Soil Nematodes: What They Are and Their Role in Soil

Soil nematodes are microscopic, unsegmented roundworms that live in the thin films of water between soil particles, and they are among the most abundant animals on Earth. A global estimate puts their numbers at roughly 4.4 × 10²⁰ individuals inhabiting surface soils worldwide, with a combined biomass of about 0.3 gigatonnes.¹ Most are invisible to the naked eye, typically less than a millimeter long, yet they drive processes that shape plant growth, nutrient availability, and the health of the soil ecosystem in ways that are only now being fully mapped.

What Soil Nematodes Actually Are

Nematodes belong to the phylum Nematoda, one of the most species-rich groups in the animal kingdom. They share a basic body plan: a tubular, unsegmented body with a fluid-filled cavity, a simple digestive tract running from mouth to anus, and a flexible outer cuticle that they molt as they grow. What separates soil-dwelling nematodes from their more famous relatives (like the intestinal parasites you might have heard of) is where they live and what they eat. Most soil nematodes are free-living, meaning they are not parasites of animals. They slide through water-filled pore spaces in soil, feeding on bacteria, fungi, other nematodes, or plant roots depending on the species.

The diversity is staggering. Molecular studies have revealed that parasitism of plants and animals has evolved independently at least 15 times within the phylum.² Plant-parasitic lineages appear to have descended from fungus-feeding ancestors, a transition supported by DNA evidence and by the evolutionary history of the stylet, the needle-like mouthpart that plant parasites use to pierce cell walls.³ Some of the genes that plant-parasitic nematodes use to break down plant tissue appear to have been acquired through horizontal gene transfer from soil bacteria, meaning the genes jumped between completely unrelated organisms rather than being inherited from a common ancestor.

Where They Live and How Many There Are

Nematode abundance varies enormously by habitat. A typical soil sample might contain a few hundred to a few thousand individuals per 100 grams of dry soil, with a global median of about 860 and a mean around 2,670, though the most heavily populated samples exceed 20,000.⁴ Cold, high-latitude soils tend to be the richest: tundra soils have a median of roughly 2,700 nematodes per 100 grams, and temperate broadleaf and boreal forests are not far behind. Hot deserts, by contrast, hold a median of just 44. Globally, sub-Arctic regions account for about 38% of all soil nematodes, compared with 24% in temperate zones and 21% in the tropics.¹

This pattern surprises many people, who expect the species-rich tropics to dominate. But nematode abundance tracks soil moisture and organic matter more than temperature. Cold, organic-rich soils in the Arctic hold enormous bacterial and fungal populations that nematodes feed on. Meanwhile, the thin, rapidly cycling soils in dry or hot environments simply cannot support the same densities.

Within any landscape, nematodes also cluster around plant roots. The rhizosphere, the narrow zone of soil directly influenced by root secretions, consistently harbors higher nematode densities than the surrounding bulk soil, a pattern observed across different cropping systems and fertilization regimes.⁵ Roots leak sugars, amino acids, and organic acids that fuel bacterial blooms, which in turn attract bacterivorous nematodes. Plant-parasitic species are drawn directly to root signals. The rhizosphere is, in effect, a bustling nematode metropolis compared to the quieter soil a few centimeters away.

The Feeding Groups and Why They Matter

Ecologists sort soil nematodes into feeding groups based on what they eat, and each group plays a different role in the soil ecosystem. The major categories are bacterivores (bacteria feeders), fungivores (fungus feeders), herbivores (plant parasites), omnivores (mixed diets), and predators (which eat other nematodes and small invertebrates). Across global datasets, bacterivores are consistently the most abundant group, while predatory nematodes are the least common.⁴

Bacterivores and fungivores are the workhorses of nutrient cycling. When they consume microbes, they release nitrogen and phosphorus in plant-available forms, a process sometimes called the “microbial loop.” Experiments comparing soils with and without nematodes have found that their presence increased plant biomass by about 9%, net nitrogen availability by about 25%, and net phosphorus availability by about 23%.⁶ Those are not small numbers. When nematodes graze on bacteria and fungi, they keep microbial populations from stagnating and they excrete surplus nutrients (especially ammonium) that plants can take up directly. The soil food web is less like a static pyramid and more like a set of conveyor belts, with nematodes keeping nutrients in motion.

