Soils of the Earth are complex mixtures of minerals, water, air, organic matter, and countless organisms that are the decaying remains of once-living things. It forms at the surface of land – it is the “skin of the earth.” Soil of the Earth is capable of supporting plant life and is vital to life on earth.
Soil of the Earth
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.
The rocky material that is transported and DEPOSITED by rivers, seas, glaciers, and the wind is called sediment. Clay, sand, and gravel are all types of sediment. Sediments build up to form features such as mud banks along rivers or dunes in deserts. Sediments deposited on the seabed often build up over millions of years to form sedimentary rocks.
The laying down of sediments in water or on the ground is called deposition. Sediments are picked up by fast-flowing water, by strong, swirling winds, or by the ice in glaciers. Sediments are deposited when flowing water, wind, or glaciers cannot carry it any further – for example, when the water or wind slows down or stops, or when the glacier’s ice melts.
FORMATION OF DUNES
The shape of a sand dune depends on the strength and direction of the wind, how much sand there is, and whether plants grow on the dune or on the ground. Barchan (crescent-shaped) dunes form when the wind blows from one direction most of the time. They move forwards at up to 30 m (100 ft) a year as the wind blows sand over the crest.