There will be a wide selection of potting mixes on offer at your local garden centre and online. Take some time to read up about the different types that are available, as that way you will be sure of choosing the best one for each container planting. There are two main types of potting mix: soil-based potting mix and soil-less potting mix.
Soil-based Potting Mixes
A soil-based potting mix is a reliable, all-purpose mix containing a combination of sterilised loam (soil), peat, sharp sand and fertilisers, making it perfect for most containers. It provides plants with a good supply of nutrients for the weeks immediately after planting and retains moisture well. It is also free-draining, which encourages roots to grow. You will see three types of soil-based potting mix for sale in the UK (information about potting mixes elsewhere), made according to formulae devised by the John Innes Institute, a well-regarded plant science research centre in Norfolk, with each one being suitable for different scenarios, as follows:
For seeds and cuttings (John Innes No. 1)
This potting mix is lightweight and low in nutrients. Seedlings will need potting into larger pots as they grow.
For annuals and perennials (John Innes No. 2)
This is a slightly heavier, richer potting mix, containing more nutrients to encourage the growth of roots and foliage. You can also get bulb potting mix which contains added horticultural sand or grit to provide a free-draining planting medium suitable for growing bulbs.
For permanent plantings (John Innes No. 3)
This potting mix is high in nutrients and free-draining, and also contains slow-release fertiliser, making it ideal for trees, shrubs and bamboos that will be growing permanently in a container.
Soil-less Potting Mixes
As the name indicates, this type of potting mix doesn’t contain any loam (soil). Instead, it is usually peat- or peat-substitute-based. If you are at all concerned about the environmental impact of removing peat from bogs to make peat-based potting mix, then choose one made from a peat substitute such as coir or wood fibre. These are now nearly as good as peat-based potting mixes.
Soil-less potting mixes are perfectly adequate for most types of plants. They have the advantage of being lighter than soil-based potting mixes, both to transport and in situ, plus they are also often cheaper and ideal for small containers. Their main drawback is that they tend to dry out very quickly and can be very difficult to rehydrate once dry. For this reason, it is a good idea to add some moisture-retaining granules to the potting mix when you plant up your containers. You will also need to feed plants grown in this type of potting mix regularly to keep nutrient levels topped up. I advise against using them for long-term container plantings.
Also available are potting mixes designed for specific types of plants. Plants that require a specialist potting mix include cacti and succulents, which need extra grit for drainage. If you can’t get hold of cacti and succulent potting mix, then just use a general-purpose potting mix and add some horticultural sand or grit to make it more free-draining. There are also ericaceous potting mixes with a pH lower than seven. These are lime-free and designed for acid-loving plants such as heathers, camellias, azaleas and rhododendrons.
Using Additives
Although not essential, it can be beneficial to incorporate various additives into the potting mix before planting your container. They improve drainage and make it more effective.
Horticultural sand or grit Use these additives to improve drainage – perhaps if you are growing alpines, herbs or cacti and succulents. If you are growing seedlings or very small plants, then use only sand or fine gr`avel to avoid damaging their delicate roots. Sand and grit can also be used to add weight to a potting mix, which may be helpful if you are growing a top-heavy plant.
Perlite This is a naturally occurring white volcanic glass that helps to aerate the potting mix and improve drainage. It is lightweight too, making it useful in containers for balconies and roof gardens where weight is a consideration. Also ideal for hanging baskets.
Vermiculite An expanded mineral, this additive is usually grey or beige in colour. Use it for the same purposes as perlite.
Water-retaining gel With watering being the key to successful container gardening, adding some water-retaining gel is a great idea. The granular material swells up when wet, providing the plants with water so you do not have to water them as often. I recommend adding this to the potting mix when planting summer containers and hanging baskets.