Fungivorous nematodes have a more complex relationship with plants. Some studies have found that when fungivores feed on mycorrhizal fungi, the symbiotic fungi that help plants absorb nutrients, they can reduce the fungi’s colonization of roots. Paradoxically, plant growth sometimes actually improves in the presence of those nematodes, possibly because the grazing speeds up nutrient turnover and the mycorrhizal fungi compensate by working harder.⁷ Under certain conditions, though, fungal-feeding nematodes can disrupt mycorrhizal partnerships enough to harm plant establishment, particularly in young seedlings that depend heavily on their fungal partners.⁸

Plant-Parasitic Nematodes and Crop Damage

The feeding group that farmers care most about is the herbivores, the plant-parasitic nematodes. These species use a stylet to puncture root cells, and in the case of sedentary endoparasites like root-knot nematodes, they manipulate the plant into building specialized feeding structures inside the root. Root-knot nematodes of the genus Meloidogyne are among the most damaging crop pests worldwide; they induce swollen “giant cells” in host roots whose cell walls are remodeled to support a continuous nutrient supply to the nematode.⁹ These feeding sites divert water and nutrients from the plant, causing stunted growth, wilting, and yield loss.

What makes sedentary endoparasites particularly sophisticated is their use of secreted proteins from glands in their esophagus. During penetration and migration through root tissue, one set of glands dominates. Once the nematode settles and begins constructing its feeding site, a different gland takes over to maintain and expand the structure.¹⁰ The nematode is, in a real sense, reprogramming the plant’s own cells to serve as a pantry.

Managing plant-parasitic nematodes in agriculture is tricky. Chemical nematicides have historically been effective but carry serious environmental and health risks, leading to the phase-out of several products. Biofumigation, where brassicaceous cover crops (mustards, radishes, and relatives) are grown and then mowed and tilled into the soil, is an alternative. The decomposing plant tissue releases compounds toxic to nematodes. The catch is that many brassica varieties are themselves hosts for root-knot nematodes, so growing them as a cover crop can actually increase nematode populations before you incorporate the crop into the soil. Selecting cultivars that are poor hosts is essential.¹¹

Predatory Nematodes and Natural Pest Suppression

Predatory and omnivorous nematodes sit at the top of the soil food web, and their presence can keep plant-parasitic species in check. Research comparing natural and agricultural soils has found that long, complex food webs with abundant predatory nematodes effectively suppress plant-parasite populations, while the simplified communities in disturbed agricultural soils do not.¹² The ratio of predators to prey matters: soils with more omnivore and predator species relative to herbivorous nematodes show stronger natural suppression of pests. This is one of the strongest arguments for management practices that build soil biological complexity, like reduced tillage and cover cropping, rather than relying solely on chemical inputs.

Entomopathogenic Nematodes as Biological Control Agents

A separate group of soil nematodes has found commercial use as living insecticides. Entomopathogenic nematodes, primarily from the genera Steinernema and Heterorhabditis, carry symbiotic bacteria inside their bodies. When a juvenile nematode finds a soil-dwelling insect larva, it enters the insect through a natural opening and releases its bacterial partner. The bacteria multiply, kill the insect within a day or two, and break down its tissues into a nutrient soup that the nematode feeds on and reproduces in.¹³ The bacterial symbionts, Xenorhabdus for Steinernema and Photorhabdus for Heterorhabditis, also produce antimicrobial compounds that prevent other microbes from colonizing the insect carcass, giving the nematode exclusive access.

These nematode-bacteria partnerships are increasingly used against grubs, weevils, and other soil-dwelling pests in agriculture and turf management. They are considered environmentally safe because they are highly specific to insects and do not persist long in the environment without a host.¹⁴ You can buy them commercially and apply them with a watering can.

Nematodes as Indicators of Soil Health

Because different nematode species thrive under different conditions, the composition of a soil’s nematode community can tell you a lot about that soil’s health. The most widely used tool for this purpose is the Maturity Index, an ecological measure based on nematode community composition. It works on a simple principle: some nematode species are “colonizers,” fast-reproducing opportunists that thrive in disturbed or enriched soils, while others are “persisters,” slow-reproducing species that require stable, undisturbed conditions. The Maturity Index assigns each species a score along this spectrum, and the community average reflects how disturbed or stable the soil environment is.¹⁵

In practice, a high Maturity Index suggests a complex, undisturbed soil food web, while a low value indicates recent disturbance or nutrient enrichment. There is a wrinkle, though. Plant-parasitic nematodes were originally excluded from the Maturity Index because they respond to different pressures than free-living species. A separate Plant Parasite Index was proposed for them. Research has shown that under nutrient-enriched conditions, these two indices move in opposite directions: the Maturity Index drops (indicating disturbance) while the Plant Parasite Index rises (as plant feeders exploit the enriched environment). Combining them into a single index would mask these divergent signals, making the tool less sensitive to environmental change.¹⁶ The ratio between the two indices has been proposed as a particularly sensitive gauge for monitoring agricultural systems.

