Trees are amazingly self-sufficient. They take in needed elements from the environment without having to move to fetch it as animals do. But that can be a limitation as well, since they are dependent on finding what they need nearby.
In the case of most trees, elements leech through the soil and into contact with the roots. Bonsai, since they’re confined to a pot, need supplements added artificially. Fortunately, supplying the right ones in the correct amounts at the appropriate times is one of the simpler tasks associated with caring for them.
Bonsai ‘food’ can readily be obtained from a local nursery or gardening section, or purchased online. The most common type contains NPK, nitrogen (N), phosphates (P) and potassium (K). The last is usually in the form of potash, a material made from the ashes of wood and plants. Beyond these three, bonsai need a number of other nutrients, including iron (Fe) and vitamins (especially B-vitamins).
In a pinch, beer makes a good home-recipe substitute for a B-complex, since it contains several B vitamins. The risk is that, unless the alcohol content is very low, you can damage the tree. Be sure to use a weak American beer and dilute it to at least half-beer, half-water before using.
Whether using beer or commercial B-complex supplements, spraying onto the leaves in cooler or moderately warm weather is an ideal way to apply. Above 85F (29C) the stomata (holes) in leaves close and the tree stops aspirating (’breathing’) – the exchange of gases and moisture through the leaves is radically lowered.
Spraying the leaves during hot weather cools them enough to open up, but they lose moisture to the hot air, which drys out the leaves and ultimately the tree. More bonsai are killed from incorrect watering practice than through any other means.
One good way to apply nutrients is to use commercially available pills. Place them on the soil to the left and right of the trunk and water daily. The water and natural leeching process will import the nutrients down through the soil.
How much nutrient to supply depends on a number of factors, including the age, size and species of the tree, how long the tree has been growing since the last re-potting, how good the drainage is, the development of the root system and others.
Some experimentation will be needed, but following the directions on the packet is a good beginning. Watch for burned leaves or drooping flowers, one sign of too much of a good thing.
Nutrient supplements should be added in larger amounts during the growing season, but small amounts are helpful during the fall (Sept-Oct in the Northern Hemisphere) to aid color enhancement. NPK 0-10-10 is useful at this time.
Feeding every 2-4 weeks is best, but don’t feed the tree immediately after re-potting. Wait three or four weeks. Feeding is best carried out while the soil is moist, but for the pill-type can be part of the watering regimen.