Layers of Soil

If you dig down through any patch of soil, you will find that it is not all the same. It comes in several different layers, called horizons, each with its own colour, texture and ingredients. Soil scientists name them with letters, from the dead leaves at the top to the unweathered rock at the bottom. Together they form what is called a soil profile: a vertical slice through the soil that tells the story of how the soil formed in that exact place over hundreds or thousands of years.

  • Main horizons5O, A, B, C, R from top to bottom
  • Topsoil depthApprox. 5 to 30 cmWhere most farming and gardening happens
  • Most nutrient-richA horizon (topsoil)Dark with broken-down organic matter
  • Most clayB horizon (subsoil)Often a different colour from above
  • Bottom layerR horizonSolid unweathered bedrock
  • Deepest known profileApprox. 10+ mIn tropical rainforests

The five main layers

  • O horizon (organic layer): the very top. A thin layer of dead leaves, twigs, needles and animal droppings. Slowly rots down into the layer below. Best developed in forests with a lot of leaf fall.
  • A horizon (topsoil): dark and rich, with lots of decomposed organic matter (called humus). Most plant roots grow here. Most farming and gardening happens in this layer. Usually approx. 5 to 30 cm deep.
  • B horizon (subsoil): lighter in colour, less organic matter. Often clay-rich, because clay particles have been washed down from above and settled here. Some deep tree roots reach down into it.
  • C horizon: weathered bits of the parent rock that have not yet broken down completely. Often coarse and stony.
  • R horizon (bedrock): the solid, unweathered rock at the bottom. The very deep foundation everything else rests on.

How the layers form

The layers develop over hundreds or thousands of years.

  1. Bedrock is slowly broken into small bits by frost, water and tree roots.
  2. Lichens, mosses and tough plants colonise the broken rock.
  3. Dead plants pile up on the surface, slowly rotting into the topsoil.
  4. Rainwater carries dissolved minerals down through the soil, sometimes leaving them in the B horizon.
  5. Worms and other soil animals constantly mix the upper layers.

Why each layer is different

Each horizon has a slightly different job to do in the soil ecosystem.

  • The O horizon feeds the soil with constant fresh organic matter.
  • The A horizon is where most plant nutrients live, where seeds germinate and where most roots grow.
  • The B horizon stores extra water and minerals washed down from above.
  • The C horizon is the slow factory that turns bedrock into more soil.
  • The R horizon anchors everything above and is the original source of soil minerals.
Fact The chernozem soils of the steppes of Ukraine and Russia are some of the most fertile in the world, with topsoil up to 1.5 m thick and rich black colour from thousands of years of grass roots adding organic matter. These soils helped feed entire empires. Today the same soils still produce a huge share of the world's wheat and other crops.

How long the layers take to develop

A full soil profile with all five horizons takes a long time to build.

  • 1 cm of topsoil: about 100 years in temperate climates.
  • A noticeable B horizon: 1,000 to 10,000 years.
  • A deeply developed profile: tens of thousands of years.

That is why protecting soil matters so much: lose it and you cannot quickly replace it.

Did you know? Soil scientists can read the history of a place from its layers. Old buried "A horizons" (called palaeosols) can show where forests once grew before being buried by floods or lava. The depth, colour and chemistry of each horizon together tell the story of climate, vegetation and human use, sometimes going back thousands of years.
Deeper dive: how to read a soil profile

Soil scientists (called pedologists, from the Greek for soil) read soil profiles like other people read a book. To study a profile, they dig a pit about a metre deep and look at the wall of the pit. Different things they record:

  • Colour: standardised using a special book called the Munsell Soil Color Chart. Dark colours usually mean lots of organic matter; red colours often mean iron oxides; grey colours suggest waterlogged conditions.
  • Texture: the mix of sand, silt and clay. Different textures hold water differently and feel different in the hand.
  • Structure: how soil particles clump together. Crumbly soil is good for plants; tightly packed soil makes drainage difficult.
  • Roots: how deep and dense they are.
  • Boundary types: gradual or abrupt? Smooth or wavy?
  • pH: acidic, neutral or alkaline? Affects which plants will grow.

From all this, a scientist can usually work out the climate the soil formed under, the original parent rock, what kinds of plants have grown there, how long the soil has been developing, and whether humans have disturbed it. Soil profiles are some of the longest history books in nature, often going back thousands of years.

For more, see types of soil and soil erosion.