How Climate Change Affects Soil Nematodes

Soil nematodes are sensitive to both temperature and moisture, and climate change is reshaping their communities. A synthesis of studies found that drought reduced total nematode abundance by about 20%.¹⁷ A meta-analysis of rainfall manipulation experiments confirmed that reduced rainfall negatively affected total nematode abundance and most feeding groups, with fungal feeders being the exception. Increased rainfall, on the other hand, boosted total nematode numbers and plant-parasitic nematodes, but only in studies lasting longer than a year.¹⁸

Temperature matters too. Rising temperatures accelerate nematode development, which can increase the number of generations per growing season and allow economically important pest species to spread into higher latitudes and elevations where they previously could not survive.¹⁹ For farmers in regions that are warming, this means plant-parasitic species that were once limited to warmer climates may become new threats.

Some nematodes have remarkable survival strategies for extreme conditions. Certain species can enter anhydrobiosis, a state of suspended animation triggered by drying. During slow desiccation, the nematode accumulates trehalose, a sugar that helps stabilize cell structures, and upregulates genes for specialized protective proteins and antioxidant enzymes.²⁰ Additional molecular defenses include antioxidant pathways, molecular chaperones, and metabolic rewiring that together allow the nematode to lose almost all its body water and revive when moisture returns.²¹ Not all species can do this, though, which means that increasing drought frequency will likely shift nematode community composition toward drought-tolerant species and away from moisture-dependent ones.

Microplastic Pollution and Soil Nematodes

An emerging concern for soil nematodes is microplastic contamination. Because nematodes live in the spaces between soil particles and filter their food from soil water, they are directly exposed to plastic fragments and the chemical additives that leach from them. Laboratory studies using the model nematode Caenorhabditis elegans found that polystyrene particles significantly reduced offspring production, and that in soil the larger particles (530 nanometers) were more harmful than smaller ones (42 nanometers), the opposite of what was observed in liquid media.²² This size-dependent flip suggests that the physical properties of soil change how plastic particles interact with organisms in ways that liquid-only lab tests miss.

Different types of plastic also affect nematodes differently. In experiments testing six common plastic types, PET fragments and polyacrylonitrile (PAN) fibers began causing significant reproductive harm at concentrations as low as 0.1% soil weight. Most other plastics caused harm at 1%, reducing offspring numbers by 20% to 44% compared with controls. Low-density polyethylene film had no detectable effect.²³ Researchers suspect that leachable chemical additives, not just the physical plastic fragments, drive much of the toxicity. Since nematodes are among the first organisms to respond to soil contamination, their reproductive health could serve as an early warning system for microplastic-related soil degradation.

How Nematodes Move Through Soil

Nematodes are poor long-distance travelers on their own. They can swim short distances through water films, but their main means of spreading across a landscape involve hitchhiking. Earthworms, for instance, act as nematode dispersal agents. As earthworms eat their way through soil, they ingest nematodes along with soil particles. The nematodes survive the passage through the earthworm’s gut and are deposited in casts at a new location. Studies have shown that increasing earthworm density and the length of time earthworms and nematodes coexist both increase the total number of nematodes dispersed, without wiping out the population in the original location.²⁴ Wind-blown soil particles, irrigation water, contaminated plant material, and even the mud on boots and equipment also carry nematodes from field to field, which is why plant-parasitic species can be so difficult to contain once established in a region.

Identifying Soil Nematodes With DNA

Traditionally, identifying soil nematodes required extracting them from soil, mounting them on slides, and examining them under a microscope, a laborious process demanding specialized expertise. DNA metabarcoding is changing that. The approach extracts all DNA from a soil sample and amplifies a target gene region (usually from 18S ribosomal DNA) to identify which nematode species are present. Purpose-built primer sets have been developed specifically for soil nematode communities, and they perform well at identifying nematodes to the genus or family level.²⁵

The technology is not perfect. Assessments of commonly used primers show that while most achieve good coverage, capturing the majority of nematode sequences in reference databases, none is highly specific to nematodes alone. They all amplify DNA from a broad range of other organisms as well. Species-level identification remains a challenge, and resolution improves with longer DNA fragments, which creates a trade-off with sequencing practicality.²⁶ Still, for monitoring soil health, tracking the effects of land-use change, or screening fields for parasitic species, metabarcoding is far faster and more scalable than microscopy, and it is likely to become the standard approach as reference databases grow more complete.