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 New Zealand  should be classified as organic. There are 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. During construction of garden areas for housing lots the topsoil should be underlain by a layer of suitably certified subsoil or clays. 

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.

The two common types of Topsoil are Bulk and Bagged Topsoil. The following table illustrates major differences between the two.

This is for a multipurpose grade and certain levels can alter with regard to soil pH. Other uses specified in the standard that allows for a variety of uses in different and specific scenarios includes: 

Acidic, Calcareous, Low Fertility, Low Fertility Acidic and Low Fertility Calcareous. These uses are limited to specific site scenarios and acceptance should be on a case by case basis for construction projects. 

Carbon to nitrogen ratio

Topsoil is the primary resource for plants to grow and crops to thrive and the main two parameters for this are Carbon and Nitrogen. The Carbon provides energy and Nitrogen is a tissue builder and plants require them in a range of ratios to enable suitable growth. An optimum figure for Topsoil  is a ratio of less than 20:1. This ensures that the soil has a suitable energy reserve as well as tissue building material to enable the plants to thrive. A sawdust typically has a carbonaceous base and this a high C:N ratio (in the order of c. 400:1) while an Alfalfa Hay has a low carbonaceous content and can typically have a C:N ratio in the order of 12:1.

Commercial application

A variety of soil mixtures are sold commercially as topsoil, usually for use in improving gardens and lawns, e.g. container gardens, potting soil and peat. Another important yet not commonly known use for topsoil is for proper surface grading near residential buildings such as homes. "The ground around the home should slope down six inches for the first ten feet away from the home. This can often be done by adding topsoil (not sand or gravel)."

Erosion

A major environmental concern known as topsoil erosion occurs when the topsoil layer is blown or washed away. Without topsoil, little plant life is possible. Conventional agriculture encourages the depletion of topsoil because the soil must be plowed and replanted each year. Sustainable techniques attempt to slow erosion through the use of cover crops in order to build organic matter in the soil. 

Conservation How much topsoil do I need 

Because of its use in commercial application and due to the environmental concerns regarding erosion, it is important for consumers to accurately determine how much topsoil they need for a given project. Topsoil is mainly sold by the cubic meter in New Zealand. To calculate the amount of topsoil you will need, "simply take your length (in meters) multiplied by your width (in meters) multiplied by your depth (in Meters so if you wanted 200mm that would be ). This will give you your cubic meters needed for the project. Ex: 10L x 10W x .200  = 20 cubic meters of topsoil needed.