The Soil Food Web Is a Whole Civilisation and You Have Met None of Them

Shrink yourself down to the size of a grain of pollen and step into a teaspoon of good soil. It is not quiet and it is not empty. There are hundreds of millions of bacteria around you. There are metres of fungal thread running off into the dark in every direction like cable under a city. Long transparent animals are swimming through films of water thinner than a hair, hunting. Something with a mouth built for one job only is grazing a fungal pasture. A worm comes through and the ground moves.

This is the soil food web. It is the set of living things in the ground and the eating relationships between them, and it is the reason plants grow at all. We are going to walk through it creature by creature, because the individual characters are far stranger and far more fun than the diagram usually lets on.

Plants are not fed by soil. Plants are fed by the things that live in soil, mostly by eating each other near a root and leaving the leftovers behind.

It Starts With a Plant Deliberately Leaking Sugar

A plant makes sugar in its leaves. You would expect it to keep it. Instead a large share of everything it makes, commonly put somewhere in the range of a fifth to a third, gets pushed out through the roots into the surrounding soil on purpose. Sugars, acids, amino acids, proteins. A plant spends a serious fraction of its income feeding the ground around itself.

That leak is the whole economy. The thin sleeve of soil that surrounds a root, called the rhizosphere, is the busiest real estate in the underground world, and it is busy because there is free food coming out of the walls. Bacteria arrive in numbers that dwarf the general population of the soil. Everything that eats bacteria follows them in. The plant is not being generous. It is building a market and it is going to collect.

The Cast, One Character at a Time

▸ Bacteria, the crowd that votes

They are the smallest and the most numerous, and they do the chemistry nothing else can. They pull nitrogen out of the air. They prise minerals off rock grains with acid. They eat things nobody else will touch. Most of them live stuck to a surface inside a shared slime they build together, which is closer to a walled town than to a swarm.

The strangest thing about them is that many species can count. They release a signalling chemical constantly and taste how much of it is around them, which tells each cell roughly how many neighbours it has. Below a threshold, nothing happens. Above it, the whole population switches behaviour at once. This is called quorum sensing, and it means a bacterial colony holds something very like a vote before it acts.

One family deserves its own line. Actinobacteria grow in threads rather than single cells, break down the tough material nothing else can digest, and produce a compound called geosmin. Geosmin is the smell of rain on dry ground. That smell you have loved your whole life is a soil organism announcing itself, and the human nose is absurdly sensitive to it, down to a few parts per trillion.

▸ Fungi, the roads and the road builders

A fungus in soil is not a mushroom. A mushroom is a fruit that appears occasionally. The organism is a network of threads called hyphae, each one narrower than a human hair, growing at the tips and branching and fusing, and there can be many metres of it in a single teaspoon.

Some of them are decomposers, taking apart wood and leaf litter that bacteria cannot manage. The interesting ones are mycorrhizal, meaning they have made a deal with a plant. Something in the order of eight or nine out of every ten land plants on Earth have such a partner, and it is one of the oldest arrangements in biology, older than roots themselves.

The deal is simple. The fungus gets sugar, which it cannot make. The plant gets water, phosphorus and other nutrients gathered from a volume of soil hundreds of times larger than its roots could ever reach, because a hypha is so much thinner than a root that it goes places a root physically cannot.

Where the exchange actually happens is worth picturing properly. In the most common type, the fungus pushes through a root cell wall and grows, inside that single living cell, a tiny branching structure shaped exactly like a tree. It is called an arbuscule, from the Latin for little tree. There is a tree growing inside a cell of a root of a tree, and it exists so two organisms can trade.

▸ Protozoa, the grazers that actually feed the plant

These are single celled hunters, and they come in three rough shapes. Flagellates whip themselves along with a tail. Amoebae pour themselves around their food and absorb it. Ciliates are covered in beating hairs, are much larger, and are the wolves of the group.

Here is the part that changes how you think about fertility. Bacteria are extremely rich in nitrogen. The protozoa that eat them are not, and cannot use anything like that much. So a protozoan eats bacteria, takes the carbon it needs, and dumps the surplus nitrogen straight out as ammonium, which is exactly the form a plant can absorb. It does this in the rhizosphere, because that is where the bacteria are, which is to say it does it right against the root.

Bacteria hoard. Predators make them let go. A large share of the nitrogen a plant takes up has been through a small hunter first, and the fertility of your garden is partly a body count. This is usually called the microbial loop, and it is the single most underappreciated fact about growing food.

