Container compost mixes
Container compost mixes vary considerably depending on what they are being used for. There are three main types:
- Potting compost – this is used when planting plants into containers.
- Cutting compost – this is used for all types of cuttings which are put into compost.
- Seed compost – this is used for sowing seeds in containers.
Please note that the term ‘compost’ is used here as a term for growing media, not meaning garden compost.
All of the different types can be purchased ready-mixed. However you may wish to make your own – which can work out quite a bit cheaper since you can buy the raw materials in bulk, then simply use them in different proportions for different mixes. Below we’ve outlined what the requirements are for each type of compost and listed different mixes which can be used (though this is by no means an exhaustive list – many gardeners like to experiment with their own mixes or use different ones for different plants). The proportions shown are by volume.
Potting compost
In addition to the main ingredients listed below, you may wish to add a liming ingredient (eg ground limestone) to lower the acidity and/or a slow release fertiliser.
Loam-based
These mixes contain loam (soil with a balanced mix of sand, clay and silt), which is usually made by stacking turf for at least 6 months until it breaks down and then sterilising the resulting loam. Loam-based composts are heavier, and therefore more difficult to handle, but are best for longer term planting as they have a strong structure.
Example mixes:
- 7:3:1 – loam:peat*:coarse (sharp) sand
- 7:3:2 – loam:peat*:coarse (sharp) sand (these are the proportions used in John Innes potting compost mixes)
Loamless
In contrast, these composts don’t contain any loam, making them lightweight and easy to use. However the structure of the compost breaks down quickly (often ’shrinking’ within the container) so is only suitable for short term planting (eg displays of annuals).
Example mix:
- 3:1 – peat*:coarse (sharp) sand
Cutting compost
The key requirement for cutting compost is that it is free draining, particularly since cuttings are often grown in humid conditions and are prone to rotting.
Example mixes:
- 1:1 coarse (sharp) sand:peat*
- 1:1 peat*:fine composted bark
- 1:1:1 peat*:fine composted bark:perlite
Seed compost
Like potting compost, seed compost can be with or without loam. It needs to be very fine, so the seedling roots can make good contact with it, so sieve it well before use.
Example mixes:
- 2:1:1 – loam:peat*:fine (silver) sand (these are the proprtions used in the John Innes seed compost)
- 3:1:1 – peat*:fine composted bark:perlite
- 3:1 – peat*:fine (silver) sand
* Where peat is indicated, then a peat substitute such as coir, or composted bracken can be used instead.
On a large scale, compost is mixed in specialist compost mixing machines. On a smaller scale, however, you can mix compost with just a little hard labour! Create a pile of all the ingredients. Next shovel everything into a new pile. Then shovel everything back to make a pile in the original place. This should mix the compost sufficiently. Obviously, with very small amounts this can just be mixed by hand in a tub or bucket.
