Best Compost Mixes for Growing Vegetables in Containers

Why multipurpose compost on its own falls short

A bag of multipurpose is a decent starting point, but it is designed to be adequate at everything and brilliant at nothing. Modern peat-free mixes are largely bark, wood fibre and coir. They break down fast, slump in the pot, and can temporarily lock up nitrogen as soil microbes work on all that woody material. By midsummer your tomatoes are sitting in a shrunken, soggy block that needs watering twice a day.

A good container mix needs three jobs done at once: hold moisture, drain freely, and feed steadily for months. That means blending your multipurpose with loam for structure and grit for air. Get the proportions right and potted vegetables will crop almost as well as those in the ground — often better, because you control everything.

The three-part blend and how to measure it

Work in parts by volume, using a bucket or an old plant pot as your measure. My standard base mix for most vegetables is 2 parts multipurpose, 1 part loam and 1 part horticultural grit. For hungry, long-season crops, shift towards 2 parts loam, 2 parts multipurpose and 1 part grit, which holds nutrients far better through the summer.

On loam: use sterilised loam, sieved topsoil from an old turf stack, or a loam-based potting compost. Avoid soil scraped straight from the veg bed — it brings weed seeds and, worse, clubroot, which persists for twenty years.

On grit: horticultural grit or sharp sand at 4–6mm is ideal. Never use fine builder's sand, which sets like concrete and often contains lime. Into every bucket of mix, stir a generous handful of slow-release fertiliser or blood, fish and bone.

Blends for different crops

One mix will not suit everything. Adjust like this:

• Tomatoes, peppers, chillies and aubergines: equal parts multipurpose, loam and grit, with slow-release feed added. One plant per 10–15 litre pot.
• Courgettes, cucumbers and squashes: richer and more moisture-retentive — 3 parts multipurpose, 2 parts loam, 1 part grit, plus a couple of handfuls of garden compost.
• Runner and French beans, peas: 2 parts multipurpose, 1 part loam, 1 part grit. Deep pots, and don't let them dry out at flowering.
• Carrots and beetroot in deep containers: 2 parts multipurpose, 1 part loam, 1 part fine grit or sand. No fresh manure, or roots will fork.
• Salad leaves, chard and spinach: 2 parts multipurpose, 1 part loam, 1 part grit. Sow little and often.
• Rosemary, thyme, oregano and sage: 1 part multipurpose, 1 part loam, 2 parts grit. Poor and sharp suits them; rich compost makes soft, tasteless growth.

Drainage, pots and watering

Forget the old layer of crocks in the bottom of the pot — it raises the water table and causes more problems than it solves. One good drainage hole, a few pot feet, and a free-draining mix is all you need. Stand pots on gravel or slabs so worms and slugs cannot walk straight in.

Mulch the surface with 2cm of garden compost or gravel to slow evaporation. Group pots together in hot spells so they shade one another, and water in the early evening to cut losses. Small pots dry out alarmingly fast — a 5 litre pot can lose its moisture in a single warm day, so err on the generous side with volume.

Feeding: the mix is only half the job

Slow-release granules last roughly six to eight weeks. After that, top-dress with a small handful of blood, fish and bone every month, scratching it into the surface. Start liquid feeding tomatoes when the first truss sets, and use a high-potash feed fortnightly. Comfrey or nettle tea works well for leafy crops. If tomato leaves yellow between the veins in late summer, a dilute Epsom salts drench usually perks them up. Herbs, by contrast, want almost nothing — overfeeding them is the commonest mistake.

Refreshing and reusing your mix

At the end of the season, spent compost is not waste. Shake out the roots and spread it on beds as a mulch, or dig it into the allotment. Do not reuse it for tomatoes or potatoes, as disease carries over, but it can be refreshed for salads: add one part fresh multipurpose, one part loam and a dash of grit, plus a little slow-release feed.

Store spare mix in a lidded bin or old compost sack, kept just damp rather than bone dry. A mix that has dried out completely is hard to rewet evenly, and you will spend the first fortnight of the season chasing dry patches. Make up a batch in spring, keep it under cover, and you will always have a wheelbarrow of good growing medium ready when the mood takes you.