Fertilizer Facts and FAQs

In this video I talked briefly about using fertilizer: The 3 Keys to Bonsai Health. But guess what: fertilizer is *not* one of my 3 key elements! Why not? Read on…

Simply put, trees do not die if you don’t use fertilizer. At least, not for several years.

In the first year, without fertilizer you won’t notice any ill effects: the tree will bud and grow out like in any other year. Maybe you might perceive slightly slower growth, maybe not.

In the second year you might actually start to notice markedly slower growth. This is because our good draining bonsai soil generally contains infertile components; baked clay, pumice, and lava are infertile. Even pine bark does not inherently contain any fertilizer. They can all store nutrients for a time.

“Think of fertilizer like vitamins for plants and trees.”

Dave Bluesky

Question: Is fertilizer plant food?

Answer: No, it is not plant food.

Real plant food is the glucose and carbohydrates manufactured by trees themselves, called photosynthate. It is photosynthesized by light on leaves, using carbon dioxide from the air, and water from the roots. 98-99% of your tree is carbon from the air plus hydrogen and oxygen from the ground, all mixed in various different compounds. A living tree is made up of about 15% carbon, 10% hydrogen, and up to 75% oxygen (by weight). So you see, fertilizer does not “feed” a plant. Think of fertilizer more like vitamins for plants and trees.

The remaining small percentage of other elements in your tree are required in the processes of growing, such as chlorophyll required for photosynthesis, and cytokinin which is a growth regulator that promotes cell divisions in roots. Curiously, auxin – the “growth hormone” that promotes cell elongation – does not contain any of the nutrients in a fertilizer; auxin is purely carbon, hydrogen and oxygen.

A good plant fertilizer includes various elements that are found in fertile soil in the earth, of course highly concentrated in the product you buy. The main three elements are nitrogen, phosphorous and potassium, in a much higher percentages than the other 15 or so elements, which are often called “micronutrients” like magnesium and iron.

Nitrogen is one of the elements in Chlorophyll which is needed in all green plantlife to trap sunlight energy and manufacture the photosynthates that build the tree. But even Cholorophyll (C₅₅H₇₂O₅N₄Mg) is only 6% nitrogen, by weight. And chlorophyll makes up only about 0.5% of the weight of a leaf, which varies between species and times of year. So the nitrogen content of a leaf is actually miniscule, but even so, it is needed just like a vitamin in animals.

Question: Can I use “Osmocote 14-14-14 slow release capsules” to thicken the branches and trunk?

Dave’s Answer:

For most trees it is okay for a long-term slow-release fertilizer but I wouldn’t use it alone if your goal is thickening up trees. Combine with another additional fertilizer.

You could sprinkle on some osmocote, but the problem is, you have no way of telling how slowly it’s releasing the chemicals. So maybe best to put a half dose of osmocote combined with another fertilizer – either organic or diluted liquid. I don’t use organic fertilizer from Spring through Summer because birds are a real problem here, and we really don’t like that smell in the garden! I use diluted liquid fertilizer in spring-summer.

I have used something similar to Osmocote here in Spain; I nearly killed a tiny sageretia bonsai by putting too much! But Sageretia Theezans are, in my experience, much more sensitive to fertilizer salts than other trees. I don’t know why. I find it sufficient to use about half or less than the dose on liquid fertilizer manufacturer’s instructions.

Question: have a sweet plum bonsai and during the fall it lost all its leaves on these branches. It is exposed to shade and I would like to know how long it will take to recover its foliage, soil and Akadama and Poussolane. Should I continue to fertilize even if it is bare without leaves.

Dave’s Answer:

Don’t use fertilizer until sometime after buds reappear and new leaves have hardened off. Put it in partial sunlight with a clear plastic bag over it, for a few weeks. The idea is to keep it very high humidity, with some sunlight but not able to dry. Give it a couple of months like that. Hopefully it will start budding again.

Fertilizer could jeopardise the recovery. Fertilizer chemicals in the soil could cause the roots to dry a bit in a process called “reverse osmosis” where the water is sucked from lower salt-concentration in the roots to higher salt concentration in the soil.

Question: I have found some acorns on the ground that have rooted, bringing them home and pot up, should I use some of the akadama in the pot and also do I just give the new seedling water or do I need to feed with a fertilizer?

Dave’s Answer: Yes, akadama is best but quite expensive for seedlings so its up to you. You could mix it with some garden centre substrate (seived to remove the fine particles) and that well help your akadama stretch further.
No, fertilizer is needed at all yet. You could start fertilizing in summer but the first year it gets enough nutrients from the acorn still.

Question: I’ve got 4 pomegranates grown from seed the oldest being 18 years old. She’s only flowered once and I’m not sure if I’m giving her the correct feed. Can I ask what you feed them?

Dave’s Answer:

Wow, I would expect more flowers from an 18 year old pommy. They can flower after as little as 5 years. I’m not a big fan of fertilizers in general, of course they need some fertilizer because they don’t get the natural N, P and K in bonsai soil in a pot. But I use much less than the manufacturers tell us to.

In spring, I don’t fertilize until the first flush of leaves has developed and hardened off. So late April or so, I use a liquid fertilizer, balanced NPK (around 6-6-6) with the normal micro nutrients, and I dilute it with more water than the manufacturers instruct on the bottle.

I fertilize developing (fattening) trees every two weeks, but small bonsai trees I do only once a month. Until mid June. Then I don’t fertilize at all until mid September. In summary, I just do a few feeds in Spring and Autumn, and that works fine. Also, nowadays I only prune the pomegranate in late summer on branches without flowers & fruit, and in winter only if needed for structural shaping.

Question: What kind of fertilizer please?

Dave’s Answer: I recommennd generic liquid fertiizer, approx NPK 6-6-6 or so. With micronutrients. Dilute in water following instructions on the bottle. If your trees are refined bonsai, they need LESS fertilizer than the manufacturer says. In that case, use about half the amount that they say on the instructions. The manufacturers want you to use more fertilizer so that they sell more product! Only use the “full” dose if your bonsai is in developement and you’re trying to grow it like crazy to fatten up the trunk.

Question: I like to fertilize my bonsais, is that wrong?

Dave’s Answer: Not wrong; you just don’t need to apply as much fertilizer as the manufacturers want you to use – and not so often – unless you want them to grow really fast.