Why Soil Is Not Just Crushed Rock

Soil is not just crushed rock because rock is only one of its ingredients, and it is not the one doing most of the interesting work. Soil is a mixture of mineral grains, air, water, and the decaying remains of plants and animals, and it is held together and kept breathing by living things: worms, fungi, bacteria and many creatures too small to see. Crush a boulder to powder and you have the mineral part. You do not have the rest, and the rest is what makes a spoonful of garden dirt behave so differently.

I came to this slowly, in the way I come to most things, by standing in the wet and looking. I keep a small notebook of puddles, and over a few winters I noticed that rain behaves differently on different ground. On the path it pools and sits. On the vegetable bed it vanishes with a soft sigh. On a patch of bare clay at the bottom of the hill it slides about like oil on a pan. The water is the same each time. The ground is what changes, and it turns out the ground is far more than rock.

What is actually in a spoonful of garden soil?

Take a level spoonful of good garden soil from a bed that has been looked after, one that crumbles in your fingers and smells of the woods after rain. Roughly speaking, and the figures vary a great deal from place to place, about half of it by volume is solid material and the other half is empty space. Of the solid half, most is mineral grains and a small fraction, often a few per cent, is organic matter. The empty half is filled with some mix of air and water that changes by the hour, depending on when it last rained.

Four ingredients, then:

• Mineral grains: the broken-down rock, in sizes from visible sand to particles too small to see.
• Air: held in the gaps between grains, and needed by roots and most soil organisms.
• Water: clinging in thin films around grains and filling the smaller gaps.
• Organic matter: the remains of leaves, roots and creatures, in every stage from "still recognisably a leaf" to a dark, crumbly material called humus.

On top of these sits the fifth thing, which is not an ingredient so much as a population. A teaspoon of healthy soil holds a staggering number of living organisms, mostly bacteria. Scientists are still working out how many kinds there are, and I will come back to that at the end.

Where the mineral part comes from

Start with the rock, because that really is where it begins. Rock breaks down in two ways, and they work together.

The first is physical. Water seeps into cracks and freezes, and because water expands as it turns to ice, it pries the crack wider. Roots push into fissures. Temperature swings make the surface expand and contract. Slowly, a solid face becomes a skirt of fragments.

The second is chemical, and it is the quieter and more transformative of the two. Rainwater picks up a little carbon dioxide on its way down and becomes very weakly acidic. Over long stretches of time it reacts with minerals, and some of them change into new ones. Feldspars, which are common in many rocks, slowly turn into clay minerals. That matters, because clay is not simply "very fine sand". It is a different material, with a different structure.

Crushing a rock with a machine copies the physical part only. The chemistry, the slow rinse of rain over centuries, is what makes the finest, most chemically active part of soil.

Sand, silt and clay: the same word for very different sizes

Soil scientists sort mineral grains by size. The usual boundaries, which differ a little between classification systems, are these:

• Sand: from about 2 mm down to about 0.05 mm. You can see and feel individual grains, and they feel gritty.
• Silt: from about 0.05 mm down to about 0.002 mm. Too small to see grain by grain, it feels smooth, like flour.
• Clay: finer than about 0.002 mm. It feels sticky when wet and can be rolled into a ribbon.

A clay particle is to a sand grain roughly as a pea is to a beach ball, and the difference is not just size. Clay minerals tend to be thin, sheet-like crystals with a great deal of surface for their volume, and their surfaces carry a small electrical charge. Many of the nutrients plants need, such as calcium, magnesium and potassium, are present in soil water as charged particles called ions. The charged surfaces of clay and humus hold on to these ions loosely, like a pantry shelf, and release them when a root needs them. Sand has very little of this. Nutrients wash straight past it.

This is one reason a cup of fine sand and a cup of garden soil, though both are "ground-up rock" in part, are different materials to a plant. One is a sieve, and the other is a sieve with a pantry attached.

Why air and water matter as much as the grains

Here is a thought experiment. Imagine filling a jar with perfectly uniform marbles. They cannot pack perfectly, so there are gaps between them, and water poured in would run through quickly. Now imagine filling another jar with marbles of many sizes, where small ones tuck into the gaps left by large ones. There is less empty space, and what there is has narrow, winding passages. Water moves slowly and some of it stays put.

That is a fair picture of the difference between coarse sand and a mixed soil, and it shows something people often get wrong. The grains are only half the story. What matters just as much is the space between them, because that space is where roots grow, where air moves, and where water is held.

Water in soil is held in two ways. Some drains away under gravity through the larger gaps. Some is held tightly in the finer ones, as thin films around grains and in tiny pores, by the same surface forces that make water climb a paper towel. Fine soils, rich in silt and clay, hold more water against gravity. Sandy soils let it go. A good garden soil has a mixture of large and small spaces, so water can drain and be kept at the same time, and air can follow the draining water in behind it.

Now take the other thing I noticed on the bottom of the hill, where bare clay slid about like oil on a pan. Clay particles are so small and flat that, with nothing to hold them in clumps, they pack tightly and leave almost no large pores. Rain cannot get in, so it runs off. Crushed rock fines behave the same way: pour water onto rock dust and it often seals into a cake. A soil with life in it is full of passages, and I will get to who dug them.

What the living things do

This is the part that sets soil apart from any heap of powder, and I think it is the most delightful.

Bacteria are the most numerous. They break down dead material and release the nutrients locked inside it. Some live around plant roots and trade with them. Some turn nitrogen from the air into forms plants can use. That last point is worth pausing on, because rock contains almost no nitrogen to speak of. Nearly all the nitrogen in soil ultimately comes from the atmosphere, and it arrives through living things. You can grind rock for a thousand years and never produce the nitrogen a leaf needs. The smell of damp earth after rain, by the way, comes in large part from a compound called geosmin, made by certain soil bacteria. It is the smell of the living part, and we are remarkably good at noticing it.

