DK Science: Soil
Much of the solid bedrock of the Earth?s crust is covered in soil. This loose, soft material is a mixture of organic matter and particles of rock, made by weathering and erosion. The organic matter is made up of dead and living plants, animals, and other organisms. Many of the living organisms are DECOMPOSERS that live on the dead plants and animals. Plants get the water and nutrients they need from the soil they grow in.
The texture of a soil depends on the size of the rock particles it contains. Clay soil feels very smooth because it is made mostly of tiny particles. Sandy soil feels gritty because it is made of larger particles of up to 2 mm (1/10 in) across. Sandy soils are dry, while clay soils tend to be wet and sticky. Loam contains a mixture of sand, clay, and silt, and is a a good soil for growing crops.
Many of the millions of organisms that live in the soil, including bacteria, fungi, insects, and earthworms, are known as decomposers. They live on the remains of dead plants and animals and break down these organic remains into simple chemicals that are released into the soil. Some of these chemicals provide nutrients for new plants to grow, so decomposers recycle plant material.
Soil seen through a microscope reveals microorganisms called bacteria. A handful of soil contains millions of bacteria and fungi, which cling to particles of rock and decaying matter. Bacteria and fungi continue the decomposition started by larger organisms such as earthworms, woodlice, and slugs.
Earthworms do two important jobs to keep soil fertile, or good for plants to grow in. First, they feed on dead plant matter, helping to decompose it. Second, as they burrow, they mix and loosen the soil, which spreads organic matter and nutrients, allows air in, and improves drainage.