Potting compost-buying season approaches as gardeners begin to sow seeds, pot-up plants and refill containers. Garden soils are a mix of minerals, organic matter, water and air. Despite holding their structure in the open ground due to natural processes involving worms and other soil organisms, they fall short when used in pots as the soil becomes compacted and the air spaces are squeezed out.

Air content is actually the most important factor in a potting compost. Garden soil is also potentially contaminated with weed seeds and disease organisms. The latter are manageable in gardens but become damaging in the relatively airless conditions of pots.

To overcome this, gardeners have traditionally made or bought potting composts with more organic matter than is usually found in garden soil. These are not actually compost at all but a mix of materials formulated for the right friable and free-draining consistency.

The organic matter provides the air spaces, and therefore the drainage, required in pots so that air can enter the root zone allowing healthy roots to grow. Organic matter also retains plant nutrients in the same way as fertile soil.

Manufactured potting composts can contain sterilised loam, a particularly fertile form of soil, either as John Innes Composts or as a minor ingredient in compost that are mostly soil-free. The ideal crumbly loam, derived from grassland soils, is hard to find nowadays and other, lesser, soil often has to be used.

Westland New Horizon peat free compost. Vegetable peat-free compost, general purpose peat-free compost. Image via Emma Dahl
Non-soil components include sand, grit, composted municipal green waste (Photo: Neil Hepworth RHS)

Non-soil components include sand, grit, composted municipal green waste, wood fibre, coir (coconut fibre) and bark. Peat-containing products should not be used owing to its harvesting impacting on the environment. Fertiliser and lime is added to get the optimum nutrient levels and acidity.

Commercial potting composts, sold in garden centres, are often formulated for particular purposes such as houseplants, seed-sowing, cuttings or vegetables, but much is “multipurpose” designed to be good enough for most uses. Check the label as some do not claim to be suitable for seeds. Except for casual use, it is wise to use products formulated for particular purposes such as houseplants.

Commercially made garden potting composts are not all the same – the best advice is to find one that suits you and stick to it. There is some variability in products from year to year, but manufacturers are getting better. Reviews are published but it is unclear how reliable they are. Those from Which? Gardening use statistically valid methods and are reliable.

Gardeners can consult the Responsible Sourcing Scheme to identify the most environmentally benign products. This scheme assigns a score to products that have been tested; A–E, A being the least harmful.

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Much of the potting compost for sale now will have been manufactured and packed late last year. Happily, it will not have deteriorated in the cool winter conditions and should be satisfactory.

Potting media made last summer may still be offered – but is likely to have rotted to some extent, which will decrease the porosity and, therefore, air content. It is better to buy fresher material. Unfortunately, few manufacturers date-stamp their bags of compost, but the degree of fading of packaging and shrinkage of the contents can give clues.

Any surplus potting compost left over from last year can be mixed in equal parts with new material and will be good enough for less demanding plants. Adding 25 per cent by volume of small chipped bark to old potting compost can add air spaces and is also worth considering for less demanding applications such as tubs and troughs. Alternatively, old potting compost can be used as mulch or soil improver.

Industry research suggests that much new fresh potting compost is used as a soil improver. This is probably because potting compost is keenly priced and gardeners see it as a cost-effective soil improving material.

The use of the word “compost”, suggesting similarity with garden compost made in compost bins, is also unfortunate. In fact, potting compost is inferior as a soil improver both to garden compost and the materials sold for that purpose – composted manure, for example.

It is interesting and fun to try making your own homemade potting composts with some gardeners claiming good results with mixtures of garden compost, leaf mould, sieved garden soil, sand and grit.

Most potting compost, however, is bought for larger containers, and here the risks are lower and significant cost savings possible. A mixture of two parts garden soil and one part garden compost is worth experimenting with, perhaps adding some sand if the garden soil is rich in clay.

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