Pick up a handful of soil from a garden and you are holding four things at once: tiny broken pieces of rock (minerals), the rotted remains of plants and animals (organic matter), water, and air. Soil is that mixture, the thin, loose layer covering much of the land, and it is where almost all of our food grows. It forms very slowly, as rock is broken down by weather and living things; a layer of topsoil just 2.5 cm (1 inch) thick can take 500 to 1,000 years to build up. Dig a deep hole and you will see soil arranged in layers called horizons, labelled O, A, B and C. Soils also differ in the size of their grains, from coarse sand to fine silt and sticky clay, and from place to place, from the black chernozem of Ukraine to the red laterite of the tropics. And when people treat soil badly, it can blow away, as it did in the American Dust Bowl of the 1930s.
What a Handful of Garden Soil Contains
Soil scientists have measured what a typical, healthy soil is made of. About half is solid and about half is empty space. The solid half is mostly minerals, with a small but very important share of organic matter. The empty spaces, called pores, are usually shared between water and air.
The water and air matter as much as the solids. Roots need air to breathe, just like you do, and they drink water from the pores. When heavy machines squash soil flat, the pores close up and plants struggle to grow.
Soil is also alive. A single gram of soil, a small pinch, can hold billions of tiny living things, mostly bacteria, belonging to thousands of different species. Many of them have never been studied. They break down dead leaves and turn them into nutrients that new plants can use.
From Bare Rock to Topsoil: 500 to 1,000 Years per 2.5 Centimetres
All soil starts as rock. The rock underneath a soil, or the sand and mud dropped by rivers and glaciers, is called the parent material. Over time it is broken down by weathering. Physical weathering cracks rock into smaller pieces, for example when water freezes in a crack and expands, or when a rock heats up by day and cools at night. Chemical weathering slowly dissolves or changes the minerals, as rainwater does to limestone. You can see how each kind works in our guide to how weathering works.
Then living things move in. Lichens and mosses grow on the rock, die and rot. Roots push into cracks. Worms, insects, fungi and bacteria mix the dead material into the broken rock, and slowly the mixture becomes soil.
Scientists say five things decide what kind of soil forms: the climate, the living organisms, the shape of the land or relief, the parent material and time. A warm, wet place breaks rock down quickly, but a cold, dry place may take far longer. That is why topsoil is so precious: it is usually only 13 to 25 cm deep, and replacing 2.5 cm of it can take 500 to 1,000 years.
Reading the O, A, B and C Horizons of a Soil Profile
If you dig a deep pit and look at its side, you will see soil in bands of different colours. A cut like this is called a soil profile, and each band is a horizon. Scientists label them with capital letters, from top to bottom:
- O horizon (organic): a thin top layer of fallen leaves, twigs and rotting plant material. In a forest you can feel it crunch underfoot.
- A horizon (topsoil): usually the darkest mineral layer, because it is mixed with humus, the dark, crumbly material left when dead plants rot. Most roots and soil animals live here.
- B horizon (subsoil): lighter or more colourful. Water trickling down carries clay, iron and other substances out of the topsoil and leaves them here. A well-developed B horizon is the sign of a truly mature soil.
- C horizon: loose, broken-up parent material that has barely changed into soil yet.
- R: the solid bedrock underneath.
Not every soil has every layer. A young soil on a new lava flow may be only a few centimetres of A horizon sitting straight on rock, while an old soil in the tropics may be many metres deep.
Sand, Silt, Clay and Loam: Sorting Soil by Grain Size
The mineral part of soil is sorted by grain size. Soil scientists only count grains smaller than 2 millimetres; anything bigger is gravel or stone.
- Sand grains are the biggest, from 0.05 mm up to 2 mm. You can feel them as gritty. Sandy soil drains fast, so it dries out quickly and nutrients wash away.
- Silt grains are 0.002 to 0.05 mm, too small to see one by one. Silt feels smooth and floury.
- Clay grains are the smallest, under 0.002 mm, and shaped like flat plates. Clay feels sticky when wet and goes hard when dry. It holds lots of water, sometimes too much.
Most soils are a mix. The United States Department of Agriculture names 12 soil textures, from "sand" and "loamy sand" to "silty clay" and "clay", depending on the share of each grain size. The one gardeners love most is loam, roughly 40% sand, 40% silt and 20% clay. It holds enough water without getting soggy and usually has more nutrients and humus than sandy or clay soils. Try this at home: wet a pinch of soil and rub it between finger and thumb. Gritty means sandy, silky means silty, and sticky means clay. There is much more on this in our guide to the types of soil.
