Multi-purpose compost is, by its nature, a general purpose compost, that will work reasonably well for most plants. But the nature of ‘general’ things is that they may well not be absolutely ideal for anything. And multi-purpose compost does have some drawbacks, most notably in that it varies hugely between manufacturers and types. There is no agreed formula for multi-purpose compost, so different manufacturers use different base materials, and different quantities of each constituent. Some may even vary the ingredients year by year. So you don’t really know exactly what you’re getting.
What’s more, there’s the peat/not peat dilemma. Do you go for a peat-based compost, which is more traditional, and some say more reliable? It will tend to dry out, and is hardly the ethical choice these days. Or do you go for another base which may be less reliable or less consistent but is more environmentally-friendly?
For container planting, you may want to consider a container-specific compost. Many, if not most, of these contain some sort of water retaining gel or crystals, to stop the compost drying out. However, these composts are generally very expensive considering that you can achieve the same result by mixing water-retaining crystals into any other compost, and so they’re not generally recommended.
Then there are the soil-based composts. Probably the best-known of these is the John Innes range, developed originally by the John Innes Institute, and formulated scientifically to meet the precise needs of plants at different stages of growth. There’s one for seed sowing, which has smaller particles, one for potting on and one for mature plants (numbers 1, 2 and 3). The big advantage of loam-, or soil-based composts is that they tend to dry out more slowly than peat-based composts.
There are several multi-purpose composts on the market that claim to have “added John Innes”. However, this phrase, like ‘multi-purpose’, doesn’t have an agreed meaning. It’s likely that they just have some loam, or soil, added to the basic multi-purpose compost. This won’t necessarily be a drawback, as it will stop the compost drying out so quickly, but these composts are unlikely to be scientifically formulated in the same way as the ‘John Innes’ numbered composts.
As an alternative to ‘compost’, you could use top soil (also written topsoil) for your containers. This may perhaps not be so good for annuals, which need more nutrients. However, it is ideal for growing perennials or shrubs, especially those that need fewer nutrients or like poorer soils, and so will not thrive in the nutrient-rich environment of multi-purpose compost. Top soil will not dry out as quickly as compost, so you won’t have to water your containers quite so often. If you are worried about the nutrient levels, and think your plants will need more food than the soil can supply, you can add slow-release fertiliser to your pots when you plant them up. It is well worth adding all the fertiliser needed for the whole season at once – the plants really seem to thrive on that, maybe because they need more feeding when they start to grow than later. And you can always feed your plants with a good general purpose soluble plant food should they start to look a bit ‘peaky’.
The other big advantage is that there are plenty of topsoil suppliers out there, and you can buy topsoil in bulk bags, and have it delivered to your door, meaning that you don’t have to make a trip to the garden centre, or indeed, own a car!
So next time you’re planning your container display, think about what growing medium to use, and try something a bit different that may suit your plants even better.
Lyndon Ogden writes on many lifestyle subjects including gardening. In this instance on the use of topsoil and compost and best topsoil suppliers