Fertilizing
I wish I were more of an authority on fertilizing, but
I can offer only its basics, and a few important or useful concepts.
Fertilizing is sometimes incorrectly call "feeding". One
doesn't feed a plant... one supplies it with required nutrients and the plant
makes its own food, feeding itself. Feel free to use the term "feed" (everyone
does), but understand the difference! This is more important than just being
picky about words. This is why one shouldn't fertilize a dormant tree.
A tree without leaves on it cannot make food. An evergreen in the midst of
winter is not active, is not making food, and so cannot make much use of
fertilizer. Likewise, a sick tree is often fertilized, thinking that is its
problem, but it ends up making things worse. A sick tree is not functioning
properly to begin with; adding fertilizer to the soil may interfere with
water absorbtion.
Fertilizing isn't only for inducing growth --- it's important
for normal health of any tree. Most bonsai soil mixes are coarse and contain
little nutrient material; fertilizing is the only source of nutrients for
many bonsai.
What to use: almost any of the widely-available fertilizer
mixes are good. Always use a fairly weak concentration, say for "houseplants",
and use often, perhaps weekly. This is much safer than infrequent strong
fertilizings. Liquids allow you to spray the foliage directly, commonly known
as "foliar feeding", which is recommended by most growers; I suppose the
nutrients are more directly and quickly absorbed directly into the cells
where they are used, for quicker and more efficient usage.
Pay attention to the percentages of nitrogen, phosphate,
and potassium listed on the fertilizer package. They are listed as "N-Ph-Po"
on the label; for example, 15-10-5 would indicate 15% nitrogen, 10% phosphorous,
and 5% potassium.
High nitrogen encourages growth; phosphorus and potassium
are important to cell structure, root development, flowering, and probably
other things. Look for fertilizers that contain "trace elements" also. These
are various elements and compounds that plants need in small amounts ("micronutrients"),
and the addition of these help prevent a deficiency that can interfere with
normal functioning of the plant's processes. (A plant with an iron deficiency,
for example, will have trouble making food no matter how much nitrogen you
dump on it!) You can add supplements to fertilizer mixes. "Superthrive" and
various seaweed extracts are examples of supplements you can add to strictly
N-Ph-Po fertilizers to enhance the mix.
Bonsai shops sell fertilizer "cakes" which are placed on
top of the soil in the pot; these give a constant, weak fertilizing at every
watering. I have experimented with breaking up large tree fertilizer stakes
into small chunks, and placing these on top of the soil as a cheap, handy
substitute. The only problem with these is that they are usually more concentrated
fertilizer and you risk "burning" the roots if you use too much or at the
wrong time. I've also used the smaller, houseplant fertilizer sticks, which
appear to be a little safer.
The best time to fertilize is in the spring. Early
spring, with high-nitrogen mix for those young trees in development, where
you want fast, strong growth. Late spring (after the initial growth
surge) with low-nitrogen mix for trees at a more mature stage. Fertilizing
should continue into the summer until the point at which trees go semi-dormant
due to hot weather. Fertilizing should be resumed when the weather
breaks, with very low-nitrogen mixes, until the onset of fall.