22/05/2026
Chairman’s Chat
Chairman's tips
Fertilizing
Bonsai live in a relatively small container where the availability of nourishment is obviously
limited. When the tree begins to grow, it absorbs the nutritive elements it needs from the soil
in the container as well as from water and the surrounding air, and, in addition from the air
that enters the soil replacing the water as the soil dries out.
There are 16 elements essential to sustain proper plant growth. Carbon, hydrogen and oxygen
are absorbed from water and the air. The remaining 13 must be absorbed by the roots from
the soil. If these elements are not present and available in the soil, the plant will not be
healthy and vigorous.
As the plant absorbs the elements that are in the soil, they must be supplemented and
replaced. The sensible way to do this is by regularly applying fertilizer. In fact, many bonsai
growers presently use a soilless mix, in which case particular care must be taken to ensure
that the necessary elements are provided. Fertilizing practice must be consistent with the soil
mix. This in part explains some of the widely differing recommendations you may encounter.
In any case law requires that the three principal macronutrient responsible for plant growth be
identified on chemical fertilizer labels. The percentage content of nitrogen, phosphorus and
potassium (often called potash) must be identified in that specific order. Thus for example a
fertilizer identified as 20-10-5 would contain 20 percent soluble nitrogen by weight, 10
percent soluble phosphorus by weight and 5 percent soluble potassium by weight. If you have
trouble recalling which number is which, remember that they are in alphabetical order. The
total of the three principal elements in the preceding sample is only 35%.
What happens to the remaining 65%? The remainder consists of a fillers and small amounts
of the remaining 10 elements boron, calcium, chlorine, copper, iron, magnesium, manganese,
molybdenum, sulphur and zinc in various chemical forms. These are called fertilizer trace
elements. If you look at the small print on the fertilizer box, you will often see these
identified.
As a practical matter, however, most bonsai growers do not bother, reading all the small
print, even if their eyesight permits. The best approach is to alternate from one brand to
another at each fertilizer application. That should provide an adequate supply of all the trace
elements. From a bonsai perspective there is not much difference between the major brands,
the basic identified percentage of nitrogen, phosphorus and potash should be your guide.
It is important to understand the role of these elements in the growth cycle of plants, so that
you may apply a fertilizer appropriate to the plants needs at that particular time of the season.
Nitrogen strongly promotes the growth of branches and leaves, so it doesn't make sense to
use a high nitrogen fertilizer in the autumn as winter dormancy approaches.
A deficiency of nitrogen in the active growing season results in decreased growth with leaves
turning a light green or yellow. However, nitrogen must be supplied carefully in some species
to avoid too lush a growth and long internodes, a characteristic not desirable in Bonsai. In the
extreme, too much nitrogen can cause fertilizer burn which may be fatal.
Phosphorus is used by trees to produce flowers, fruit and seeds, eight in the process of
photosynthesis and stimulate root growth. Plants just don't grow well without sufficient
availability of phosphorus. One of the recognizable signs of phosphorus deficiency is the
development of purple or red areas on leaves caused by the impairment of photosynthesis and
resulting loss of chlorophyll.
Potassium insures general plant vigor, increased resistance to disease, activates enzymes
which will help the plant use other elements and in a sense counters the tendency of nitrogen
to produce too much growth. Potassium also helps the plant and your winter temperatures.
The first sign of potassium deficiency is yellowing at the edges of the leaves.
Calcium, magnesium and sulphur also play major roles but are normally present in adequate
measures in most soil mixes, seldom do these elements need to be supplemented.
The remaining elements are micro-nutrients, required in only very small amounts. By
alternating fertilizers, an adequate supply is usually obtained.
In spite of a broad diversity of opinions, there are still some general practices that most
growers of Bonsai accept.
Following is a list of some fertilizer dos and don’ts.
Do fertilize on a regular schedule.
Do consider the porosity of your soil mix and your watering practices.
Do alternate fertilizers.
Don't fertilize flowering trees when they are in flower.
Don't fertilize a newly re-potted plant until new growth has started, at least three or four
weeks later.
Don't fertilize a sick plant.
Do fertilize Young trees more than older ones.
Do ease up on nitrogen in the autumn.
Do use fertilizers higher in phosphorus and potash in the autumn.
Don't fertilize dormant plants in the winter.
Do resume fertilizing when spring growth starts.
Bonsai are pretty resourceful; they survive and flourish under a wide variety of fertilizing
practices. Settle on routine which suits you and your plants best. If one plant rebels, you will
know it.
Remember as John Naka used to say "trees want to live too".
Robert
