How often should I fertilize my tomato plants?

The interval changes across the season, and so does the fertiliser

Tomatoes want different things at different stages, and feeding them one product all summer is the usual reason for lush green plants carrying four tomatoes.

At planting: work compost into the bed, plus a balanced or slightly phosphorus-forward starter. This is the moment for phosphorus, because it moves slowly through soil and roots need to find it where they are growing.

Early growth, before flowers: a balanced feed every 2–3 weeks. Some nitrogen here is right — you are building the plant that will carry the crop.

From first flowers onward: drop the nitrogen and raise potassium. Nitrogen after this point pushes leaf and stem growth at the expense of fruit. A tomato-specific feed, or anything with a high final number in the N-P-K ratio, is what you want.

From first fruit set to end of season: every 2 weeks in the ground, weekly at half strength in pots, right through until harvest finishes.

Containers are a different regime entirely

A tomato in a pot is on life support and you are the life support. Every watering flushes soluble nutrients out of the drainage holes, and in high summer a large tomato plant in a container may need watering daily. Under that regime, a monthly feed is close to nothing.

The standard approach is a dilute liquid feed weekly — half the strength on the bottle, twice as often — which keeps availability steady rather than spiking it. A slow-release granular fertiliser mixed into the compost at planting gives a baseline underneath the liquid feeds, topped up every 6–8 weeks.

Consistency of watering matters as much as feeding, and the two interact. Calcium uptake depends on steady moisture, and blossom-end rot — the sunken black patch on the bottom of the fruit — is usually a watering-consistency problem rather than a calcium deficiency in the soil. Adding calcium to soil that already has some does not fix it. Even watering and a mulch does.

Reading the plant

• Too much nitrogen: dark green, thick, fast-growing foliage, curled leaf tips, few flowers, or flowers that drop. This is the most common tomato feeding mistake by a wide margin
• Nitrogen shortage: older lower leaves yellowing uniformly while the plant stalls
• Potassium shortage: yellowing and scorching at leaf margins, uneven ripening, blotchy shoulders on the fruit
• Magnesium shortage: yellowing between the veins on older leaves while the veins stay green — common in containers, and easily corrected with Epsom salts
• Phosphorus shortage: purple tinting on stems and leaf undersides, usually in cold soil early in the season, and often it resolves as the soil warms without any intervention

Practical cautions

Never feed a dry plant. Water first, then feed, or you risk burning roots with concentrated fertiliser. Feed in the morning or evening rather than in full midday sun, and keep liquid feed off the leaves.

More is not better. Over-fertilised tomatoes grow fast, soft and attractive to aphids, and salt buildup in container compost is a real risk over a season — flush pots occasionally with plain water.

And do the free thing first: a soil test. Most established garden beds are not short of phosphorus, and feeding what is already there costs money and produces nothing.