Feeding

Besides taking water from the soil, the roots also absorb the nutrients needed to feed bonsai. It is quite obvious that the small amount of compost in a bonsai tray is inadequate to supply enough nutrients to ensure the development and survival of the plant. So regular feeding your bonsai with fertilizer is vital.

Which fertilizer?

It is hardly surprising that the bonsai tradition frowns upon the use of synthetic fertilizers, particularly liquid fertilizers and mineral-based powders. Apart from any consideration of dismissing them as 'modern', it should be noted that bonsai trees derive most benefit from slow-acting fertilizer, which means an organic fertilizer with a slow decomposition rate.

An ideal bonsai fertilizer contains about 50% nitrates, 30% phosphate, and 20% potash. It may be based on bone meal, fishmeal, powdered horn or dried blood.

It may come in powder form and be sprinkled over the surface of the soil and raked in, or in the form of pellets, which are simply placed on the surface and absorbed by capillary action. The traditional preference is for pellets, but these cannot be used if the surface of the soil has a moss covering, which the pellets may 'scorch'.

There is no single rule about fertilizing. It is a little like watering, as amount and frequency depend entirely on the species of plant and size of tray. You should bear in mind that too much fertilizer does more damage to bonsai than too little. Fertilizers are not intended to make bonsai grow, but to ensure their survival. Too much fertilizer may counteract the effort to dwarf the tree, besides which it may 'burn' the roots and lead to the death of bonsai.

Feed your plants during the tree's growing season, that is, from spring to autumn. Do not give flowering or fruiting trees any fertilizer until after they have flowered. For deciduous trees, continue feeding until the leaves drop, but conifers should not be given fertilizer after mid-autumn. If you use powdered fertilizer, two doses (two teaspoonfuls) per month should suffice. If you fertilize with pellets, wait until they have dissolved before replacing them and do not put the new pellets in the same places as the old ones. Always place the pellets about half way between the edge of the container and the trunk. Make sure, however, that they are slightly nearer the trunk, as they can damage the roots of bonsai. Do not bother to feed in winter, when the bonsai roots absorb very little nutrients.

A foliar feed could be added to the water used to mist over the foliage. However, always use your discretion when tempted to use products advertised commercially for house plant care. Some products, such as leaf polishes, can prove harmful to certain types of plant. Do not confuse bonsai with indoor plants. Even though they are grown in rather similar ways, they need different care. Finally, note that indoor bonsai should not be given any fertilizer for three months after they have been repotted. With bonsai, you must be patient.

Your choice of fertilizers should contain the following minerals:

Nitrogen

This element controls the growth of the leaves and shoots in bonsai. Too much nitrogen is harmful, for the plant develops large, very dark green leaves and delays the production of blossom and the maturation of the wood. Plants are also more susceptible to disease. Nitrogen deficiency is suggested by the pale color and poor growth of the leaves. Nitrogen is usually present in sufficient quantity in fertilizers. Otherwise it can also be added as calcium nitrate or, in the case of plants that are particularly acid- loving, as potassium nitrate.

Phosphorus

This encourages the growth of roots and flowers. Unlike nitrogen bonsai will not take up more phosphorus than it needs. A lack of it is indicated by a reddish tinge to the leaves which also point upwards. Super phosphate applied at the beginning of summer for about two months, in the correct quantities for bonsai, will improve the blossom.

Potash

This helps in the maturing of the wood and increases the absorptive capacity of the roots; it makes the plants more robust overall. A lack of potash is evident from brownish flecks and curling edges on the leaves.

Calcium

This controls the absorption of phosphorous and potash and affects the growth of bonsai roots. A lack of lime causes the vegetative points to die back. Soil test kits are available to test the pH level, and if this is low carbonate of lime can be applied to rectify things.
Over-fertilizing with lime causes a yellow tinge to appear on the leaves (lime chlorosis), for lime prevents the plant from absorbing iron from the soil.

Iron

Among the trace elements such as magnesium, iron, sulphur, manganese, boron, copper, zinc, molybdenum and others, most of which are catalysers in plant metabolism, iron is of particular importance. It is responsible for the development of chlorophyll, thus a lack of iron produces chlorosis of the leaves. This can be rectified by watering bonsai with a solution of chelated or sequestered iron.