<Table> <Tr> <Td> </Td> <Td> This article's tone or style may not reflect the encyclopedic tone used on Wikipedia . See Wikipedia's guide to writing better articles for suggestions . (September 2017) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article's tone or style may not reflect the encyclopedic tone used on Wikipedia . See Wikipedia's guide to writing better articles for suggestions . (September 2017) (Learn how and when to remove this template message) </Td> </Tr> <P> 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 . </P> <P> Plants generally concentrate their roots in and obtain most of their vital nutrients from this layer . Actual depth of the topsoil layer can be measured as the depth from the surface to the first densely packed soil layer known as subsoil . </P>

What is the upper most layer of soil