A good bonsai fertilizer does not need to be a special tree fertilizer and can use the same plant food as other plants. The best fertilizer for bonsai trees, however, should be balanced between the three main nutrients. Furthermore, you should not use a bonsai fertilizer that is too rich. Foliar fertilizers, which are liquid garden fertilizers sprayed on the foliage, and soluble soil fertilizers are best for use in the growth season. For evergreens, slow-release pellets are best to use in the winter.
All trees and plants, including bonsai trees, require the same nutrients: nitrogen, phosphates and potash. Nitrogen promotes strong stems and healthy leaves. Phosphates feed and strengthen the roots. Potash promotes resistance to fungal disease and gives a plant resistance to harsh climate conditions. All labels on plant food or fertilizers refer to these three nutrients in this order: nitrogen (N), then potassium (P), then potash (K).
Bonsai trees need these same three nutrients, as well as minute trace elements such as molybdenum, magnesium, zinc and boron. Most fertilizers have these trace elements as well as the three main elements. The three main elements need to be balanced in a bonsai fertilizer for general use. In the autumn, however, you should use a fertilizer without nitrogen for broad-leaved species in order to allow any shoots to harden before winter. Evergreen trees, on the other hand, need a low-strength, slow-release fertilizer for winter because they remain slightly active during the cold months.
Fertilizer labels will contain the letters "NPK," referring to the three main nutrients, followed by three numbers that represent the percentage by weight of each of the three nutrients. The best bonsai fertilizer will have an NPK rating of "7:8:8." This is a balance of all the nutrients, and it will not be too strong or too rich. Higher NPK ratings, such as "20:15:15" are too rich and will cause the bonsai tree's roots to burn. Do not use houseplant fertilizers, because nearly all of these are too rich.
A bonsai tree requires fertilizing at half the strength directed on the fertilizer or plant food label. On the other hand, fertilizing should take place twice as often as the label directs. Do not overfeed a bonsai. Do not fertilize a bonsai tree during the winter months. Following these simple rules for using a bonsai fertilizer will produce a well-fed bonsai tree.