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A small area of soil may be full of living things. Soil is a vital part of the natural world. It consists of fragments of rock, such as sand grains, mixed with humus, the rotting remains of leaves, animal droppings and other plant and animal matter. Between the particles are large air spaces. These are important because they allow water to drain through the soil. They also allow oxygen to reach both the plant roots and the organisms (living things) that inhabit the soil. An average soil will consist of about 45% rock fragments, 25% water, 25% air and 5% organic material. Nutrients that are essential to plant life, such as nitrogen (in the form of nitrates) and phosphorus (phosphates), are found in the soil. 

A cross-section through soil, showing the layers of leaf litter (1), topsoil (2), subsoil (3) and bedrock (4)

Soil layers

A slice through the ground reveals the different soil layers, called horizons. On top is leaf litter with old leaves, twigs and feathers (the O horizon). Below is topsoil (A horizon), rich in dead and decaying remains of plants and animals, the humus, and home to small soil creatures. It also contains the thin roots of small plants. Next is the subsoil, the B horizon. This has less organic matter, and more and larger rock fragments. The roots of bushes and trees grow into the subsoil for firm anchorage. Below, in the C horizon, the rock fragments dominate until the bedrock itself is reached.
Water seeps down through the topsoil, a process known as percolation. As it does so, it takes with it soluble substances, such as mineral salts, which it deposits in the subsoil. This is known as leaching. 

Living in 1 square kilometre (0.4 square miles) of soil will be around 250 million spiders.


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