Why Good Soil Crumbles and Poor Soil Turns to Dust

Take a handful of soil from under a hedge, where nobody walks. Squeeze it, then open your hand.

It falls apart into little rounded lumps, somewhere between a breadcrumb and a peppercorn. It does not turn into a ball and it does not turn into powder. It breaks into pieces, and the pieces have a shape.

Now do exactly the same thing on a bare trodden path a few metres away. Same field, same rock underneath, same rain falling on both.

This time you get dust, or a hard grey lump that will not break at all.

Something is in one handful that is missing from the other, and it is not a mineral.

Soil is a building, not a pile

Start with what soil is made of, because "dirt" is doing a lot of hiding.

There is broken rock in it — sand, silt and clay, which are just the same rock in three different sizes. There is dead plant matter. There is water, and there is air, and the air matters more than anyone expects.

And there is an enormous amount of life. A single teaspoon of good soil holds billions of bacteria, along with metres of fungal thread, plus mites and springtails and worms and things nobody has got round to naming.

That is not a decoration on top of the soil. It is part of the structure, and here is why.

The grains are glued into crumbs

Grains of rock do not stick to each other. Sand poured on sand stays a pile.

What holds a crumb together is alive, or used to be, and there are four things doing it at once.

Fungal threads grow out through the soil in every direction and physically wrap around bundles of grains, the way a net holds a load of shopping. They are not sticking the grains; they are tying them.

Root hairs do the same job on a smaller scale, threading between grains and binding them where they pass.

Bacteria leave sticky substances behind them as they feed, and those cement grains together at the points where they touch.

Earthworms eat soil and pass it out again, and what they leave behind is a ready-made crumb, already bound and already the right size.

So a crumb of soil is not a lump of rock. It is grains of rock held together by living things. Which means it can be taken apart — and once you know that, you can guess what a bare trodden path is.

Two sizes of space, and that is the whole trick

Now the part that actually matters, and it is not about the crumbs at all. It is about the gaps.

When soil is built into crumbs, you get two completely different sizes of space, and each one does a different job.

Small spaces hold water. Inside a crumb the gaps are so narrow that water clings in them and will not drain out, the same way a drop of water stays in the corner of a straw. Gravity pulls, and the water stays. That is the soil's store.

Big spaces carry air. The gap between one crumb and the next is far too wide to hold on to water, so rain runs straight through it and air fills it up behind. That is the soil breathing.

A root needs both, and that is the bit that surprises people. Everyone knows a root drinks. Fewer people know that a root also has to breathe, all the time, with actual oxygen, the same as you do. A plant that is standing in waterlogged ground does not die of too much water. It suffocates, because every air gap is full and nothing is getting through.

Crumbs give a plant a drink and a breath from the same handful of soil. That is the whole point of them.

Dust does neither

Now break the crumbs. Walk on the soil, drive on it, dig it about when it is soaking wet, or simply take away every living thing that was rebuilding the crumbs and wait.

The grains come apart and settle in one even mass, like sugar poured into a jar. Every gap is now the same size, and that size is small.

So the soil can no longer do either job properly. When it rains, every gap fills and there is nowhere for air to go, and the roots are underwater. When it dries, the whole thing shrinks and sets into a slab, and a root cannot push through it at all.

There is a third problem, and it is the one you can see from a distance. Water no longer soaks into a surface like that — it runs off the top instead, and takes the surface with it. That brown water in a ditch after a storm is soil leaving.

Test both handfuls yourself

This one is worth doing, because the result is fast and it is not subtle.

Here is what you should see. The lump from under the hedge sits on the bottom and mostly stays a lump. It may fizz a little as air escapes from inside it, and it may fray at the edges, but an hour later it is still recognisably a lump.

The lump from the path slumps. Often within seconds it lets go completely and rolls out across the bottom of the glass in a cloud, and the water goes cloudy from top to bottom.

You are watching the glue. The lump that held together had something living tying it up. The lump that collapsed had nothing left holding it but the fact that it was dry.

The next time you are standing on a worn path with a hedge beside it, you are standing between two soils that were the same soil not very long ago. One of them is still being held together by things you cannot see. The other one is waiting for them to come back.