There are three basic nutrients that a plant needs to thrive: nitrogen, phosphorous and potassium. Organic soils can have these nutrients washed out of them, and often inorganic soils may not have them to begin with. It can be assured that the plant is getting enough of these nutrients by the application of fertilizer. While many specialty shops and nurseries will sell fertilizer supposedly specially designed for bonsai, this is not necessary. Bonsai are plant like any other, and regular fertilizer can be used.
All fertilizer will list the proportion of the three basic nutrients on the packaging, in the order given above. Thus 20:20:20 would mean twenty parts nitrogen to 20 parts phosphorous to 20 parts potassium. Generally, fertilizer should be used that is in equal parts, although this will vary depending on the variety of plant and the season. The application directions on fertilizer packaging should be followed to most effectively apply the fertilizer, usually it is mixed with water poured over the surface of the soil mix. There are also solid fertilizers that are used, particularly in Japan. These are placed on top of the soil and the the nutrients are released when the plant is watered.
The basic elements of fertilizer – nitrogen, phosphorous and potassium, can be translated into a formula for what parts of the plant they effect: nitrogen – leaves and branches; phosphorous – flowers and roots; and potassium – general well-being of the plant. Thus it will sometimes be desirable to use a different proportion fertilizer depending upon the time of year and needs of the plant. For example, in spring, a 40:20:20 fertilizer may be used to encourage growth. Conversely, a 20:40:40 fertilizer would be appropriate for a plant in the fall to prepare it for the coming winter. Bonsai should never be fertilized in winter.
Care should be taken to avoid over-feeding bonsai plants. Err on the side of caution. Again, the manufacturer’s instructions on the packaging of fertilizer should be followed carefully and not modified. Caution should also be used in applying fertilizer to plants that are unhealthy, as this can damage weak roots.