Why Make Your Own Seed Compost?

There is something quietly satisfying about raising a tray of seedlings in a mix you built yourself from your own land. But the practical case is just as strong. Bags of seed compost have crept up in price, peat-free versions vary enormously from batch to batch, and a smallholding that can produce its own growing medium is a smallholding that needs one less trip to the garden centre in March.

The good news is that the traditional recipe is not complicated. It relies on three ingredients, most of which you can either make on site or buy cheaply by the sack. Get the proportions right and your seedlings will be every bit as sturdy as anything raised in a bought bag.

The Three Ingredients, And What Each One Does

Seed compost is not about richness. Seeds carry their own food store, and too much nutrition in the mix encourages soft, sappy growth and algae on the surface. What you want is a fine, free-draining, moisture-retentive medium that holds together just enough around a germinating root.

• Loam — rotted turf or old pasture topsoil. This is the body of the mix: it holds water and nutrients, and gives the compost substance so it does not slump when you water it.
• Leaf mould — two-year-old, crumbly, sieved. This is the lightener, keeping the mix open and letting roots move freely. It also holds moisture without becoming sodden.
• Grit — horticultural grit or sharp sand, roughly 3–4mm. This is the drainage. Avoid builder's sand, which contains fine silt and salts, and never use beach sand.

The Mix: Two Parts Loam, One Leaf Mould, One Grit

The classic seed mix is measured by volume, not weight, which makes it easy to knock up with a bucket on the potting bench:

• 2 buckets sieved loam
• 1 bucket sieved leaf mould
• 1 bucket horticultural grit or sharp sand

Tip them onto a hard surface and turn them three or four times until the colour is even and no streaks remain. If your loam is a heavy clay, push the grit up to one and a half parts; if it is already light and sandy, hold back a little. The mix should feel friable and slightly gritty between your fingers, never sticky.

Do not add fertiliser, blood, fish and bone, or sieved garden compost to a seed mix. Save those for a potting-on blend, where you can safely use roughly seven parts loam, three parts leaf mould and two parts grit, with a handful of slow-release feed per bucket.

Making Loam From Turf

If you have not made loam before, start this autumn. Cut turf from a spare patch of old pasture — about 5cm thick — and stack the turves grass-side down in a heap roughly a metre square. Leave them for six to twelve months, covering the stack with a tarpaulin or old tin sheets to stop rain leaching the goodness out and weeds seeding into it.

By the following season the grass will have rotted and the whole stack will break apart in your hands. Riddle it through a 6mm sieve for seed work, keeping the coarser material back for potting mixes or for filling gaps in the vegetable beds. Any loam you cannot use straight away will keep for years in sacks under cover.

Mixing, Storing and Keeping Things Clean

Damping off is the main risk with a homemade seed mix, and it is almost always a hygiene problem rather than a recipe problem. Use clean trays, wash your hands, and avoid reusing last year's compost for seed sowing. Sieving thoroughly helps too, since it removes undecomposed fragments that harbour mould.

Once mixed, dampen the compost to the point where a squeezed handful holds its shape but no water drips from your fist. Store it in a lidded bin somewhere cool and dark, and use it within twelve months — after that, leaf mould continues breaking down and the mix loses its open structure. If you have had trouble with damping off in the past, fill your trays, pour boiling water through them, and let them drain and cool overnight before sowing.

Sowing In Your Own Mix — And Tweaking It Next Year

Fill trays loosely, firm gently with a flat block or your fingers, and water before sowing rather than after. Sow thinly, cover with a fine layer of the same mix, and keep the trays somewhere bright with gentle bottom heat at around 15–18°C. Water from below where you can, and give plenty of ventilation once the first pair of seed leaves opens.

Your mix will probably be a touch heavier than a commercial one, which suits beans, brassicas, tomatoes and most hardy annuals very well. For slow germinators like parsnips and carrots, add an extra handful of grit per tray. Keep a note in the shed diary of what you sowed in which blend and how it performed, then adjust the ratio slightly the following season. Within two or three years you will have a mix tuned precisely to your soil and your climate — and a shelf full of strong, stocky seedlings raised entirely from your own ground.