Name the Top Two Layers of Soil

Significance

• The O horizon is composed of all the debris that accumulates on the Earth's surface. This includes vegetation, bits of plant leaves, twigs and roots, and any other type of organic materials deposited on the soil's surface. Some of it stays where it falls from trees, while other matter is brought in by the wind. This layer is best exemplified in a wooded area, where the forest floor is littered with debris from trees and plants. In cultivated fields lacking this layer of debris, the A, or topsoil, horizon constitutes the first layer, under which there is found the subsoil, or B, layer, composed of lighter, less fertile, soil.

Considerations

• The O horizon can be further broken down into several sublayers that are composed of only partially decomposed material containing large to moderate amounts of plant fiber, or completely decomposed materials that contain very little raw fiber. The A horizon is the most fertile layer and benefits from eluviation, or the downward movement of materials decomposing on the soil's surface. It can also contain a layer that has been disturbed by plowing, logging or other human activity. This layer may also continue to release nutrients downward to the B horizon below it.

Soil Characteristics

• Topsoil contains large amounts of humus, or decayed material, and is usually the darkest of all the soil layers. It is usually loose and crumbles easily, and is the best layer of soil for plant root formation. Subsoil is lighter colored, denser and contains less organic matter than topsoil. Below that lies the C layer, also called Parent Material, that is the last horizon before bedrock, or R level, which is solid rock lying deep beneath the soil. At this level, the organic matter and the minerals have not yet joined forces to become soil, a process that can take hundreds of years.

How Soil Forms

• Soil starts out as parent material composed of clearly distinct substances. It can include volcanic ash, sediment that has been placed there by water or wind action, or glacial deposits. Climatic forces, such as temperature and rain water, go to work on the material, breaking it down into finer particles in what is called the weathering process. Animals break up the soil by digging through it, allowing more air and water to filter through. As plants and animals die, they continue adding more organic material to the parent material, and microorganisms, such as fungi and bacteria, take over by breaking the materials down into smaller and smaller bits.