In the olden days, gardeners had to create their own potting compost types and it could be incredibly variable. After the Second World War, standardised ‘recipes’ emerged, and today there are dozens of types available in the shops.
But all that choice can be confusing and overwhelming. It’s hard to tell what the differences are and if you are using the right compost.
So here’s a handy guide to potting compost types, mixes and uses.
Note on the word ‘compost’
By compost, I don’t mean the black recycled compost you get out of the compost bin. This is a soil improver and should never be used to fill pots or seed trays. It would be like putting young plants in pure manure mixture, and will definitely kill them.
You need the bags of compost that are designed for pots, which is why it is called potting compost.
And you also don’t want to use topsoil in containers. This is designed for topping up borders and filling raised beds. Ordinary garden soil does not have enough nutrients or water retention to be used in pots.
Seed compost is a mixture designed for sowing seeds into. It’s finer and less lumpy than standard potting compost, and is also light and well-drained.
You can also use seed compost to repot seedlings and young plants. But it doesn’t have enough nutrients to support larger plants.
This is also called general purpose compost. It’s a middling mix between seed compost and potting compost, and can be used as a substitute for either.
Multi-purpose compost is the best choice if you’re planting small patio pots, hanging baskets, herbs, leafy salads and flowering bedding plants.
But it has less nutrients than potting compost, so be prepared to start adding fertilisers sooner if you’re planting larger varieties. It’s not suitable for established trees and shrubs.
Multi-purpose compost is also available peat-free – please see below for more details on using peat.
This is designed for supporting established plants in pots. There are two main types – soil-based and soilless.
Soil-based composts are designed to mimic loamy garden soil. These are called John Innes.
Many people think John Innes is a brand, but it’s actually a recipe from a gardener by the name of John Innes, and any brand can make it. There are a few different types of John Innes compost with distinct uses.
The JI seed compost is used for sowing seeds. Then there are three JI numbers, each denoting a different recipe. The higher the number, the more nutrients it has and the bigger the plants that can grow in it.
JI No1 is used for potting up small, young plants. When they need repotting in a larger container, you then need JI No2. A year or so later, you should repot the plant into a larger container filled with JI No3 and it won’t need repotting again for a couple of years.
Soil-based composts are good choices for established plants because they are rich in nutrients and retain moisture well. But they are dense and heavy and drainage is not too good, making them the wrong choice for seedlings and containers like hanging baskets.
Be aware that John Innes composts do contain some peat, if you are trying to avoid it.
Peat or peat-free compost
The alternative soilless composts were until recently based on peat, a high-nutrient organic matter found in peatland bogs. But the use of peat in gardening led to a dramatic decline of the natural wildlife habitats found in peat bogs, and ethical concerns have made it less desirable.
You can now buy reduced-peat and peat-free compost mixes. When these originally appeared, many were poor quality and that has put some gardeners off. But modern peat-free composts are much better. They are based on compost bark, coir or green manure and waste materials from parks.
Most compost types used to contain peat so look for peat-free labels. If it doesn’t say peat-free, it will contain some level of peat. If you want the best of both worlds, you can always create a half and half mix of soil-based and peat-free.
The composts above will suit most plants and growing situations. But there are other types of compost for specific purposes and plants.
Ericaceous compost is designed for acid-loving, lime-hating plants like camellia, azalea and gardenia.
Container and hanging basket compost is supplemented with water-retaining crystals and/or a slow-release fertiliser. This helps plants to last longer and is a good choice if you struggle to water and feed as often. But you can also buy water crystals and feed separately and add to multi-purpose compost.
Houseplant composts are usually in smaller bags for indoor gardeners. You can also buy gritty cactus compost and coarse orchid compost if you need to repot houseplants.
Tree-planting compost is not potting compost – it’s a soil improver that can be dug into borders when planting a tree. Do not use this for potting up trees – use John Innes or peat-free potting compost instead.
Potting compost tips
Don’t buy too much compost. Only get as much as you will need to use in six to eight weeks. Seal up the bag after use to stop insects and weed seeds getting in.
Store bags of compost in a dry place where they won’t get waterlogged.
Refresh the compost in pots every year or so to replace nutrients. You also need to feed container plants to add extra nutrients to the soil. Apply a liquid fertiliser every two to four weeks during the growing season, or mix in slow-release granules at planting time. Read more on NPK fertilisers.