Topsoil

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. It also houses many organisms.

Importance

Plants generally concentrate their roots in and obtain most of their vital nutrients from this layer. The 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.

Classification

In soil classification systems, topsoil is known as the "O Horizon or A Horizon," therefore, it is the very top layer.

Evaluation

When starting a gardening project, it is crucial to check whether or not the soil is satisfactory. Different types of plants vary in their nutrient needs and preferred soil conditions, many are strongly adapted to particular conditions. However, some general guidelines for "desired levels of Topsoil nutrients" have been made, broadly suitable for many plants.