Multi-purpose compost, which many countries call potting soil, is ready-fertilised compost for pots, window boxes and beds. Seed compost is finer and contains almost no nutrients, made for seeds that are about to germinate. Houseplant compost is a potting compost for plants that will stay in the same pot for several years. The right bag depends on what is going to happen in the compost over the next 4-6 weeks.
The compost types side by side
The bags differ on three points: the amount of fertiliser, the structure and how acidic the compost is. Most growing media are limed to around pH 5.5-6.5, within the 5.5-7.5 range where most plants take up nutrients well.
| Compost | Nutrients | Structure | pH | Suitable for |
|---|---|---|---|---|
| Seed compost | Low | Fine, without lumps | 5.5-6.5 | Sowing seeds and taking cuttings |
| Multi-purpose compost | Medium | Medium-coarse and airy | 5.5-6.5 | Pricked-out seedlings, window boxes and summer bedding in pots |
| Houseplant compost | Medium | Stable structure, often with clay pebbles | 5.5-6.5 | Indoor pot plants that stay in the same pot for several years |
| Raised bed compost | High, often with manure or garden compost | Coarser and rich in organic matter | See the bag | Pallet collars (stackable wooden bed frames) and raised beds |
| Cactus compost | Low | Gritty, with sand or pumice | See the bag | Cacti, aloe vera and other succulents |
| Citrus compost | High | Stable structure, often with clay | Slightly acidic | Citrus trees in pots |
| Ericaceous compost | Low to medium, little lime | Coarse and fibrous | 4.5-5.5 | Rhododendrons, azaleas, heathers and blueberries |
At Florea we use multi-purpose compost for almost everything except sowing, and only buy a special compost when a plant needs acidic or lean soil.
How to choose the right bag
Start from what the compost has to do, not from what is printed in large letters on the front.
If something is to germinate: seed compost. A 10-litre bag goes a long way and fills just over 30 fibre pots of 6 x 10 cm.
If something is to grow for one season: multi-purpose compost, for pricked-out seedlings, window boxes and summer bedding.
If something is to stay in a pot for several years: houseplant compost, or a special compost if the plant wants acidic or lean soil.
If something is to grow in an outdoor bed: raised bed compost, or multi-purpose compost mixed with garden compost.
If you are buying several bags at once, first work out how many litres you need and decide on the type for each bag.
How much compost you need
A standard pot that is about as tall as it is wide and tapers towards the base holds roughly the cube of the diameter in decimetres, divided by two. A 30 cm pot gives 3 x 3 x 3 divided by 2, so 13-14 litres.
| Pot or bed | Compost needed |
|---|---|
| Fibre pot, 6 x 10 cm | Just under 0.3 litres |
| Pot, 20 cm | About 4 litres |
| Pot, 30 cm | About 13-14 litres |
| Pot, 40 cm | About 32 litres |
| Pallet collar, 80 x 60 x 20 cm | Just under 100 litres |
| Pallet collar, 120 x 80 x 20 cm | Just under 200 litres |
| Double pallet collar, 120 x 80 x 40 cm | Just under 400 litres |
A 120 x 80 cm pallet collar therefore takes five 40-litre bags. The compost settles a few centimetres when it is watered in, so allow one extra bag. There is more about base layers and crops in the article on growing in a pallet collar, and varieties that cope with 20 cm of soil depth are among our seeds for raised beds.
Why seeds need lean seed compost
A seed carries its own food in the seed leaves or the endosperm, and it lasts until the first true leaves appear. Fertiliser dissolves in the soil water as salts, and young seedling roots tolerate high salt levels less well than the roots of an established plant. In standardised growing media, a seed and cutting compost is therefore given 1-1.5 grams of fertiliser per litre, while compost for hungry pot plants gets up to 3 grams. Seed compost is also finely sieved, so that small seeds have moisture all around them instead of ending up in an air pocket.
We move the seedlings into multi-purpose compost when they have their first or second pair of true leaves, usually 4-6 weeks after sowing. The whole process is in the article on starting seeds indoors.
Peat and peat-free compost
Most bagged composts are based on sphagnum peat. Peat is light and airy and soaks up a lot of water without leaving the roots standing in the wet. It is also low in nutrients and acidic to begin with. The manufacturer limes it, typically with around 5 kg of lime per cubic metre, and adds a measured amount of fertiliser.
The downside is the climate. Peatlands cover around 3 per cent of the world's land surface but store roughly twice as much carbon as all the world's forests. When a peatland is drained and dug, the carbon is released as carbon dioxide, and peat only grows by about 1 mm a year. That is why many gardeners choose peat-free compost, and in 2021 the government in England committed to ending the use of peat in amateur gardening. In peat-free compost the peat is usually replaced by a mix:
Coir: holds water much like peat and has a pH of 5.5-6.5. One kilo of dry coir swells to around 15 litres. The fibre lacks calcium and magnesium and is therefore mixed with other materials.
Garden compost: adds nutrients and microbial life. No more than equal parts garden compost and structural material is the usual recommendation.
Composted bark and wood fibre: give a coarse structure and air.
We have noticed that peat-free compost can look dry on the surface even though it is moist 2-3 cm down, so feel with your finger before you water.
Perlite, vermiculite and biochar
Perlite: volcanic glass that has been heated to 850-900 degrees Celsius and has expanded to 7-16 times its volume. The grains hold a little water and a lot of air. We mix 1 part perlite into 3-4 parts multi-purpose compost for rosemary, pelargoniums and other plants that cannot tolerate wet soil.
Vermiculite: a mineral that absorbs water and nutrients and releases them slowly. A 2-3 mm layer over the seeds keeps the surface evenly moist.
Biochar: charred plant material that holds on to moisture and nutrients. Florea Biochar is mixed in at 5-10 per cent of the soil volume, which comes to 10-20 litres in a 120 x 80 cm pallet collar.
Sand and grit: for cacti and succulents, multi-purpose compost can be mixed with up to half coarse sand.
When the compost needs replacing or topping up
The fertiliser in fresh bagged compost runs out long before the compost itself. We start giving liquid feed 4-6 weeks after repotting, and for tomatoes and chillies in pots we use Florea Fertilizer, 2-3 tablespoons per litre of water twice a week.
The compost also compacts over time. The signs are that water runs straight through down the side of the pot, or that the roots fill the whole pot.
Indoor pot plants: new compost every other year, ideally in March or April, and a pot 2-4 cm wider if the roots have filled the old one.
Large, heavy pots: replace the top 5 cm every spring.
Annuals in pots: new compost every year. The old compost can be mixed into the pallet collar.
The pallet collar: top up with 5-10 cm of garden compost or new compost in April, as the soil sinks when the organic matter breaks down.
Common mistakes with bagged compost
Sowing in multi-purpose compost: the fertiliser is too strong for the seedling roots. Sow in seed compost and save the multi-purpose compost for pricking out.
Letting peat-based compost dry out completely: dried-out peat shrinks, pulls away from the side of the pot and repels water, so it runs straight through. Stand the pot in a saucer of water for 20-30 minutes.
Filling the pot to the brim: leave a 1-2 cm gap for watering, or the water will run over.
Giving ordinary compost to acid-loving plants: rhododendrons and azaleas want a pH of around 4.5-5.5 and easily get yellow leaves in limed multi-purpose compost. If they are going into a border, first find out which of the different soil types you have in your garden.
Pressing the compost down hard: it loses the air the roots need. Tap the pot on the table 2-3 times and water the compost in instead.
Author: Emma Vogiatzi
Fact-checked by Erik Hoekstra
Last updated 2026-11-04