What is Topsoil?
Topsoil is the fertile upper layer of soil, usually 2 to 8 inches (5.1cm to 20cm) in depth. The formation of topsoil is a very slow process; typically, it takes around 100 years per inch. Topsoil is formed by the weathering of rocks and the subsequent addition of organic matter from decaying plants, animals, mosses, etc. Decaying plant matter enriches the soil, adding the nutrients and minerals essential to support plant life.
Soil composition varies considerable from area to area and is constantly changing due to the surrounding environment. Sandy loam topsoil, the soil most often used in garden projects to facilitate free drainage, is usually 50-60% sand, 34-45% silt and 10-15% clay. Soil pH varies depending on the source and location of the bedrock. As a guide, a good quality topsoil should be in the range of 5.5 to 7.5, i.e. mildly acidic to slightly alkaline.
It is important to ensure that all topsoil you use is high quality, as this represents the foundation of any garden project. High quality sandy loam soil is ideal. A quick test to check the quality of the topsoil is to ensure that the soil is a dark rich colour, lightweight and friable, i.e. hard to roll up and quick to break up if rolled into a ball. However, if you have any doubts about the quality of the top soil is may be best to have it professionally tested.
All topsoil is natural, but it can be enriched and improved with soil improvers and composts, increasing fertility and improving soil structure by increasing the essential organic matter. Soils from skip yards should be avoided if possible due to the high levels of brick, glass and other contagions. Soils should be selected from soil screening operations that only accept virgin and site clearance soils.
Some suppliers talk extensively about organic topsoils, but this is generally misleading. True organic topsoil would need to be taken from an organic certified field, which would be prohibitively expensive. A better decision would be screened soil which has been improved by blending in organic composts to give it a higher organic matter content.
The process of screening topsoil involves working the soil over or through a large mesh or screen to sieve stones and lumps out of the soil, a process that also acts to aerate the soil. Screen sizes should be 10mm or less for high quality soils and up to 25mm for infill.
When selecting your topsoil supplier it is important to ensure that they are acting ethically. Digging virgin green field soil or field stripping does not have good environmental sense. As mentioned previously, top soils is a field take years to develop and it is far more environmentally friendly to use top soils taken from sites that are to be cleared prior to development, which would otherwise go to waste.
The key to a great topsoil is its organic matter. It is this matter that plants feed on and the essential micro-organisms live off. Organic matter also improves soil structure, improves drainage and locks in moisture and nutrients around plant roots.
