility by forming mutually beneficial partnerships with plants and increasing the availability of nitrogen and other plant nutrients.

Negative Effects

• Although soil organisms provide a number of benefits for soil fertility, some of them can pose problems. For example, some types of nematodes, fungi, bacteria and insects can cause extensive damage to plants by feeding on roots. Other fungi and bacteria can cause root rot, wilt and other negative plant symptoms. Root knot nematodes cause the formation of galls on plant roots, which prevent water and oxygen flow to plant roots. And, of course, organisms such as mites and grubs can cause extensive damage to plants by chewing on leaves and roots.