Technical
Over the last five years there has been a massive increase in the number of suppliers of bagged topsoil. With this proliferation has come a big variation in quality. Hopefully, this article will help you make the right choice and buy what is fit for your intended purpose
THE British Standard for TOPSOIL BS3882:2007 is designed to help you when buying a topsoil. So, get familiar with it. It will help. All good suppliers of topsoil should be able to provide you with a “Declaration to BS3882:2007” which says that they provide a quality controlled product
Good topsoil is a balanced mixture of mineral particles, water, nutrients, organic matter, air and living organisms.
Soil Texture
This is the proportion of sand, silt and clay within a topsoil.
Sandy Loam, sand 70%, Silt 18% and Clay 12% is an ideal textural classification for a general purpose topsoil.
Organic Matter
Organic matter (OM) consists of living organisms and is very important to soil function and plant growth as it influences soil structure and, hence aeration, drainage and root growth, water-holding capacity and soil fertility.
OM helps to bind mineral particles into granular or crumb structures and the proportion of water that is available for plant uptake. It is a major source of essential plant nutrients, including nitrogen, phosphorus, potassium and sulphur.
It is also the food for soil organisms and, without it, biochemical activity, which is essential for ecosystem functioning, would cease.
However, too much OM, especially if immature, can lead to:
- A deficiency in nitrogen as the OM uses the nitrogen in the soil to fuel the composting cycle
- Unstable ground and planting conditions
- Waterlogging through the retention of too much water
A good quality topsoil should have OM content of between 3% to 20%.
pH
pH is the measure of the acidity or alkalinity of a soil. It affects nearly all soil properties and is a major factor in determining where trees, shrubs and grasses will grow. It influences structural stability, plant nutrient availability, microbe activity and soil pollutant mobility.
96 PC DECEMBER/JANUARY 2013
What's In Your Bag?
“Organic matter is the food for soil organisms and, without it, biochemical activity, which is essential for ecosystem functioning, would cease”
A good quality topsoil should have pH 5.5-8.5.
Electrical Conductivity
Electrical conductivity is the general measure of the soluble salt content or salinity of a soil. A good quality topsoil should have an electrical conductivity value within the range of 100-1500uS/cm (soil : water extract).
Nutrients
A good topsoil should be adequately supplied with the major nutrients - Nitrogen, Phosphorus, Potassium and Magnesium.
Carbon/Nitrogen (C/N) ratios
Topsoils that have a C/N of 30:1 and over, have too little N to allow for rapid decomposition. Therefore, the microorganisms will take ammonium and nitrate out of the soil to fuel decomposition. This depletes the soil of nitrate and ammonium.
Topsoils with low C/N ratios (20:1 and less) have sufficient N for the microorganisms to decompose the residues without taking from the soil. A good topsoil should have a C/N ratio of <20:1
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