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Soil

Soil is not a mixture. It is a structure, and the organisms living in it are what build it.

Mineral particles, organic matter, water, air and living organisms — bound into crumbs rather than lying loose. The structure is built, mostly by roots, fungi and microbes, and it is what makes soil different from ground-up rock.

The question people ask is what soil is made of, and the honest first answer is that the proportions vary so much that quoting a single figure is misleading. A peat soil is almost entirely organic; a desert soil is almost entirely mineral; a fertile loam sits between with a large fraction of its volume as pore space filled with air and water. What all of them share is the list of components rather than the recipe: mineral particles from weathered rock, organic matter in every stage of decay, water, air, and organisms. The part worth understanding is that soil is not a mixture — it is a structure. Mineral particles and organic matter bind into microscopic crumbs, and those bind into larger ones, held by roots, by fungal threads growing through them, and by the sticky products of microbial metabolism. That architecture creates the pore space that holds air and water, and it is why the same material compacted into a solid mass is no longer functioning soil. The organisms build the house they live in. This also supplies the modern answer to why some soil carbon is thousands of years old. It is not made of specially indestructible molecules — the old picture, in which decomposition manufactures stable humic substances, does not survive analysis that examines organic matter in place instead of extracting it with alkali first. What persists is ordinary material that decomposers cannot get at: locked inside aggregates, or bound tightly to mineral surfaces. Persistence is a circumstance rather than a chemistry, which has an uncomfortable corollary. Break the structure apart and carbon that was stable for decades becomes available again, without anything about the molecules having changed.

Early coverage · 58% complete · reviewed 2026-09-04

What this page covers

Soil is a habitat rather than a taxon, and it holds an enormous and incompletely described diversity of bacteria, archaea, fungi, protists and animals — much of it never formally named.

Often confused with: Dead plant material, which is one component and not the largest; A uniform substance with fixed proportions, when soils differ enormously; A passive medium roots sit in, rather than a structure organisms build

Quick facts

No universal recipe
Mineral, organic matter, water, air and life — in wildly varying proportions
Built, not mixed
Crumbs bound by roots, fungal threads and microbial glues
Why old carbon lasts
Not indestructible — unreachable
And why disturbance releases it
Break the aggregates and the enzymes can reach it again

What soil is actually made of

Five components, and no fixed recipe.

The components, and why the proportions are not worth memorising
ComponentWhat it isHow much it varies
Mineral particlesWeathered rock: sand, silt, clayNear-total in a desert soil, nearly absent in peat
Organic matterDead material at every stage of decayA few per cent to almost all of it
WaterHeld in pores and films around particlesChanges by the hour
AirThe same pore space, when not full of waterInversely with water — and its loss is what waterlogging means
OrganismsBacteria, archaea, fungi, protists, animals, rootsEnormous, and mostly undescribed

Diagram

There is no universal recipe

Three soils, and how differently the same five components combine.

There is no universal recipeTemperate topsoilMineralWaterAirDesert soilMineralAirPeat soilOrganicWaterIllustrative proportions. Water and air trade places by the hour as soil wets and drains.
The same explanation in words

Three horizontal bars show the composition of different soils by mineral, organic matter, water and air. A temperate topsoil is mostly mineral with a small organic fraction and roughly half its volume as pore space. A desert soil is overwhelmingly mineral with almost no organic matter. A peat soil is overwhelmingly organic with very little mineral content. A note records that the proportions are illustrative, and that water and air trade places by the hour as soil wets and drains.

Textbook pie charts giving soil as roughly forty-five per cent mineral, five per cent organic and half pore space describe one kind of temperate agricultural topsoil. They are a reasonable starting picture and a poor general fact, and the variation is not noise: it is what distinguishes a peat bog from a chalk grassland.

Soil is not loose particles. Minerals and organic matter bind into crumbs, held together by roots, fungal threads and microbial glues — and the carbon inside those crumbs survives because enzymes cannot reach it.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Soil aggregation is hierarchical: mineral particles and organic matter form microaggregates, bound into macroaggregates by transient agents including roots, fungal hyphae and microbial polysaccharides. Organic matter occluded within aggregates is physically protected from decomposition, and disruption of aggregates increases its availability.

Who this applies to
Structured soils; aggregate hierarchy is better established in some soil types than others.
Studied in
Fungi, Bacteria, Plantae, Animalia
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Aggregate structure and its protective effect are directly observable, and the response to physical disturbance is well documented.

How far it can be extended

Documented across many soils, with much evidence from agricultural systems.

Caveats

  • No single organism builds soil structure; roots, fungi, microbes and burrowing animals all contribute and their relative importance varies.
  • Much aggregate research comes from agricultural soils, whose disturbance regimes are unlike natural systems.

Still unanswered

  • How quickly aggregate structure and its protected carbon recover after physical disruption, which differs greatly between soils.

Last reviewed 2026-09-04

The evidence (2 studies)

Layers, where there are layers

A representative profile, with the caveat that matters.

A representative temperate profile — not a universal one
LayerWhat it isWhat is happening
OOrganic material on the surfaceLitter arriving and decomposing
ATopsoil: mineral mixed with organic matterMost root and biological activity
ELeached layer, where presentMaterial washed downward out of it
BSubsoilReceives what was washed down from above
CWeathered parent materialRock breaking down into mineral particles

Diagram

A representative profile — not a universal one

The layers, with the one that is often absent marked as such.

