What is Topsoil?

Legal Definition
Topsoil is the upper, outermost layer of soil, usually the top 2 inches (5.1 cm) to 8 inches (20 cm). It has the highest concentration of organic matter and microorganisms and is where most of the Earth's biological soil activity occurs. Four elements constitute the composition of soil. Those elements are mineral particles, organic matter, water, and air. The volume of top soil consists of 50 to 80 percent of these particles which form the skeletal structure of most soils. This composition allows the soil to sustain its own weight, and other internal matter such as water and overlying landscape. Organic matter, another important element, varies on quantity on different soils. This provokes positive and negative effects or reactions on the soil. 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 skeletal structure becomes affected once the soil is dewatered. The soil's volume substantially decreases. It decomposes and suffers wind erosion.
-- Wikipedia
Legal Definition
Soil on the surface to an average depth of plod to use to plant in.