Fungi grow as fine threads that thread through the soil and weave around roots. Many plants form partnerships with them, in which the fungus reaches into tiny spaces and gathers water and nutrients while the plant provides sugars made in its leaves. The threads also act as a kind of net, binding crumbs of soil together.

Worms are the ones we can see. In 1881 Charles Darwin published a whole book about earthworms, written after decades of watching them. He was convinced that they were moving remarkable amounts of soil over time, and in the broad strokes he was right. Worms eat their way through soil and organic matter, mixing the two, leaving behind casts that are rich and crumbly, and leaving tunnels that let air and water travel deep. Each tunnel is a little road for rain.

There are plenty of others, such as tiny mites, springtails, beetle larvae and nematodes, and each has its own job in the slow business of turning a fallen leaf back into soil.

How a crumb is built

If you take a handful of good soil and gently squeeze it, it breaks into small, rounded crumbs rather than powder or a sticky lump. Soil scientists call these crumbs aggregates, and they are among the most important things in the soil, because they create the large and small pores I described.

Aggregates form when mineral grains are glued together. The glue comes from living things: sticky substances released by bacteria and by plant roots, the threads of fungi winding around the grains, and the humus left behind by decay. Worm casts are aggregates too. A freshly dug garden bed that has been well fed with compost over a few seasons is full of them, and it shows in the way it falls from the fork.

I should be honest that I am still learning the finer points of this. The details of exactly how different organisms and compounds bind soil together are an active area of study, and I would rather say so than pretend to a tidiness the subject does not have. What I can say with confidence is the broad picture: living things build the structure, and the structure is what lets a soil hold air and water together.

So a spoonful of rock flour and a spoonful of garden soil differ in at least four ways. The rock flour has no organic matter, no nitrogen to speak of, no community of organisms, and no crumb structure. Its grains may be chemically the same as some of the grains in the garden soil. Everything else is different.

Try a jar test at home

You can see a good deal of this for yourself with a jar, some water and a little patience. This test sorts a soil by grain size. It gives a rough idea rather than a precise result, and it is the kind of thing that rewards doing twice.

You will need: a clear, straight-sided jar with a lid (a tall one is best), a trowel, a ruler, a marker pen or strips of tape, and a teaspoon of dishwashing liquid if you have it. The detergent helps clumps break apart, though it is optional.

Method:

1. Dig a sample from a few centimetres below the surface, avoiding fresh compost and surface litter. Pick out stones and roots, and crumble any lumps.
2. Fill the jar about one third full with soil.
3. Add water until the jar is nearly full, and add the detergent if you are using it. Put the lid on firmly.
4. Shake for a good few minutes, until no clumps remain. Then set the jar on a level surface where it will not be knocked.
5. After about a minute, mark the level of the settled material on the outside. That is mostly sand, which falls fastest.
6. After a couple of hours, mark the new, higher level. The new layer is mostly silt.
7. After a day or two, mark the level again. The finest layer on top is clay, and it may take longer. The water may stay a little cloudy even then.

You will probably see a layer of dark, light flecks floating on top. That is organic matter, which is less dense than water, so it rises rather than sinking.

Measure each layer and compare it with the total depth of settled soil. That gives you a rough proportion of sand, silt and clay. A soil with a fairly even mixture of the three is what gardeners call a loam, and a thick bottom layer means sandy soil, while a thick top layer means clay. Do not worry if your jar looks nothing like a textbook picture. The layers blur into each other, and the clay can be stubborn. My first attempt, years ago, looked like muddy soup for three days, and I wrongly decided I had failed. I had only been impatient. Soil is not in a hurry.

For a comparison, do the same with a spoonful of builder's sand or fine gravel dust from a garden shop, or with fine sand from the beach. It will settle almost at once into a clean, gritty layer with no cloud, no floating specks, and no smell.

You can also compare how each behaves when wet. Put a spoonful of each into a cup with holes, or a small funnel lined with a coffee filter, and pour a little water over each. Notice how fast the water passes through, how much it holds on to, and whether the surface seals or stays open.

Why the grains sort themselves in the jar

The jar test works because of something physicists worked out long ago. A small particle falling through water speeds up until the drag from the water balances its weight, and after that it falls at a steady pace. For small particles, that pace depends strongly on size: it goes with the square of the particle's diameter. A grain ten times smaller falls about a hundred times more slowly. The rule has a name, Stokes' law, and it holds best for fine particles like silt and clay. For larger sand grains it is only approximate, but the ordering still holds.

So if a bit of silt takes a few hours to settle through the water in your jar, a clay particle a tenth its size takes about a hundred times as long, which is why that cloudy water hangs about for days. A little physics from the study of fluids ends up telling you what is in your garden. I find that quietly wonderful: the same idea that explains how mud settles in a puddle also explains how a river sorts its stones, with the coarsest dropped first and the finest carried furthest.

Something to wonder about

I mentioned that scientists are still working out how many kinds of organisms live in soil. That is true, and it is a strange thing to be able to say. We have mapped the surface of the Moon in detail, yet a single teaspoon of earth contains a community that has only been partly described. Many of the bacteria in soil have never been grown in a laboratory, and researchers are still finding out what they do and how they depend on one another.

So next time it rains, perhaps on a cold grey morning when the hillside is dark and the air smells of leaf mould, you might crouch down and look at the ground for a moment. Somewhere beneath your boots, rain is moving through passages dug by creatures you will never see, past grains that were rock a very long time ago, towards roots that are waiting for it. How much of that world is still unnamed? And what might the next jar of soil on your kitchen bench have to tell us?