A representative profile — not a universal oneOLitter on the surfaceATopsoil — mineral plus organic matterELeached layer, where presentBSubsoil — receives what washed downCWeathering parent materialThe dashed layer is often absent entirely, and a deeply mixed soil may show almost no layers.
The same explanation in words

Five layers are stacked from the surface downward. O: litter on the surface. A: topsoil, mineral mixed with organic matter. E: a leached layer, drawn with a dashed outline to mark that it is often absent entirely. B: subsoil, receiving what washed down from above. C: weathering parent material. A note records that the dashed layer is frequently missing, and that a deeply mixed soil may show almost no layers at all.

The caveat is not a formality. Many soils have no E horizon at all; young soils on new surfaces may have almost no profile; a deeply mixed soil — by earthworms, by ploughing, by burrowing mammals — can have its upper horizons blended into one. A diagram showing O-A-E-B-C is a teaching device drawn from a particular kind of temperate forest soil, and reading it as what soil looks like is the commonest error in this subject.

Why some soil carbon is thousands of years old

Not because it is indestructible.

Old soil carbon is not made of indestructible molecules. It persists because decomposers cannot reach it — locked in aggregates or bound to minerals. Break the soil apart and it becomes available again.

Well supported

Good evidence backs this, though some details remain open.

Soil organic matter is a continuum of progressively decomposing fragments rather than a set of distinct stable humic macromolecules. Persistence arises from physical protection within aggregates, association with mineral surfaces and consequent inaccessibility to decomposer enzymes, rather than from intrinsic molecular recalcitrance.

Who this applies to
Soil organic matter generally; the relative weight of each mechanism varies by soil.
Studied in
Bacteria, Fungi, Plantae
Why we rate it this way, and what the caveats are
Well supportedHigh confidence

The shift is well established in the current soil-science literature, though quantifying each protection mechanism in a given soil remains difficult.

How far it can be extended

Supported by spectroscopic and isotopic evidence across soil types, and by the response of soils to physical disturbance.

Caveats

  • The older extraction-based literature is large, and some disagreement about terminology and mechanism continues.
  • Chemistry has not become irrelevant: it influences how readily material is used, just not how long it ultimately survives.

Still unanswered

  • How to quantify the relative contribution of mineral association, aggregate occlusion and other protection in a given soil.

Last reviewed 2026-09-04

The evidence (2 studies)

The older account is worth stating because it is still taught. Decomposition was supposed to build humus — large, dark, chemically distinct molecules, stable because nothing could digest them. Much of the evidence for those molecules came from extracting soil with strong alkali and examining what came out, and methods that look at organic matter in place instead find a continuum of ordinary fragments getting smaller rather than a distinct class of stable substances.

What replaced it is less tidy and more useful. Persistence depends on whether a decomposer can physically reach the material: bound to a clay surface, or sealed inside an aggregate too small for anything to enter. That explains why carbon dates in soil can run to thousands of years for material that is not chemically remarkable, and why ploughing or erosion releases carbon that had been safe for decades.

Related

Claims about this, checked

Things people have heard, and what the evidence actually supports.

The research behind this page

8 studies, newest first. Each one has a page explaining what it found and what it could not show.

2017Nature Ecology & Evolution

A test of the hierarchical model of litter decomposition

Local factors — soil conditions and decomposer community — explained substantially more variation in decomposition than the hierarchical model predicts, and climate alone was a poorer predictor at the scales tested than commonly assumed.

2015Nature

The contentious nature of soil organic matter

Soil organic matter is better described as a continuum of progressively decomposing fragments than as discrete humic macromolecules.

2014Nature

Belowground biodiversity and ecosystem functioning

Soil hosts an extraordinary and incompletely described diversity of organisms.

2009Plant and Soil

Carbon flow in the rhizosphere: carbon trading at the soil–root interface

A substantial share of photosynthetically fixed carbon is released below ground by several distinct processes, of which passive leakage along a concentration gradient is a major one.

2008Science

The global stoichiometry of litter nitrogen mineralization

Decomposing litter initially immobilises nitrogen — taking it from the surrounding soil — when its carbon-to-nitrogen ratio is high, and only begins releasing nitrogen once decomposition has narrowed that ratio sufficiently.

2005Annual Review of Ecology, Evolution, and Systematics

Biodiversity and litter decomposition in terrestrial ecosystems

Mixing litter species frequently produces decomposition rates that differ from those predicted by the species alone, in both directions.

2004Soil and Tillage Research

A history of research on the link between (micro)aggregates, soil biota, and soil organic matter dynamics

Aggregation operates hierarchically: mineral particles and organic matter bind into microaggregates, which are held into macroaggregates by transient binding agents including roots and fungal hyphae.

2004Science

Ecological linkages between aboveground and belowground biota

Above-ground and below-ground communities are reciprocally linked.

This page is a stop on a longer route

A guided journey reads several subjects in a deliberate order, with an argument for why one follows another. You can join in the middle.

Where to go from here

Each of these follows from something on this page — a relationship in the evidence, a claim people ask about, or the next mechanism along.

How complete this page is, and what it is still missing

NatureHQ publishes its own gaps. This page is at 58% completeness against what we would call a finished subject, and was last reviewed on 2026-09-04. It carries 6 claims and answers 4 mapped search questions.

  • no research from the last few years is attached — check for newer work
  • more experiments could be explained in plain English
  • Soil formation — parent material, climate, topography and time — is described only in passing.
  • Soil classification systems are not covered at all, deliberately.
  • Agricultural soil management is out of scope; this page is about what soil is, not how to work it.