Soil Food Web

The digestive system of nature

The clearest way to understand soil life is to see it as the stomach and gut of a landscape. A plant cannot eat a fallen leaf. The nitrogen, phosphorus and sulphur in that leaf are bound in large organic molecules that a root cannot absorb. Soil life does the digesting. Fungi and bacteria break the material down with their enzymes, step by step, until the nutrients are released in simple mineral form. Soil science calls this mineralisation. What a stomach does for an animal, the soil does for the whole plant community.

A second step makes the picture even more fitting. Bacteria store a great deal of nitrogen in their own bodies, more than the protozoa and nematodes that graze on them actually need. When these grazers eat bacteria, they excrete the surplus as ammonium, and they do so mostly where bacteria are densest: around the roots. Nutrients are released at exactly the point where a plant can take them up.

The plant is not a passive diner in this process. Through its roots it releases sugars, amino acids and organic acids into the soil, a substantial share of what it produces in its leaves. With this it feeds a teeming community in the thin zone around each root, the rhizosphere. The microbes living there unlock nutrients from minerals and organic matter, and the plant takes them up. Put simply, the plant feeds its gut flora, and the gut flora feeds the plant.

Two partnerships extend this system. Mycorrhizal fungi grow into the roots of most land plants and spin a web of threads far beyond the root zone, bringing back phosphorus and water in exchange for sugars and fats. This is described in more detail under mycorrhiza. And root-nodule bacteria in legumes take nitrogen directly from the air, the basis of nitrogen-fixing plants.

Like every image, this one has a limit: a stomach has a place, while soil life is spread through every crumb of ground. But the function is the same. Without this digestion, a forest would choke on its own fallen leaves and starve at the same time. With it, forests and meadows have fed themselves for millions of years without anyone adding fertiliser. On the way, soil life also builds humus and glues mineral particles into the crumbs that give soil its structure.

The wood wide web: what is proven and what is not

In 1997 the journal Nature published a field experiment by the Canadian forest ecologist Suzanne Simard: carbon had moved from birch to Douglas fir, apparently through shared mycorrhizal fungi. The journal's cover called it the "wood wide web", and the image went around the world. Since then, books and films have told of trees that talk to each other through fungi, warn one another and feed their young.

It is worth separating the solid from the speculative here. Firmly established is the exchange between plant and fungus: the plant supplies carbon, the fungus supplies water and nutrients. Also shown is that the threads of one fungus can connect several plants, and that substances can pass between plants in experiments. Contested is everything beyond that. In 2023 three mycorrhiza researchers reviewed the field studies in Nature Ecology & Evolution. Their conclusion: how widespread such networks are in forests, and whether transfer through them actually helps seedlings, is insufficiently supported. For the claim that mother trees preferentially supply their offspring, they found no peer-reviewed evidence at all. They also observed that unsupported claims had become more frequent in the literature over 25 years.

This does not make the fungal network less important. It is simply wise to admire it for what it demonstrably does. It remains the most important supply line between soil and plant, especially on dry ground.

How soil life is weakened, and why fertiliser then becomes necessary

Mineral fertiliser has a history of its own. In the 1840s the chemist Justus von Liebig showed that plants take up nutrients in mineral form. This laid the foundation for mineral fertiliser, and with the Haber-Bosch process, used industrially from 1913, nitrogen fertiliser became a mass product. The aim was to replace what harvests carry off the field and to feed a growing population.

What research shows today is a different, self-reinforcing loop. When soluble phosphorus is abundant, many plants invest less in their fungal partners: a meta-analysis of field experiments found mycorrhizal fungi reduced by about a third on average under phosphorus fertilisation. A European study of 150 cereal fields found that mycorrhizal fungi from arable soils delivered markedly less phosphorus to plants than fungi from grassland; among the arable soils, fungicide use was the factor that best explained how much phosphorus the fungi still delivered. Ploughing tears fungal threads apart, and studies across Europe show that intensive land use thins out the soil food web, with larger soil animals disappearing first.

Put together, this creates a spiral. The weaker the soil's own digestion, the less it supplies on its own, and the more nutrients have to come from the bag. The more readily nutrients come from the bag, the less a plant needs its partners underground. In the image of the stomach: where digestion is damaged, food has to arrive pre-digested from outside. The spiral can be turned, but slowly, through permanent ground cover, many different roots and as little disturbance as possible.

How Terra Humana understands soil life

For Terra Humana, soil life is not a side topic of restoration but its engine. Whatever a slope is meant to carry later is first digested underground. That is why the most important question on degraded land is not which fertiliser it needs, but how its soil life can be brought back to work.

From this follow the practical choices: never leave ground bare, return prunings to the surface as mulch, dig as little as possible, plant many different species with different roots, and do without synthetic fungicides. None of this is new. It is the way forests have always fed themselves.