Top soil contains minerals and nutrients.
Topsoil is the exterior most layer of soil. It provides an environment rich in nutrients, minerals and water to plants and crops. The topsoil is also known as the A horizon in soil classification systems. According to "Barron's AP Environmental Science," healthy topsoil should consist of about 45 percent minerals (in combination with clay, silt and sand), 5 percent organic matter, 25 percent water and 25 percent air.
Minerals & Nutrients
Minerals are the main constituents of topsoil. Topsoil minerals are derived from their bedrock parent material, which include rocks and meta-sedimentary rocks--such as gneiss, schist and granite. Phosphorous (P), potassium (K) and nitrogen (N) are common topsoil minerals and nutrients.
Water
Water exists in topsoil and functions in the decomposition of mineral grains. Soil solution is a mixture of dissolved nutrients and minerals in water. This nutrient-rich soil solution carries essential substances to plants via their root hairs and facilitates the movement of soil bacteria.
Air
Air (oxygen) is a component of topsoil, which provides a conducive environment for aerobic bacterial activities. Aerobic bacteria decompose dead organic matter into essential soil nutrients. This decomposing takes place in the presence of oxygen, which is provided by the air. Topsoil air pores also act as water reservoirs and can hold immense quantities of underground water.
Organic Matter
According to "The Basics of Earth Science," the organic layer of topsoil is also called its O layer. This surface layer is made up of organic compounds and substances that support plant life. Carbon-rich organic substances are broken down in the topsoil to produce humus. Humus acts as a binding agent (or the glue) and brings together all other components of the topsoil.
Organisms & Microorganisms
According to "The Basics of Earth Science," topsoil contains trillions of mites, insects, worms, protozoa, fungi, bacteria and algae. Certain helpful bacteria convert atmospheric nitrogen into a form that can be easily usable by plants. An example of a nitrogen-fixing bacteria that exists in the topsoil is the Rhizobium, which recycles nutrients by decomposing litter and dead organic matter.
Sand and Silt
The topsoil also consists of a layer of loam, clay, silt and sand. Clay is present as fine particles that clump when wet, have a low permeability to water and cause water logging. Silt is a sedimentary material that is made of fine particles that get easily displaced by water. Sand is coarser than silt, and is useful for plants and crops that require low water. Loam comprises of humus, silt, sand and clay. It is rich in nutrients, retains water but prevents water logging.
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