What Is Soil?

Soil is the thin layer of broken-down rock, decaying plant and animal matter, water and air that covers most of Earth's land. It is one of the most important things on the planet: almost every land plant grows in it, almost every building stands on it, and almost every drop of fresh water has filtered through it. Soil takes hundreds or thousands of years to form (about 1 cm every 100 years) but can be washed away in a single bad rainstorm. A single handful of healthy soil contains more living organisms than there are humans on Earth.

  • Time to form100s to 1,000s of yearsApprox. 1 cm every 100 years
  • Main ingredients4Minerals, organic matter, water, air
  • Organisms per gramApprox. 1 billionMostly bacteria, fungi, mites
  • Most fertile soilLoamMix of sand, silt and clay
  • % of land coveredApprox. 80%Only the very cold or rocky bits have no soil
  • Deepest soilTropical rainforestCan be over 10 m deep

What soil is made of

Healthy soil has four main ingredients in roughly equal amounts.

  • Mineral particles (approx. 45%): tiny bits of broken-down rock. The three sizes are sand (biggest), silt (medium) and clay (smallest).
  • Organic matter (approx. 5%): partly rotted plant and animal remains. The dark stuff that gives soil its rich brown colour.
  • Water (approx. 25%): held in the spaces between the mineral particles, where roots can drink it.
  • Air (approx. 25%): also in those spaces, providing oxygen for roots and soil organisms.

How soil forms

Soil starts as solid rock. Over very long periods, weather slowly breaks the rock down.

  • Frost: water seeps into cracks, freezes, expands and shatters the rock.
  • Rain: dissolves some minerals and washes others away.
  • Wind: blows fine particles around and sand-blasts exposed rock.
  • Plants: roots grow into cracks and prise rocks apart, while dying leaves add organic matter.
  • Microbes: bacteria and fungi break down dead matter into soil-friendly chemicals.

Building 1 cm of soil takes roughly 100 years in a temperate climate. A really deep soil profile (1 m or more) may have taken 10,000 years to form.

The hidden life in soil

Soil looks like dead stuff, but it is bursting with life. A single teaspoon of healthy garden soil contains about a billion bacteria, plus fungi, nematodes, mites, worms, beetles and many other small creatures. They:

  • Break down dead leaves and animals.
  • Recycle nutrients so new plants can use them.
  • Mix and aerate the soil (especially earthworms).
  • Form partnerships with plant roots (especially mycorrhizal fungi, which help most plants absorb water and nutrients).
Fact The world's land surface holds more carbon in its soil than in all the world's plants and the entire atmosphere combined. Soil is one of the planet's biggest natural carbon stores, and looking after it (rather than ploughing or eroding it) is one of the most important things we can do to slow climate change.

Why soil matters

  • Food: 95% of the world's food is grown in soil.
  • Water: soil filters rain on its way to rivers and underground stores.
  • Carbon storage: soil holds vast amounts of carbon and helps regulate the climate.
  • Habitat: home to roughly a quarter of all species on Earth.
  • Building: every building, road and farm depends on stable soil.
Did you know? A single earthworm can move its own body weight in soil every day. A typical British meadow contains approx. 2.5 million worms per hectare, together moving tonnes of soil each year. Charles Darwin spent decades studying earthworms and wrote a whole book about them in 1881, arguing they were the most important small creature in shaping the landscape.
Deeper dive: the soil food web and "wood wide web"

Most plants do not live alone underground. Almost all land plants form partnerships with special fungi in the soil called mycorrhizae. The fungi wrap around (and sometimes inside) the plant's roots, vastly extending the area the roots can reach. The fungi share water and minerals from a much larger volume of soil than the roots could ever reach alone; in return, the plant shares sugars from photosynthesis.

Even more remarkably, these fungal networks often link many plants together. A single network can span dozens or hundreds of trees across a forest. Scientists have shown that plants can actually send signals and even sugars to each other through the network. Big "mother trees" feed sugar to young saplings struggling in the shade. Trees being attacked by insects send chemical warnings to their neighbours through the fungus, and those neighbours start producing defensive chemicals before the insects arrive.

The system is sometimes called the "wood wide web". It changes how scientists think about forests: not as collections of separate trees competing for light, but as connected communities sharing resources through the soil. Healthy soil with healthy fungi is essential for healthy forests, which is one more reason that protecting the soil matters so much.

For more, see layers of soil, types of soil and composting.