Why What Goes in the Pot Matters So Much

In a border, a plant that dislikes its spot can send roots sideways in search of something better. In a 12-litre pot on a sunny patio, there is nowhere to go. Whatever you fill that pot with becomes the whole world for your tomatoes, beans and salad leaves — their only source of water, air and food for months. Get the mix right and container growing feels almost effortless. Get it wrong and you spend the summer fighting yellowing leaves, collapsing foliage and fruit that never quite ripens.

Good growing compost has to do three jobs at once: hold enough water to see a thirsty plant through a hot afternoon, drain freely enough that roots never sit in a stagnant puddle, and release nutrients steadily rather than in one brief flush. No single ingredient manages all three, which is why commercial mixes are blends.

The Three Main Types You'll Find on the Shelf

Most garden centres sell variations on three themes. Each has a personality, and each suits certain jobs better than others.

• Multi-purpose compost. The workhorse. Usually a blend of peat or peat-free materials with added fertiliser, it is light, easy to handle and fine for most short-term crops. Its nutrients typically last four to six weeks, so it needs supplementary feeding later on. Excellent for salads, herbs and young plants being potted on.
• Loam-based compost. Heavier, darker stuff made from sterilised loam, sand and organic matter. It holds water and nutrients far better, drains well thanks to the grit content, and gives weighty pots stability — handy for tall tomatoes or a wigwam of runner beans. It is the better choice for anything living in the same pot all season. Do not confuse it with garden soil dug straight from the border, which compacts badly in pots and can harbour pests and disease.
• Peat-free mixes. Made from coir, bark, wood fibre, green waste or a blend, these are now the default for most gardeners. Quality varies considerably. Wood-fibre mixes can dry out at the surface while staying wet lower down, and they sometimes lock up nitrogen in the early weeks. They work well, but reward a little extra attention.

Matching the Mix to the Crop

Different vegetables make different demands, and a little tailoring pays off handsomely.

• Tomatoes. Hungry, thirsty and prone to blossom end rot when watering is erratic. Give each plant at least 10–15 litres of loam-based or good-quality multi-purpose compost and keep the moisture even. Add a handful of slow-release fertiliser at planting, then start liquid feeding once the first truss has set.
• Beans, French and runner. They resent waterlogged roots and will drop flowers if the pot swings between drought and swamp. A free-draining mix with a little horticultural grit or perlite stirred through suits them well. Don't overfeed — a very rich compost gives you mountains of leaf and disappointingly few pods.
• Leafy greens. Salad leaves, spinach and chard want nitrogen and steady moisture in equal measure. Multi-purpose or peat-free mixes are ideal, but be ready to feed fortnightly with a nitrogen-rich liquid feed, particularly in peat-free blends where young growth can look pale. They are shallow-rooted, so they are happy in troughs and window boxes as long as you never let them dry out completely.

Drainage, Watering and the Crock Myth

Putting a layer of broken pots or stones in the base of a container does not improve drainage — it raises the saturated zone and can actually make matters worse. What matters is a free-draining mix and generous drainage holes. Cover the holes with a single piece of broken pot or a scrap of mesh to stop compost washing out, and leave it at that.

Watering style matters as much as the mix itself. Little and often beats an occasional flood, especially in peat-free compost, which can turn water-repellent if it dries out completely. Water slowly until it runs from the base, then stop. Stand pots on feet or stones so they are not sitting in a puddle, and mulch the surface with a centimetre of compost or bark to slow evaporation on hot days.

Feeding, Refreshing and Next Year

Assume that the nutrients in bagged compost are spent by midsummer. From June onwards, feed hungry crops weekly with a liquid tomato feed and leafy crops with something higher in nitrogen. If leaves yellow from the bottom up, that is usually hunger rather than disease.

At the end of the season, spent compost can be tipped into the border or used as a mulch, but do not reuse it for next year's tomatoes — nutrients are exhausted and soil-borne problems can build up. For a fresh start, buy new compost each spring or blend your own: roughly two parts peat-free multi-purpose, one part sieved garden compost and a good handful of grit per bucket makes an excellent all-round mix.

Get these basics right and container growing becomes genuinely productive — a few pots against a sunny wall can easily supply salad all summer, with a decent crop of tomatoes besides.