Some nematodes eat roots, others eat fungi and algae. Predatory nematodes up to 2 inches long eat protozoa and other nematodes. Species of nematodes as small as 1/500 of an inch long eat bacteria and are among the most beneficial nematodes found in gardening and agricultural soil.
Impact of Bacteria
Bacteria decompose organic matter in the soil. Nitrogen-fixing bacteria convert nitrogen from the atmosphere into forms of nitrogen that plants can use. Bacteria also convert ammonia into nitrate, a form more useful to plants. Bacteria often produce sticky substances that help bind soil. In soils that are compact or that have not been tilled, species of bacteria convert nitrate into gases that escape into the atmosphere. Some bacteria, usually in tropical or subtropical soils, can cause root rot and block the xylem vessels that carry plant nutrients.
Nematodes that eat bacteria are called bacterivores. Their stoma or "mouth" is at one end of a hollow tube that takes in bacteria and extrudes waste at the other end. Many bacteria-eating nematodes are in the order Rhabditidahae. They feed on bacteria in decaying organic matter, and their life cycles can be as short as three-and-a-half days. Bacteria contain more nitrogen than bacteria-eating nematodes require. When nematodes eat bacteria, their waste includes ammonium, a necessary plant nutrient.
As nematodes move about through the soil and among roots they carry bacteria in their digestive systems. This helps distribute bacteria in the soil that are necessary for the decomposition of organic matter.
The number of bacteria-eating nematodes depends on the amount of available bacteria. Nematodes that eat bacteria congregate near the roots where most bacteria are found. There are more nematodes in soils with a coarse texture. While there may be several hundred nematodes in a teaspoon of forest soil, there will likely be fewer than 100 in a teaspoon of agricultural soil.
When small populations of nematodes eat bacteria, the population of bacteria increases; when large numbers of nematodes feed on bacteria, the number of bacteria decreases. Reducing soil bacteria that decompose organic matter is harmful to plants. Predatory nematodes that eat bacteria-eating nematodes prevent the bacteria-eaters from overgrazing. An acre of soil will have from 400 to 4,000 pounds of bacteria and from 5 to 50 pounds of nematodes.