Soil nematodes are microscopic roundworms that inhabit nearly all soil environments across the globe. These worms are highly diverse, with over 20,000 identified species, and millions can be found in a single handful of soil. They are an integral part of the complex web of life beneath our feet, influencing soil health and plant growth.
Understanding Soil Nematodes
Soil nematodes are tiny, typically measuring between 0.1 to 2 millimeters in length, though some can reach up to 7 millimeters. Their slender, cylindrical bodies are often adorned with ridges, rings, or bristles. They are commonly referred to as roundworms or eelworms.
They require a thin film of water to move and thrive in various soil types, from deserts to oceans. They are most prevalent in the top 6 to 8 inches of soil, particularly around plant roots. While some are plant parasites, the majority are free-living, meaning they do not parasitise plants, animals, or humans. Instead, they feed on microorganisms and organic debris, forming distinct feeding groups such as bacterial-feeders, fungal-feeders, predators, and omnivores, identifiable by their specialized mouthparts.
Essential Roles in Soil Ecosystems
Nematodes contribute significantly to the health and fertility of soil ecosystems. They do not directly decompose organic matter, but feed on living material, including bacteria, fungi, and other nematodes. This grazing regulates microbial populations, preventing overgrowth and promoting a balanced, diverse microbial community.
Through their feeding, nematodes play a substantial role in nutrient cycling, particularly the mineralization of nitrogen. As they consume bacteria and fungi, which have a lower carbon-to-nitrogen ratio than nematodes, they excrete excess nitrogen in plant-available forms, such as ammonium (NH₄⁺). This process makes nutrients locked in microbial biomass accessible to plants, enhancing soil fertility. Nematode movement through the soil also contributes to improving soil structure, creating microscopic channels that enhance aeration and water infiltration, which are both beneficial for root development.
Interactions with Plants
The relationship between nematodes and plants is complex, encompassing both detrimental and beneficial interactions. Plant-parasitic nematodes can cause substantial crop losses, estimated at millions of pounds annually. These nematodes possess a needle-like mouthpart called a stylet, which they use to puncture plant cells and feed on root tissue.
Feeding by plant-parasitic nematodes can lead to symptoms in plants, including stunted growth, yellowing leaves, wilting, and reduced nutrient and water uptake. Some species, like root-knot nematodes (Meloidogyne species), induce galls or swellings on roots, while others, such as lesion nematodes (Pratylenchus species), cause internal browning and root lesions. Beyond direct damage, plant-parasitic nematodes can also create entry points for secondary infections by fungi and bacteria. Some species, like dagger, needle, and stubby root nematodes, can also transmit plant viruses.
Conversely, beneficial nematodes offer natural pest control against insect pests without harming plants, humans, or other beneficial organisms. These entomopathogenic nematodes seek out insect hosts in the soil, entering their bodies through natural openings or the cuticle. Once inside, they release symbiotic bacteria that produce toxins, typically killing the insect within 24 to 48 hours. Examples include Heterorhabditis bacteriophora for grub control and Steinernema feltiae for fungus gnat larvae and thrips pupae. These beneficial nematodes also contribute to plant health by reducing root damage caused by insect pests, thereby improving the plant’s ability to absorb nutrients.
Managing Nematode Populations
Managing nematode populations involves strategies to suppress harmful species while encouraging beneficial ones. For controlling plant-parasitic nematodes, crop rotation is a widely used practice. It involves planting non-host crops or resistant varieties to break the nematode life cycle. For instance, certain grasses like rye, crotalaria, and velvet bean are generally resistant to root-knot nematodes.
Soil solarization is another method, particularly effective in hot, sunny climates. This technique involves covering moist, cultivated soil with clear polyethylene sheeting for several weeks to months. The trapped solar heat raises soil temperatures to levels lethal to many plant-parasitic nematodes and weed seeds.
To encourage beneficial nematode populations, increasing soil organic matter by adding compost or manure is effective. Organic matter improves soil structure and provides food sources for beneficial nematodes, enhancing their survival and reproduction. Minimizing tillage also helps conserve beneficial nematodes, which are sensitive to soil disturbance. Avoiding broad-spectrum chemical pesticides is also important, as these can harm beneficial nematodes and disrupt the soil ecosystem’s natural balance. Instead, adopting integrated pest management (IPM) practices that favor biological controls supports a thriving community of beneficial organisms.