What Is a Nematode?

• A nematode is a microscopic worm, tube-shaped and often transparent, also known as a roundworm. It is the most plentiful of the physical decomposers at work in soil and compost. There are many species of nematodes. Some feed on decaying vegetation; some feed on bacteria, fungi and other nematodes; and others drink the juices of plant roots.
  
  

Harmful Nematodes

• The best-known harmful nematodes are plant nematodes, which parasitize plants. These are considered serious garden pests, especially in sandy soil in the South, and also cause serious plant and crop damage.
  
  Plant nematodes invade the roots of plants, forming growths known as root knots. These root knot nematodes can multiply in the soil and destroy a crop. Harmful nematodes can appear in gardens, as well, feeding on the roots of vegetables like beans, tomatoes, spinach and celery.
  
  

Beneficial Nematodes

• There are two genera of beneficial nematodes: Heterorhabditis and Steinernema. For gardeners, these "good" nematodes are welcome because they act as effective, environmentally-friendly, biological pesticides. They help control common garden pests like rootworms and fleas.
  
  Compost nematodes are a boon for gardeners because they help create nitrogen for plant use. They also consume harmful plant nematodes when both are present in the same soil.
  
  

Controlling Harmful Nematodes

• Soil that is rich in organic matter works well as a defense against plant nematode infections. Rotating crops and garden plants each year is also important to managing "good" and "bad" nematodes in soil and, ultimately, your compost. Some gardeners remove roots of plants that are infected with harmful nematodes and put them in a separate compost pile. It's also helpful to solarize your soil, using the sun's heat to kill seeds that might draw destructive nematodes. Till the soil, water it, and cover it with plastic sheets held down with rocks or soil. This allows the sun to heat the soil; the process can take up to eight weeks.