surface layer of soil that may be rich in nutrients from decaying plants and bacterial material. often removed when sites are graded in preparation for construction or development.

Merriam-Webster Online Dictionary
topsoil (noun)
surface soil usually including the organic layer in which plants have most of their roots and which the farmer turns over in plowing
topsoil (Wikipedia)
Surface runoff, a type of nonpoint source pollution, from a farm field in Iowa during a rain storm.
Topsoil as well as farm fertilizers and other potential pollutants run off unprotected farm fields when heavy rains occur.

Topsoil is the upper, outermost layer of soil, usually the top 5 inches (13 cm) to 10 inches (25 cm). It has the highest concentration of organic matter and microorganisms and is where most of the Earth's biological soil activity occurs. Topsoil is composed of mineral particles, organic matter, water, and air. Organic matter varies in quantity on different soils. The strength of soil structure decreases with the presence of organic matter, creating weak bearing capacities. Organic matter condenses and settles in different ways under certain conditions, such as roadbeds and foundations. The structure becomes affected once the soil is dewatered. The soil's volume substantially decreases. It decomposes and suffers wind erosion.

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