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.
Importance
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.
Classification
In soil classification systems, topsoil is known as the "O Horizon or A Horizon," therefore, it is the very top layer.
Commercially available topsoil (manufactured or naturally occurring) in the United Kingdom should be classified to British Standard BS 3882 with the current version dated 2015. The standard has several classifications of topsoil with the final classification requiring material to meet certain threshold criteria such as Nutrient Content, Extractable Phytotoxic Elements, Particle Size Distribution, Organic Matter Content, Carbon:Nitrogen ratio, Electrical Conductivity, Loss on Ignition, pH, Chemical and Physical Contamination. The topsoil should be sampled in accordance with the British Standard and European Norm BS EN 12579:2013 Soil improvers and growing media - Sampling.[3] During construction of garden areas for housing plots the topsoil should be underlain by a layer of suitably certified subsoil that conforms to the British Standard BS 8601:2013 Specification for subsoil and requirements for use.[4]
It is always recommended that for construction projects that topsoil is placed in accordance with the DEFRA report Construction Code of Practice for the Sustainable Use of Soils on Construction Sites
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