▸ Nematodes, sorted by mouth

Nematodes are microscopic worms, and there are a lot of them. They are the most numerous animals on Earth by a wide margin. In soil biology they are grouped not by family but by what their mouth is built for, which is a wonderfully direct way to organise a population.

• ▸Bacterial feeders have a simple open tube of a mouth and vacuum up bacteria, releasing nitrogen the same way protozoa do.
• ▸Fungal feeders carry a hollow spear, puncture a fungal thread and drink the contents out of it.
• ▸Predators have teeth, or a wider spear, and hunt other nematodes and protozoa.
• ▸Root feeders carry the same spear and use it on your plants. These are the ones gardeners have heard of, and they are a minority of the population in healthy ground.

A soil scientist can look at the ratio between those four mouths and tell you how disturbed the ground is, how far along in recovery it is, and whether it is cycling nutrients well. The mouths are a diagnostic.

▸ Springtails and mites, the livestock and the leopards

Move up a size class and you get small animals you can actually see if you look hard at leaf litter. Springtails graze fungal threads in herds and are named for a folded tail they release like a spring to fling themselves out of danger. Mites are everywhere and do everything, some shredding dead leaves, some grazing, some hunting springtails at speed.

Their real job is demolition. They chew tough material into smaller pieces with far more surface area, which hands the bacteria and fungi a much easier meal. Nothing in this system eats alone. Everything is preparing food for something smaller.

▸ Earthworms, the civil engineers

Worms work at three different depths and it is worth knowing which is which. Some live in the surface litter and never enter mineral soil. Some make horizontal burrows and live in the topsoil. Some dig permanent vertical shafts metres deep, come up at night to pull leaves down, and use the same shaft for their whole lives.

Soil goes into a worm and comes out different. It comes out with more available nutrients, better structure, and a substantially larger microbial population than went in, because the gut is a moving fermentation vessel. The burrows are permanent infrastructure for water and air, which is a real part of why compacted ground floods and worm rich ground does not.

▸ The oddities living in the water film

Almost everything small in soil is aquatic. It lives in the microscopic film of water clinging to particles, and at that scale water is not a liquid you swim through so much as a structure you are held inside, because surface tension is overwhelming. When the film dries, that world closes.

Which is why the survival strategies are extreme. Tardigrades, the famous eight legged water bears, expel almost all their water, curl into a barrel and shut down, and can wait years for the rain. Rotifers do something similar. Bacteria form spores. Fungi go dormant as thread. A dry soil is not a dead soil. It is a soil holding its breath, and it starts again in minutes when it is wet.

The Network Between Plants, and What Is Actually Proven

Because one fungus can connect to more than one plant, the roots of separate plants end up physically linked through shared fungal thread. Popular accounts call this the wood wide web, and the 2019 documentary Fantastic Fungi, featuring Paul Stamets, put the image in front of a very large audience.

The physical structure is real and well documented. Trees in a forest are connected underground. Substances do move through those connections. That much is not in dispute.

The bigger claims are worth handling honestly, because they are contested. The idea that mature trees deliberately nurse their seedlings through the network, or that trees send each other warnings as a kind of forest wide message system, has been challenged in the scientific literature. Reviews published in recent years have argued that the popular version has run well ahead of the evidence, and that some widely repeated findings do not hold up as strongly as the retelling suggests.

The soil is not a commune. It is a market, with hard bargaining, cheating, and partners who charge more when they can get away with it. That is a better story than harmony, because it is the one that actually holds up.

Experiments have shown fungi giving better service to plants that pay better, and moving resources toward wherever the return is highest. Some plants cheat the system entirely, including species with no chlorophyll at all that simply tap the network and take. None of that makes the underground world less astonishing. It makes it a functioning economy rather than a fable.

What This Means for Anyone Growing Anything

Once the food web is the thing you are managing, the standard advice stops sounding like a list of rules and starts sounding obvious.

• ▸Heavy tilling cuts the fungal network into pieces and collapses the burrows. You are demolishing the roads and then wondering why deliveries stopped.
• ▸Bare ground bakes and dries, the water film goes, and the small hunters that release nitrogen stop working. Cover is habitat.
• ▸Living roots in the ground for as much of the year as possible keep the sugar leak running, which is the only income the whole system has.
• ▸Heavy soluble fertiliser tells a plant it no longer needs to pay for a fungal partner, and the partnership weakens. You can buy your way out of the relationship and then have to keep buying.
• ▸Compost is not plant food. It is a delivery of population and habitat, which then makes plant food out of what is already there.

The gardener who understands this is not feeding plants. They are keeping several hundred million employees in decent working conditions and letting the plants handle their own negotiations.