Once the first flower opens, nutrition changes its logic: you're no longer feeding growth, you're feeding the fruit that's being built. By the end of this step, you'll know how to spot the right moment to switch to a fruiting fertiliser, how to read an EC/pH dosage chart the way it's meant to be read, how to use a PK booster without ever stacking two of them, how to recognise blossom end rot and fix the real cause rather than the symptom, and how to tell a genuine deficiency from one manufactured by a badly set pH.
What changes when fruiting starts
Tomatoes, chillies and peppers are day-neutral: they don't fruit because you shorten their days, unlike other crops driven by a light signal. The real marker is the plant's stage, not a date or a timer setting: the first open flower, then fruit set, the moment the fruit really starts forming behind the fertilised flower. That stage is what tells you it's time to change the menu, not your calendar.
Three elements swap roles
The change concerns three elements.
N, the engine of foliage
Gives up the lead role.
K, takes over
It's what loads the fruit with sugars and keeps the plant rigid as the fruit gets heavier.
Ca, becomes critical at this precise stage
It literally acts as cement for the cell walls of the developing fruit; a local shortage is the number one cause of blossom end rot, covered further down.
On a fertiliser label, this shift shows up in the three NPK numbers, which form a ratio between themselves, not a percentage to be maximised.
The NPK label: a ratio, not a ration
Careful what those three numbers lead you to believe: they describe a relationship between elements, not a ration.
Growth fertiliser: balanced profile
Fruiting fertiliser: K takes the lead, depending on the range
And here's the trap the label sets for you
In the reference nutrient solution for soilless tomato, nitrogen doesn't drop when the fruit arrives: it rises, from around 90 mg/L on a young plant to 190 mg/L on a plant in full load, while potassium goes from 144 to 350.
The pepper nuance. On peppers there is a nuance: after fruit set, potassium ↑ towards 270 to 300 mg/L · nitrates ↓ around 180.
About the units. These mg/L of element are a lab unit rather than a bottle-label one: to convert them into mL/L of your own fertiliser, use the converter below, starting from the concentration stated on your product.
As for the switch itself, it's less delicate than people make out. What matters is the plant's stage, not the way you change bottles:
Settled debate. Chilli growers who compared a clean switch with a gradual transition never found any difference between the two, and some even run the fruiting formula from start to finish. No need to treat it like clockwork.
The real risk lies elsewhere: in the dose, and in the nitrogen still present in an already loaded substrate. Conversely, keeping a nitrogen-rich fertiliser going for too long maintains the foliage at the expense of flowers and fruit: on chilli-grower forums, that's the number one symptom of a plant that won't fruit — lots of dark green leaves, lots of flowers, no fruit set. Outdoor gardeners follow the same logic in their own way: a nitrogen-rich feed to launch the plant after transplanting, then a switch towards potassium as soon as fruit set begins, comfrey tea or a natural potassium sulphate such as patentkali.
🌶️🍅 Before blaming the fertiliser if flowers drop without setting, first rule out a common non-nutritional cause indoors: temperature — above 32 °C or below 15 °C at night, fertilisation stalls. The other common cause, the absence of pollination, deserves its own section: it comes up next.
