It is common practice to use a standard tomato fertilizer for feeding chilli plants. This works well, but it is important to recognise that tomato feeds are not all created equal. There is significant variation in the ratio of nitrogen to potassium between brands, and this will affect the growth of your chilli plants.
Typical N:P:K (nitrogen:phosphate:potash) analyses for tomato feed are 1:1:3 1:1:2 or 2:1:3. Bear in mind that 1:1:3 might be shown as 4:4:12 on the packet to make the feed sound stronger; in reality it is the same thing. So how will each analysis affect the growth of chilli plants ?
1:1:3 has the highest ratio of potash to nitrogen - this will give strong stems and smaller, compact plants which fruit early. They will withstand cool temperatures better and the chillies will have a good flavour. This is ideal for windowsill plants, patio plants and in the restricted space of an amateur greenhouse which has to hold as many different chillies as possible. It is also appropriate if you hope to overwinter your chilli plants.
1:1:2 is also suitable but will give larger plants, with larger leaves and softer growth. The total yield of chillies will be greater and they will be produced over a longer season, but not starting quite so early. The chilli plants will need rather better growing conditions; more warmth and more light and more consistent watering. The plants will need stronger supports.
2:1:3 is a high nitrogen feed more suited to a professional growing environment with an elaborate support system.
Feeding your chilli plants too early will encourage the production of leaf and stem, and delay flowering. The base dressing in the potting compost will keep the plants going until they have started to fruit, and if they start to get hungry that will promote early flowering. Once chillies are starting to form, however, they will be taking a lot of the energy from the plant, and it will be safe to start feeding. When the chillies are full grown and starting to ripen, this places even more demands on the plant and more generous feeding may be required to keep the plant growing and producing more flower.
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