The organic humus in the A horizon aids in holding water and nutrients, making it the most fertile layer. These organisms decompose the organic material from dead organisms and animal waste into inorganic nutrients useable by plants. fungi, bacteria), earthworms and insects. The O and A horizons in typical mature soils have an abundance of microorganisms (e.g. The O horizon protects the underlying topsoil from erosion and moisture loss by evaporation. The bedrock is the source of the parent inorganic materials found in the soil. The C horizon sits atop bedrock and therefore is made up of weathered rock fragments. It consists mostly of inorganic rock materials such as sand, silt and clay. The B horizon, or sub-soil is the next layer down from the surface. This layer contains organic humus, which usually gives it a distinctive dark color. Below the O horizon is the A horizon, which is also known as topsoil.

A distinct O horizon may not exist in all soil environments (e.g., desert soil). It consists of surface litter, such as fallen leaves, sticks and other plant material, animal waste and dead organisms. The O horizon is the top layer at the earth's surface. A typical soil has a soil profile consisting of four horizons, which are designated: O, A, B and C. These layers are known as soil horizons, and each has a distinct texture and composition. Soil microorganisms or microflora can help to modify or destroy environmental pollutants. In the hydrologic cycle, soil helps to mediate the flow of precipitation from the land surface into the groundwater or can control stormwater runoff into lakes, streams, bays, and oceans. Global cycling of key nutrients, such as Carbon (C), Nitrogen (N), Sulfur (S), and Phosphorous (P), all pass through soil. Soil plays a role in nearly all natural cycles on the earth's surface.

This formation process is very slow (hundreds to thousands of years), and thus soil loss or degradation can be very detrimental to a community. Infiltrating rainwater and air also contribute to the mixture and become trapped in pore spaces. There they are mixed with organic matter called humus, which results from the decomposition of the waste and dead tissue of organisms. These small grains of rock accumulate on the surface of the earth. Its formation begins with the weathering of bedrock or the transport of sediments from another area. Soil is a complex mixture of inorganic materials, organic materials, microorganisms, water and air. Furthermore, soil quality is often a limiting factor for population growth in ecosystems. In order for a community of producers and consumers to become established on land, soil must be present.

Soil plays an important role in land ecosystems. In agriculture and horticulture, soil generally refers to the medium for plant growth, typically material within the upper meter or two. The word ' soil' has been defined differently by different scientific disciplines.

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