Soil is a vital part of the natural world. It consists of fragments of rock mixed with humus. Humus is the rotting remains of leaves, animal droppings and other organic matter. Between the soil particles are large air spaces. These allow water to drain through the soil. They also allow oxygen to reach both the plant roots and the living things in the soil. Nutrients that are essential to plant life, such as nitrates and phosphates, are also found in the soil.

Soil layers

A slice through the ground reveals the different soil layers, called horizons. At the top is leaf litter: old leaves, twigs and feathers. Below is the topsoil. This layer is rich in organic matter: dead and decaying remains of plants and animals. Home to soil creatures, it also contains the roots of small plants.

Soil contains a mass of living things, most of which are too tiny for us to see. But they perform a vital task: recycling the remains of dead animals and plants into nutrients. These nutrients are then taken up by the roots of living plants. The tiniest of the workers are bacteria. A handful of soil will contain over 10 billion of them—more than there are people living in the world.

Earthworms also have a vital role to play. They tunnel through the soil, letting air in, helping water drain through and mixing up the different soil layers. Insects, spiders, mites and centipedes hunt in the soil, and larger animals like moles and voles make their burrows there.