Soil

Soil is the thin layer of broken-down rock and organic material that covers most of the Earth's land. It supports almost every plant on the planet, holds up almost every building, filters our water, and stores enormous amounts of carbon. A handful of healthy soil contains more living organisms than there are humans on Earth: bacteria, fungi, insects, worms, mites and many others. Soil takes hundreds or thousands of years to form, but it can be lost in a single bad rainstorm. Without it, life on land could not exist.

  • Time to form100s to 1000s of yearsAbout 1 cm of soil per 100 years
  • Main ingredients4Minerals, organic matter, water, air
  • Layers (horizons)O, A, B, C, RFrom topsoil down to bedrock
  • Organisms per gramApprox. 1 billionMostly bacteria
  • % of soil lost each yearApprox. 1% globallyTo erosion, climate change and farming
  • Best topsoil depthApprox. 30 cmWhere most plant roots live

What soil is made of

Healthy soil has four main ingredients in roughly equal amounts.

  • Minerals (approx. 45%): tiny bits of broken-down rock, sand, silt and clay.
  • Organic matter (approx. 5%): rotting plant and animal remains, plus living organisms.
  • Water (approx. 25%): held in the spaces between the bits of mineral.
  • Air (approx. 25%): also in those spaces.

The layers of soil

If you dig down through soil, you find several distinct layers called horizons.

  • O horizon (top): dead leaves, twigs and other organic matter, sometimes called the litter layer.
  • A horizon (topsoil): dark and rich with nutrients. Where most plant roots live and most farming happens.
  • B horizon (subsoil): lighter, with fewer roots and less life. Often has minerals washed down from above.
  • C horizon: broken-down parent rock, partly weathered.
  • R horizon (bedrock): solid unweathered rock.
Fact A single teaspoon of healthy garden soil contains more living organisms than there are humans on Earth, around 8 billion bacteria plus mites, fungi, nematodes, protozoa and other tiny life. The whole soil ecosystem is essential for keeping plants alive: many plants rely on partnerships with soil fungi (called mycorrhizae) to absorb water and nutrients from the ground.

Pick a topic below to find out more.

What Is Soil?The loose layer covering most of the land. A mix of crumbled rock, dead plants and animals, water, air, and billions of tiny living things.
Layers of SoilA handful of soil from the ground is the top of a stack of layers. The top is topsoil, then subsoil, then weathered rock, then solid rock.
Types of SoilSandy soil drains fast. Clay soil holds water. Loam is a mix of sand, silt and clay, and is the best for growing.
Soil ErosionWhen wind and rain wash topsoil away, often faster than nature can rebuild it. A huge problem for farming around the world.
CompostingTurning dead leaves, food scraps, and grass clippings back into rich soil. Speeds up what nature does naturally.