What do the NPK numbers on fertiliser mean?

They are the percentages by weight of the three nutrients plants need in the largest quantities: nitrogen (N), phosphorus (P) and potassium (K), always in that order.

A fertiliser labelled 10-5-5 contains 10% nitrogen, 5% phosphorus and 5% potassium. The remainder is carriers, fillers and usually trace elements.

Each nutrient does broadly different work.

Nitrogen drives leafy green growth. High-nitrogen feeds suit lawns, brassicas and leafy crops. Too much produces lush foliage at the expense of flowers and fruit, and soft growth that attracts aphids and suffers in frost — the classic cause of a tomato plant that is enormous and unproductive.

Phosphorus supports root development and establishment, which is why starter and rooting feeds are higher in it.

Potassium supports flowering, fruiting and general hardiness. This is why tomato feed is high in potassium, and why the same feed works well for any flowering or fruiting plant — peppers, chillies, strawberries, roses and container flowers all benefit.

The practical shorthand: N for leaves, P for roots, K for flowers and fruit.

A balanced feed such as 7-7-7 is a reasonable general-purpose choice. Match the ratio to what you want the plant to do at that moment: nitrogen early for growth, potassium later for cropping.

Two cautions. More is not better — excess fertiliser scorches roots and leaches into watercourses. And feeding will not fix a problem caused by drainage, light or pH, which are far more common limiting factors than nutrients.

A soil test is worth doing before adding phosphorus, which many soils already hold in surplus.