Organic Gardening: Protozoa & Nematodes
Soil, or at least healthy soil, is teeming with life. Soil life lives in a symbiotic and mutualistic way, offering their services to help the whole system work. Bacteria and fungi build the soil structure and help make nutrients in the soil available to plants, and also store valuable nutrients in their bodies. Protozoa, nematodes and other larger members of the soil food web consume these microbes, and in doing so release the nutrients to the plants. This process allows natural systems to maintain themselves with the help of the soil food web.
Protozoa are single-celled animals that feed primarily on bacteria, but also eat other protozoa, soluble organic matter, and sometimes fungi. They are several times larger than bacteria, ranging from 1/5000 to 1/50 of an inch in diameter. Both protozoa and nematodes are aquatic and live and move in soil water films and water-filled pores of soil aggregates.
Protozoa are found in greatest abundance near the surface of the soil, particularly in the upper 15 cm (six inches). There they play an important role in mineralizing nutrients, making them available for use by plants and other soil organisms. Protozoa (and nematodes) have a lower concentration of carbon (C) and nitrogen (N) in their cells than the bacteria they eat. Bacteria eaten by protozoa contain too much N for the amount of C protozoa need. They release the excess N in the form of ammonium (NH4+). This usually occurs near the root system of a plant.
Nematodes or roundworms are non-segmented worms with tapered ends and are typically 1/20 of an inch (1 mm) in length. They have a head, and a tail with a well developed central nervous and fertility system with a complete digestive system, so they are considered the most primitive animal. They are small enough to fit in most soil pores and soil aggregates.
There are several species of nematodes that are responsible for plant diseases and can be very harmful (known as detrimental nematodes), but far less is known about the majority of the nematode community that plays beneficial roles in soil. Many beneficial nematodes serve as biological pest control agents in managed systems and others regulate the natural ecosystem and soil nutrient cycling. Some feed on the plants and algae, others are grazers that feed on bacteria and fungi, and some feed on other nematodes. A variety of nematodes function at several trophic levels of the soil food web. Nematodes are most abundant in the surface soil horizon.
Like protozoa, nematodes are important in mineralizing, or releasing, nutrients in plant-available forms. When nematodes eat bacteria or fungi, ammonium (NH4+) is released because bacteria and fungi contain much more N than the nematodes require. At low nematode densities, feeding by nematodes stimulates the growth rate of bacteria populations. Small or low root consumption by nematodes may stimulate plant root growth like air pruning, increasing root biomass.