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Soil and life

Soil Structure

Terra Humana editorial teamLast updated: 2026-10-126 min read

Crumbly, living soil next to a dense, smooth clod of compacted earth

In brief

The way soil particles are arranged into crumbs and the pores between them. It decides how a soil breathes, takes in water and lets roots through.

What is soil structure?

Pick up a handful of good garden soil and it falls apart into crumbs: small, rounded, porous lumps of different sizes. Take a handful from a compacted field track and you hold a dense, sharp-edged block. Both may consist of the same sand, silt and clay. What separates them is structure: the way the particles are assembled into larger units, and the spaces left between them.

Soil science describes these units by shape. Crumb and granular structure consists of small, rounded, very porous aggregates, typical of topsoil rich in life. Blocky structure forms angular or rounded blocks, platy structure forms horizontal plates, prismatic and columnar structure forms upright pillars. At the extremes are soils without structure: loose single grains, as in dune sand, or one coherent dense mass.

The pores matter as much as the particles. Large pores let water drain quickly and bring air to the roots. Medium pores hold the water that plants can actually use. The finest pores hold water so tightly that no root can extract it. A well-structured soil has all three sizes, so that after rain it can drain and breathe and still keep a reserve.

Humus is the material,,structure the buildingLearning panel: humus is the material, structure the building. Soil structure describes how soil particles are assembled into crumbs, with pores between them. Good garden soil falls apart into small, rounded, porous crumbs; a compacted field track is a dense, sharp-edged block. Both may consist of the same sand, silt and clay. The builders are living organisms: clay and long-lasting organic matter hold the smallest aggregates together, roots and fungal threads enmesh them like a net, bacteria and fungi glue them with their secretions, and earthworms deposit stable casts. This is why a soil can contain humus and still be badly structured. Three forces destroy structure most often: rain on bare ground, which bursts crumbs and leaves a crust; pressure from wheels, hooves and feet, most of all when the soil is wet; and frequent tillage, which breaks crumbs and fungal threads. Glass test: crumbs that living things have glued together keep their shape in water, often for hours; crumbs held together only by drying collapse within seconds. Lost structure takes seasons to return, not days. TERRAWIKI · LEARNING PANEL Humus is the material,structure the building How soil particles are assembled into crumbs, with pores between. 1SAME MATERIAL, DIFFERENT BUILDING Crumbs good garden soil: small,rounded, porous lumps Block compacted field track:dense and sharp-edged Both may consist of the same sand, siltand clay. 2THE BUILDERS ARE LIVING ORGANISMS clay and organic matter holdthe smallest aggregatesroots and fungal threadsenmesh them like a netbacteria and fungi gluethem with secretionsearthworms depositstable casts This is why a soil can contain humusand still be badly structured. 3HOW STRUCTURE IS LOST Rain on bare groundbursts crumbs,a crust forms Pressure wheels, hooves,feet, most of allwhen the soilis wet Tillage when frequent,it breaks crumbsand fungalthreads Glass test: glued crumbs keeptheir shape in water, oftenfor hours. Crumbs held only bydrying collapse in seconds. gluedonly dried Lost structure takes seasons to return, not days. Basis: Tisdall & Oades, Journal of Soil Science 33 (1982) · TerraWiki: Soil Structure

Structure and humus: material and building

Soil structure is easily confused with humus, because the two usually appear together. But they are different things. Humus is a substance, the dark and stable part of the organic matter. Structure is an architecture, the way all the components, humus included, are put together. Humus is one of the most important building materials; structure is the building.

The builders are living organisms. Research since the 1980s has shown that crumbs are built in stages. The smallest aggregates are held together by clay and long-lasting organic matter. Larger crumbs are bound by roots and fungal threads, which enmesh particles like a net, and by the sticky secretions of bacteria and fungi. Mycorrhizal fungi play a particular part here. Earthworms pass soil through their gut and deposit it as stable casts. This is why a soil can contain humus and still be badly structured, if pressure or tillage has destroyed what the organisms built.

How structure is lost

Three forces destroy it most often. The first is rain on bare ground. Raindrops strike unprotected crumbs with surprising force, burst them, and wash the fine particles into the pores. When the surface dries, it forms a hard crust, and the next rain runs off instead of soaking in. The second is pressure: wheels, hooves and feet squeeze pores together, most of all when the soil is wet. The third is frequent tillage, which breaks aggregates and fungal threads and exposes the organic glue to faster decay.

How stable the crumbs of a soil are can be seen with a simple glass of water. Drop a few dry, pea-sized crumbs gently into the water and watch. Crumbs that living things have glued together keep their shape for a long time, often for hours. Crumbs held together only by drying collapse within seconds into a small heap of mud. Soil scientists measure the same thing more precisely as aggregate stability, and it is one of the most sensitive indicators of how a soil is being treated.

Structure that has been lost returns only as fast as living things can rebuild it. That takes seasons, not days.

