Peat-free compost choices
Peat-free potting composts contain mixtures of organic materials – e.g. composted bark, coir (coconut fibre), woodfibre and green compost – mixed with inorganic materials such as grit, sharp sand, rock wool and perlite. A mix of coarse and fine particles is needed to create a balanced compost containing enough water and air, which are essential for root growth.
Preferably, choose peat-free compost with good on-label information. Read and follow the instructions on the packaging about the suitability of the mix for particular purposes.
Peat-free brands often recommend specific fertilisers for use with their compost: this is not a marketing ploy, as different formulations have different balances of nutrients. Use either the recommended product or one with a similar nutrient balance – compare fertilisers by checking the NPK ratio and the trace-element content quoted on the packaging.
Materials used in peat-free compost
Most peat-free composts contain wood-based materials as their primary ingredient, e.g. woodfibre, composted bark, sawdust, wood or paper waste. Wood-based mixes can be tailored to the requirements of most plants as they have excellent drainage properties as well as a low pH.
Coconut fibre or ‘coir’ is mainly imported from Sri Lanka. Coir is a waste product. It has excellent natural water-holding ability and a sufficient mix of fine and coarse fibres to hold air in its pore spaces, making it a good growing medium. It does not hold nutrients well, however. The environmental credentials of coir-based products are questioned due to the distance they must be transported to the UK, but this is balanced out by the fact that it is a genuine waste material.
Many local authorities and private companies are collecting and composting green waste. The resultant compost tends to have a high nutrient content and a high pH making it an excellent soil improver or mulch. There is an industry standard (British Standards Institution PAS100) for green compost that enforces consistent and regulated processing in order to encourage its use in potting composts. Due to its high pH and high levels of nutrients, green compost tends to be mixed with other materials to make potting compost – it is usually no more than 30% of the overall product.
Locally available materials
Research is ongoing into a number of materials that, while locally available, may be useful ingredients in blended products, e.g arable straw waste, wool waste, carpet waste and paper and cardboard production waste.
Gardeners can mix well-rotted, home-made compost, leafmould and inorganic materials (loam and sand) to make their own peat-free growing media, but results tend to be variable. It is difficult to standardise pH, moisture retention and available nutrients, and to ensure that the final mix is weed-free. However, as the materials are all made locally, this compost has a low carbon footprint.
Home-made potting media are best avoided for seed sowing because of the potential for them to contain fungal pathogens that can harm seedlings or cause damping off.