This article describes what topsoil is and is not.
The term “topsoil” is used widely, often reverentially, by people referring to the value of Australia’s soils. The following are the top five misconceptions about topsoil and its role as a resource:
- Differing definitions people have for “topsoil”.
Using an exercise with farmer groups to identify soil layers, it became apparent that people do not all mean the same thing when they say “topsoil”. Some identified topsoil as the organically stained surface horizon. Others included all the layers above a clay layer. Others saw topsoil as the whole soil that contained an abundance of plant roots, sometimes extending to depths below a meter.
Most soil scientists identify topsoil as the “A” horizon, that is, the surface and subsurface layers above a higher clay content horizon or a parent material. Regardless of the soil layer in question, the important thing is to understand the properties and constraints it presents, and to manage, treat and amend accordingly.
- That topsoil is synonymous with good soil.
Many Australian topsoils are very poor indeed. Hard setting, infertile, compacted, water repellent, and/or erodible topsoils are very common. Many need serious amounts of amendments such as gypsum, clays, or fertilisers just to be productive. In some instances it has been shown that crops could be higher yielding when a particularly poor topsoil was removed altogether, and a more friendly subsoil exposed. While not a recommended activity by any means, it was once a common saying in the Murray Mallee cropping belt- “where it blows, she grows”, referring to yield increases following wind erosion. Work in Southern Australia has demonstrated that yields could be doubled by mixing infertile topsoil layers with the subsoil.
- That plants only grow in topsoil.
Plant roots usually grow well below the topsoil into the subsoil, with most plants making use of soil within hospitable layers down to substantial depths. It is common for the majority of a soils water holding capacity to exist below the topsoil.
- That topsoil contains all the soil’s nutrients.
While a lot of nutrients do accumulate and cycle in the surface soil, many of the more mobile nutrients such as nitrate and sulphate can be found throughout the soil profile. In many soils that have a light texture over a clay subsoil, the bulk of the soil’s potassium store can be found in the subsoil.
- That topsoil is irreplaceable. Actually, in practical terms, this is true. Soil formation is very slow and is usually measured over geological time periods.