Beneficial nematodes supply up to 30% of crop nitrogen

Beneficial nematodes can supply between 9% and 30% of the nitrogen available to plants when the soil food web is in balance, according to Sheila Storey, MSc (Agric), consultant for Nemlab. That figure, from one of South Africa's leading soil biology laboratories, frames a growing argument that fertiliser programmes alone cannot sustain long-term productivity.

Storey's message to producers is direct: soil biology should now be regarded as equally important as soil chemistry in modern farming systems. As climate variability, rising input costs and declining soil quality continue to pressure South African agriculture, the living ecosystem beneath the surface determines whether applied nutrients actually reach the crop.

What does the biology actually deliver?

A balanced soil food web provides ecosystem services that fertiliser bags cannot replicate. "Provided the soil food web is balanced, it provides many ecosystem services. The most important of these is nutrient cycling," Storey said.

The headline number is nitrogen. "Beneficial nematodes can provide between 9% and 30% of the nitrogen available to plants when the biology is balanced," she said. Mycorrhizal fungi do similar work with phosphorus. "Mycorrhizae exchange phosphorus, which the fungus has 'mined', in return for carbon that the plant provides. The carbon is then stored by the mycorrhizae in the soil."

Biology also breaks down organic matter and regulates pests and diseases. "Biology is responsible for breaking down organic matter. Microbes can also regulate pests and diseases," Storey said.

Why roots must stay in the ground

Maintaining living roots throughout the year is one of the six principles of soil health. "Roots 24/7 is one of the six principles of soil health. The plant roots provide food for the biology in the form of carbohydrates," Storey said.

Those root exudates feed the microorganisms that drive nutrient cycling and build structure. Diversity above ground drives diversity below it. "The greater the diversity of cover crops, the greater the diversity of exudates and so too the greater the diversity of soil biology. Above-ground diversity begets below-ground diversity," she said.

Storey advises planting mixtures rather than a single cover crop species wherever possible. A wider range of plant species creates a wider range of food sources for soil organisms.

Cover the soil, bank the water

Soil should never sit bare, whether the cover comes from mulch, crop residues or living cover crops. "Mulching regulates soil temperatures, provides carbon and covers the soil. We always say, 'Cover the soil to protect the biology beneath'," Storey said.

The carbon payoff is measurable. "The carbon creates the humus which, in turn, acts like a sponge and increases water-holding capacity," she said. For South African producers facing prolonged dry spells, heatwaves and erratic rainfall, that moisture buffer carries real value. Mulch also moderates soil temperatures, protecting organisms sensitive to extreme heat and fluctuating moisture.

Where do legumes fit?

Nitrogen-fixing crops contribute nitrogen naturally, but only when the correct symbiotic bacteria are present. Storey cautions against treating legumes as a full substitute for synthetic fertiliser. "On their own they cannot reduce the farmer's reliance on synthetic fertilisers, but they can definitely cut the amount of nitrogen required," she said.

Which nematodes matter?

Only a relatively small proportion of nematodes are harmful. Plant-parasitic nematodes damage roots, but bacterivores, fungivores, omnivores, predators and entomopathogenic nematodes all have roles in the soil food web. "This makes these nematodes excellent bio-indicators of soil health through a nematode community profile," Storey said. A nematode community profile gives farmers a clearer picture of how well their soils function biologically.

No quick fixes from a container

The most common mistakes, according to Storey, are expecting one product or practice to fix soil health and planting only one cover crop species when a mix is required. She is blunt about microbial products: "Believing that adding a microbe out of a container will improve soil health is another misconception. Most of these products are there to improve root health, but they don't provide the complexity required for soil health."

Soil health depends on applying all six principles together: living roots year-round, root diversity, soil cover, minimised chemical, physical and biological disturbance, livestock where appropriate, and management adapted to each farm, orchard or field. The result is stronger water infiltration, greater water-holding capacity and less run-off. "The soil is more resilient. With improved water-holding capacity and infiltration there is less run-off," Storey said. "Cover crops reduce and stabilise temperature. Input costs are decreased."

Farmers should also look beyond conventional soil tests. Simple field checks include infiltration tests, counting small organisms such as springtails, feeling soil structure by hand and using a penetrometer to measure compaction. "To refine the tests, use a laboratory such as Nemlab," Storey said.

For this season, Storey recommends a simple start: choose two cover crops suited to the land or orchard, use them to provide roots 24/7, cover the soil and reduce disturbance. There is no universal formula, because every land and orchard differs. Farmers wanting a baseline should begin with a nematode community profile before the next planting window.