The depth of the topsoil layer is measured as the depth of the surface to the first densely packed soil layer, known as subsoil. Soil horizons are layers parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. In soil classification systems, topsoil is known as the O Horizon or A Horizon. A Victorian open-cut coal mine was rehabilitated with low-quality artificial topsoil made from local materials. Manufactured topsoil based on minerals, biosolids, compost and/or paper mill sludge is available commercially. More traditional examples of artificial plant-growth media include terra preta and potting mix. It is possible to create artificial topsoil which supports some of the engineering or biological uses of topsoil. The current rate of use and erosion outpaces soil generation. Natural topsoil is mined and conditioned for human use and makes up the bulk of commercial topsoil available. Topsoil is naturally produced in the process of soil formation or pedogenesis. Dehydrated topsoil volume substantially decreases and may suffer wind erosion.
The structure becomes affected once the soil is dehydrated. It condenses and settles over time in different ways depending upon conditions such as beneath roadbeds and foundations vs uncovered and exposed to the elements. Organic matter provides nutrition for living organisms and varies in quantity between different soils with the strength of the soil structure decreasing when more is present. A healthy topsoil layer is a very rich microbiome that hosts a wide array of species. Many species directly contribute to the health of the soil resulting in stronger plants. Insects also play important roles in breaking down material and aerating and rotating the soil. Bacteria and fungi can be essential in facilitating nutrient exchange with plants and in breaking down organic matter into a form that roots can absorb. It also plays host to significant bacterial, fungal and entomological activity without which soil quality would degrade and become less suitable for plants. There are generally a high concentration of roots in topsoil since this is where plants obtain most of their vital nutrients. Together these make a substrate capable of holding water and air which encourages biological activity. Topsoil is composed of mineral particles and organic matter and usually extends to a depth of 5-10 inches (13–25 cm).
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