Nature · Earth Science
The living skin of the planet
Soil is the thin, living layer of loose material covering the land, made of weathered rock, water, air and decayed organic matter. It anchors plants, stores water and carbon and teems with life — a single handful can hold more organisms than there are people on Earth.
Soil forms painfully slowly as rock is broken down by weather, water and living things, and mixed with humus from decaying plants and animals. Over centuries it settles into distinct layers, or horizons, whose character depends on the parent rock, climate, organisms, slope and time.
From the organic topsoil down through subsoil and weathered rock to solid bedrock — the layered profile geologists call horizons.
Coarse, light and fast-draining
Made of large particles with big gaps between them, sandy soil drains freely and warms quickly, but water and nutrients wash straight through it.
Fine, heavy and moisture-holding
Clay's minute particles cling together, trapping water and nutrients. It is fertile but heavy, waterlogging when wet and cracking hard when dry.
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The gardener's perfect balance
Loam blends sand, silt and clay in near-perfect proportion, combining good drainage with moisture and nutrient retention — the most prized soil for farming and gardens.
Dark, spongy and carbon-rich
Formed in waterlogged bogs where plants decay only partly, peaty soil is dark and spongy, holding water like a sponge and locking away enormous stores of carbon.