How do I mix my own potting compost?

Garden compost by itself slumps into an airless block in a pot, and topsoil alone sets solid. Every recipe here is doing the same job: adding structure that keeps air in the mix after months of watering.

What you'll need

• Sieved garden compost or leaf mould
• Loam or sterilised topsoil
• Sharp sand, perlite or vermiculite
• Sieve
• Slow-release fertiliser (optional)

Step by step

1. Decide which job the mix is for.Seed mixes are fine, low in nutrients and free-draining. Potting-on mixes are richer. Long-term container mixes need loam for weight and stability.
2. Sieve everything for seed compost.Seedlings cannot push past lumps and the surface must be fine enough for small seed to sit in contact with it.
3. Mix a seed compost.Roughly two parts fine leaf mould or sieved compost to one part sharp sand or vermiculite, with no fertiliser. Seeds carry their own food; feed comes after the first true leaves.
4. Mix a general potting compost.Around two parts sieved garden compost, one part loam, one part sharp sand or perlite, plus a measured dose of slow-release fertiliser.
5. Mix a long-term container compost.Increase the loam to roughly a third. Loam gives weight so pots do not blow over, holds nutrients, and does not shrink away from the sides after a year.
6. Make an ericaceous mix if you need one.Composted bark and leaf mould from oak or beech leaves, with sharp sand, and no lime or mushroom compost. Water with rainwater.
7. Test the drainage before using it.Fill a pot, water it, and see how fast it runs through. It should drain freely within a minute or two and not sit as a puddle on top.
8. Squeeze-test the moisture behaviour.A handful squeezed should hold together loosely and break apart when prodded. If it stays as a solid ball it needs more grit; if it will not cohere at all it needs more organic matter.
9. Do not sterilise at home casually.Oven-baking soil smells appalling, is easy to overdo, and kills beneficial organisms as well as pathogens. Buying a bag of sterilised loam for seed mixes is more reliable.

Tips

• Molehill soil is finely worked, weed-free loam, and it is free.
• Seed compost with fertiliser in it produces soft, disease-prone seedlings.
• Loam is what stops a tall pot blowing over and what stops the mix shrinking away from the sides.

Common mistakes

• Filling pots with garden compost alone — It slumps, excludes air and holds water at the base, and roots rot. Garden compost is a soil improver; a potting mix needs structure adding to it.
• Using builders' sand instead of sharp sand — Builders' sand has fine rounded particles that fill the air spaces rather than creating them, and it often contains salt or lime. Horticultural sharp sand or grit is the right material.

If it doesn't work

Questions people ask