The best fertiliser for tomatoes in Australia (and when to switch)

Tomatoes are the most rewarding crop in the Australian backyard and the clearest example of two-phase feeding. Get the switch right and everything else is easy.

Stage 1: planting to first flowers (grow)

From transplant until the first flower trusses appear, the job is building a big, healthy vine and root system. Feed 2.5 mL of Medigrow per litre of water every 1 to 2 weeks; young seedlings take it at half strength (our seedling feeding guide covers this stage in detail). In most of Australia this window runs from September or October planting through to early summer.

Stage 2: first flowers to last harvest (bloom)

The moment you see flowers, switch. From here the plant's energy should go into setting and swelling fruit, and that runs on phosphorus and potassium, not nitrogen. Feed 2.5 mL of Medibud per litre every fortnight, a full 9 L watering can (about 22 mL) per established plant, watered around the base or sprayed over the foliage in the cool of the day. Potassium is also the flavour nutrient: it moves sugars into the fruit, which is why well-fed home tomatoes taste like tomatoes.

The rest of the recipe

• Consistent water: tomatoes split and get blossom-end rot when moisture swings wildly. Deep, even watering plus mulch prevents most of it. (Blossom-end rot is a calcium delivery problem caused by uneven water, and steady watering fixes it more reliably than any additive.)
• Sun: 6 to 8 hours minimum.
• Support and pruning: stake early, pinch laterals on tall varieties, and keep lower leaves off the soil.

Common questions

Why do I get flowers but no fruit?

Usually heat (fruit set stalls above about 32 degrees), or too much nitrogen. Keep the bloom feed going; set resumes when the weather settles. A gentle midday shake of the trellis also helps pollination in still weather.

What about tomatoes in pots?

Same two-stage routine, but weekly light feeding instead of fortnightly, because pots leach nutrients with every watering. Choose at least a 40 cm pot.

Granular or liquid for tomatoes?

Liquid, for one reason above all: you can change the diet the week the plant changes phase. Granular feeds lock you into one ratio for months. The full comparison is in liquid vs granular fertiliser.