Earthworms and Charles Darwin's 1881 Book
The best-known helpers in the soil are earthworms. As they tunnel, they let air and water in. They eat soil and dead leaves and leave behind droppings called casts, which are rich in nutrients. The common earthworm, Lumbricus terrestris, is usually 11 to 20 cm long, and in much of Europe it is the largest native earthworm.
The naturalist Charles Darwin was fascinated by worms for most of his life. His very last scientific book, published in 1881, was about them. It was called The Formation of Vegetable Mould through the Action of Worms; "vegetable mould" was the old name for topsoil. At his home, Down House in Kent, Darwin spread broken chalk over a pasture field in 1842 and, in a 29-year experiment, measured how deep worm casts buried it. He calculated that there were 53,767 earthworms in a single acre of land. He even kept worms in pots indoors and tested what they liked to eat; raw carrots were their favourite. He showed that strong light sent them back into their burrows, but heat and sound had no effect.
Darwin also visited Stonehenge and Roman ruins in England, and showed that worms had slowly buried fallen stones and old floors by piling up casts around them. His big idea was that tiny changes, repeated for a very long time, can make huge differences.
Chernozem, the Black Earth of Ukraine
Some soils are prized for their richness. The best known is chernozem, from the Russian words for "black" and "earth". It is black because it is full of humus, between 4% and 16%, much more than most soils. It formed under deep-rooted grasses on wide, open plains called steppes, where roots died and rotted year after year.
The first scientist to study chernozem in detail was Vasily Dokuchaev, who surveyed the steppe soils of the Poltava region in 1883. He is often called the father of soil science. Today, as much as two-thirds of Ukraine's land is black earth, which is a big reason the country is called the "Breadbasket of Europe" for its wheat and other grains. In parts of Ukraine the black layer is about 1.5 metres thick. Chernozem covers about 230 million hectares worldwide, mostly in two belts: one across the steppes of Ukraine, Russia and Kazakhstan, and another from the Canadian Prairies to Kansas in the United States.
Laterite, the Red Soil Cut into Bricks at Angkor
In hot, wet parts of the world, soil can turn a rusty red. This is laterite. Heavy rain and heat break the rock down so completely, over such a long time, that most minerals are washed away. What remains is rich in iron and aluminium, and the iron gives it its red colour, much like rust.
In 1807, a Scottish surgeon and botanist named Francis Buchanan-Hamilton described a laterite in southern India. He named it from the Latin word later, meaning "brick", because the hard, packed material could be cut into blocks for building. People had been doing exactly that for centuries. After about 1000 AD, the builders of Angkor Wat and other temples in Cambodia used laterite, along with brick and sandstone, in their walls and enclosures. In Madhya Pradesh, India, a cap of laterite on top of a plateau is 30 metres thick.
The Dust Bowl of the 1930s: When Soil Blew Away
Soil takes centuries to form but can vanish in a few years. The best-known example is the Dust Bowl on the Great Plains of the United States and Canada. In the 1920s, a run of wet years and new tractors led farmers to deep-plough huge areas of prairie grassland for crops. The deep grass roots that had held the soil in place for thousands of years were gone. Then came terrible droughts, in 1934, 1936 and 1939 to 1940. The dry, bare topsoil was picked up by the wind.
Giant dust storms, called "black blizzards", rolled across the land. One of the worst came on 14 April 1935, a day remembered as Black Sunday. It is estimated to have moved about 300,000 tons of topsoil. Between 1930 and 1940, about 3.5 million people moved out of the Plains states, many of them heading west to California. In August 1933 the US government set up the Soil Erosion Service, led by Hugh Hammond Bennett, which became the Soil Conservation Service. The government taught farmers contour ploughing, strip farming and crop rotation, and the Civilian Conservation Corps planted the Great Plains Shelterbelt, more than 200 million trees from Canada to Abilene, Texas, to break the wind and hold the soil in place.
The lesson still matters. Every 5 December is World Soil Day, a reminder that the ground beneath our feet is a living, precious resource. To see how wind and water carry soil away, and what people do to stop it, read our guide to erosion for kids.