Soil & weathering

Dig your hand into a garden bed: the dark, crumbly earth there is not dead matter but the result of thousands of years of work. Soil forms when rock slowly breaks down into tiny grains and mixes with the remains of dead plants and animals. Without soil there would be no forests, no fields and nothing to eat.

From rock to crumb

It all starts with bare rock. Weathering takes it apart piece by piece: water seeps into cracks, freezes in frosty weather and bursts the stone open, because ice needs more space than water. Rain dissolves substances out of the rock, and roots push into gaps and widen them. Dead leaves, roots and animals turn into humus, the dark ingredient of soil that is full of nutrients. This is how soil grows in layers called horizons. Dig a hole and look at these layers from the side and you see a soil profile: the dark humus horizon on top, the lighter subsoil below it, and the parent rock at the very bottom.

What soil does for us

Soil is a sponge and a pantry at the same time. Between its crumbs there are tiny hollows where it stores rainwater and slowly releases it back to plants. Roots draw nutrients from the humus. Soil is also teeming with life: a handful of earth contains more living things than there are people on our planet. Earthworms, fungi and bacteria shred dead material and turn it into fresh humus.

Protecting the soil

Soil grows extremely slowly: nature often needs about a hundred years for a single centimetre. It can disappear very quickly though. When wind and rain hit bare earth, they carry the precious humus layer away. This is called erosion. Plants are the best protection: their roots hold the soil like a net, and their leaves slow the rain down. That is why farmers keep hedges, cover their fields with plants in winter too, and plough across slopes instead of downhill.