How Terra Humana understands soil structure

For Terra Humana, structure is the point where soil life becomes visible. Nobody can see bacteria at work, but anyone can see crumbs. A slope whose soil crumbles, drains and holds water is a slope whose underground community is working.

From this follows a simple order: protect first, build second. Every crumb that a raindrop does not destroy is worth more than one that has to be rebuilt. That is why on dry slopes ground cover comes before every other measure, and why we dig as little as possible. Roots, fungi and earthworms build structure more patiently and more durably than any tool.

The valley of Sa Medada near Jerzu, Sardinia, with olive grove and macchia

Sa Medada

What this looks like at Sa Medada

At Sa Medada the dry season runs from May to September, and the rain of the cooler months then falls on ground that has been exposed to the sun for a long time. Bare soil seals up under heavy rain: its crumbs burst and the surface closes. That is why Project Eden begins with pioneer plants, ground cover and holding water, so that the crumbs are shielded from the impact of the drops and roots can keep the pores open.

More about Project Eden

Frequently asked

What is soil structure?

Soil structure is the way the solid particles of a soil, sand, silt, clay and organic matter, are arranged into larger units, and the pores that remain between them. Soil scientists distinguish several forms: crumbly and granular, blocky, platy, prismatic, and structureless soils that are either loose single grains or one dense mass. Structure matters because roots, water and air all move through the pores. Two soils with exactly the same composition can behave completely differently depending on how their particles are put together.

Is soil structure the same as humus?

No, though the two are closely linked. Humus is a substance: the dark, stable part of the organic matter in soil. Soil structure is an arrangement: how all the particles, humus included, are built into crumbs with pores between them. Think of humus as one of the most important building materials and of structure as the building. Humus helps glue mineral particles into stable crumbs, and soil life does the gluing. A soil can contain humus and still be badly structured, for instance after compaction by heavy machinery.

How can you recognise good soil structure?

With a spade and your hands. Good structure crumbles when you lift a clod and let it fall: it breaks into rounded, porous crumbs of different sizes rather than into sharp-edged blocks or flat plates. Roots run through it in all directions, earthworm channels are visible, and the soil smells of forest floor. Warning signs are horizontal plates, a dense layer that roots bend away from, standing water after rain, or a hard surface crust that forms on bare ground after heavy showers.

What destroys soil structure?

Above all three things. Heavy rain on bare ground breaks crumbs apart, and the fine particles wash into the pores and seal the surface. Pressure from machines, hooves or feet squeezes pores together, especially when the soil is wet. And frequent tillage breaks up aggregates and fungal threads and speeds the loss of organic matter that holds the crumbs together. In each case water then runs off instead of soaking in, and the soil dries out and erodes more easily.

How can soil structure be improved?

By letting the soil build it itself. Permanent cover protects crumbs from rain and sun. Many living roots and their fungal partners bind particles together and leave channels behind when they die. Organic matter on the surface feeds earthworms and microbes, whose secretions act as glue. Avoiding traffic on wet ground prevents new damage. Rebuilding structure takes time, but every season with living roots and covered ground moves it in the right direction.

Water drops beading on parched soil instead of soaking inNext in this themeSoil Water RepellencyA soil surface that repels water after fire or severe drying. Rain beads up and runs off instead of soaking in.

Related terms

Hands holding dark, crumbly soil with fine rootsHumusThe dark, stable fraction of organic matter in soil, the part that holds water and nutrients and gives a living soil its colour, smell and structure.A cut through living topsoil with roots, white fungal threads and an earthworm beneath a layer of leavesSoil Food WebAll the organisms living in soil, from bacteria and fungi to mites and earthworms. Together they digest dead matter and turn it into food for plants.Deep tractor ruts in a wet field where rainwater stands instead of soaking inSoil CompactionThe loss of pore space through pressure, from machines, hooves or feet. Compacted soil lets less water and air in and blocks roots.Close-up: white fungal threads wrapping fine tree rootsMycorrhizaThe underground partnership between roots and fungi: the fungi reach water and phosphorus, the plant returns sugars and vitamins.Hands holding porous pieces of biochar above dark soilBiocharA stable, charcoal-like material made by heating plant matter with little oxygen, mixed into soil to improve its structure, water-holding and living activity.Water drops beading on parched soil instead of soaking inSoil Water RepellencyA soil surface that repels water after fire or severe drying. Rain beads up and runs off instead of soaking in.

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Sources

  • FAO, Guidelines for Soil Description, 4th edition (2006)
  • Judith Tisdall, J. Malcolm Oades, Organic matter and water-stable aggregates in soils, Journal of Soil Science 33 (1982)
  • Johan Six et al., A history of research on the link between (micro)aggregates, soil biota, and soil organic matter dynamics, Soil and Tillage Research 79 (2004)
  • Matthias Rillig, Daniel Mummey, Mycorrhizas and soil structure, New Phytologist 171 (2006)
  • Carol Bronick, Rattan Lal, Soil structure and management: a review, Geoderma 124 (2005)

Last updated: 2026-10-12