Patricia DeVore, age 13, of Longview, Wash., for her question:
HOW IS SOIL FORMED?
Soil is the covering over most of the land surface of the earth that is made up of rock and mineral particles mixed with living things and their remains. Soil is formed in two general ways: through the action of weather on rocks or through the movement of the surface by wind, water or glaciers.
The action of weather on rocks causes them to decompose, or break down. Little by little the surface of the rock becomes softer and decays to become soil. Such soil is called residual soil.
Soil surfaces may also be formed when wind, water or glaciers carry soil from one place to another. Soil deposited by rivers and streams is called alluvium. Soil moved by glaciers is called till. Fine soil, blown about by the wind before it is deposited, is called loess. This soil is finer than sand but coarser than clay.
Many soils, when they are first deposited, will not support much plant life. They usually do not have enough food in them to allow complex plants, such as corn or wheat, to thrive. They are ready to grow crops for the farmer only after they undergo many changes.
Air helps to make soil because it contains oxygen, carbon dioxide, nitrogen and moisture. The oxygen, carbon dioxide and water combine with the chemical elements in the rocks. This causes the rocks to decay and break down into small particles.
Plants, which also help to break down rocks, need the air's nitrogen and weight. For example, the root of a tree can crack a rock.
Wind helps to make soil when it blows sand against rocks, wearing the rocks into tiny fragments.
The force of running water alone can wear away hard rock. But nearly all running water carries sand or gravel with it and these particles also help to wear the rock away. Rain and snow help break rocks into small pieces.
When water freezes in cracks in the rocks, it expands and causes the rock to break. The constant process of freezing and thawing of water on the surface of rocks helps to wear away the rock and make soil. This process is called weathering.
Oxygen in the water, like oxygen in the air, combines chemically with the substances that have been dissolved from rocks.
Glaciers help to make soil by scraping up loose rocks from the earths surface, carrying them along and grinding them into small pieces. When the glacier melts, the soil remains behind.
Plants increase the value of the soil in two ways. They send their roots through the soil, which breaks it up and makes it better for growing things. When plants die, they decay and form humus, an organic material that makes the soil more fertile.
Animals also help to keep the soil fertile. Body wastes of many animals enrich the soil and when an animal dies, its body decays and adds important minerals to the soil.