What soil and feeding regime do tomatoes need in containers?

Container tomatoes fail for two reasons far more often than any other: the pot is too small and the feed does not change through the season.

Size first. A determinate or bush variety needs at least twenty litres; an indeterminate cordon wants thirty to forty. Smaller pots dry out faster than you can keep up with in July, and the resulting cycle of drought and flood is what causes blossom end rot — that sunken leathery patch at the base of the fruit is a calcium transport failure driven by inconsistent water, not by a calcium shortage in the soil.

Use a soil-based compost, or a peat-free multipurpose mix with around twenty per cent added loam or garden soil for buffering, plus perlite for drainage. Pure multipurpose compost in a large pot compacts by midsummer and holds water badly at exactly the wrong moment.

Feeding changes as the plant develops. Until the first truss sets, the plant is building leaves and stem, and a balanced feed is right. Once fruit begins to form, switch to a high-potassium tomato feed — potassium drives fruit development and flavour, while continued high nitrogen gives you a magnificent green plant with very little on it. Feed weekly at label strength, and stop feeding roughly two weeks before you expect the last harvest.

Water consistently rather than copiously. A daily light watering in hot weather beats a heavy soak every third day. Mulch the surface with bark or straw to slow evaporation.

One thing worth doing that most people skip: remove the lowest leaves once fruit has set on the first truss. It improves airflow at soil level and cuts blight risk substantially.