Feeding regularly during the growth season is crucial for your Bonsai to survive. Normal trees are able to extend their root system looking for nutrients; Bonsai however are planted in rather small pots and need to be fertilized in order to replenish the soil's nutritional content.
The three basic elements of any fertilizer are Nitrogen (N), Phosphorous (P) and Potassium (K), with each element serving different purposes. Nitrogen increases growth of leaves and stems, Phosphorus encourages healthy root growth and Potassium encourages growth of fruits and flowers. Different ratios of NPK are being used for different trees at different times of year, which is very important to take into account when fertilizing Bonsai.
Fertilize during the entire growth season of the tree; from early spring till mid autumn. Indoor trees can be fertilized around the year. Although sometimes disputed, do not fertilize repotted trees for about a month; also do not fertilize sick trees.
It is very important to choose the right fertilizer for your bonsai plants: during the early spring time use a fertilizer with a relatively high Nitrogen content (something like NPK 12:6:6) to boost the tree’s growth. During the summer use a more balanced fertilizer (like NPK 10:10:10) while during the autumn use a fertilizer to harden off the tree for the coming winter (like NPK 3:10:10). The mentioned NPK values are ratios, meaning 5:5:5 is in fact no different from 10:10:10 etc.
A few exceptions are worth pointing out: to encourage Bonsai to flower use a fertilizer with a high Potassium (K) content (like NPK 6:6:12) and for older trees you might want to use fertilizer with a slightly lower Nitrogen (N) content or reduce the quantity of fertilizer applied.
Although “Bonsai fertilizer” is fertilizer like any other, buying from (online) Bonsai shops will help you find the right NPK values. Any fertilizer with the right NPK value is perfectly fine. You can choose to use either liquid or solid fertilizer; which doesn't matter much, just follow the application guidelines as stated on the product’s packaging.
Left: Solid Bonsai fertilizer. Right: Fertilizer covers
Feed your Bonsai using the quantities and frequency as stated on the fertilizer’s packaging. You can choose to reduce the recommended quantity slightly for trees that are not in training anymore, to balance their growth instead of stimulating it. When using solid fertilizer (see photo 1, above) it helps to use fertilizer covers, which make sure the fertilizer stays in place (see photo 2, above). Never overfeed your trees, as this will have serious consequences for their health.