An exposed area of soilSoil 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.
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.
Below the topsoil is the subsoil. This has less organic matter, and more and larger rock fragments. The roots of bushes and trees grow into the subsoil. Going deeper into the subsoil, the rock fragments become larger and more frequent until the bedrock itself is reached.
Living in 1 square kilometre (0.4 square miles) of soil will be around 250 million spiders.
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