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Bonsai for beginners, Issue #013 Bonsai fertilizer
March 02, 2020

Bonsai Fertilizer

Hi everyone It is still summer in New Zealand with temperatures reaching 30 Celsius a lot of time has been spent watering trees. I did set up a sprinkler system for a short time with the intention of having some time away, however you are never sure of how much water each tree is getting, if any and the water wastage seems criminal, so I have returned to hand watering every night for my own peace of mind. A friend sent me some photos of some old Japanese trees a while ago and in the photos I noticed small cages on top of the soil, these may look like some kind of diabolical insect trap but the are in fact fertilizer baskets. I have been meaning to give this a go for sometime now so I thought it would be a good demonstration for this newsletter.

First the basics

Should I feed my Bonsai?

A tree that had been re-potted and root pruned should be able to survive 12 months without feeding. You may however wish to boost growth or stimulate other characteristics of your tree. If you have an older tree that you do not re-pot each year, feeding may be required.

When should I feed my Bonsai?

There are several times that you should not feed. Don't feed directly after root pruning, wait at least 8 weeks - you may use a soil mix that includes a slow release fertilizer and it will do it no harm. Don't feed sick or diseased plants. This is one of the most common mistakes I hear through emails. "My tree was sick so I loaded it with fertilizer to try and bring it back to health." It won't if your tree is diseased, it will not be taking up food and at best the fertilizer will be leached through and wasted, or at worst it will build up and prevent water uptake compounding the trees problems. Don't feed during heavy rain, our pots are only small with very good drainage and if you fertilize during rainy periods, liquid fertilizer especially will be leached out. Do water before by all means, about 25% moisture level is ideal. Don't feed during hot weather, this is not quite so obvious but at high temperatures most trees will shut down and stop taking up nutrients. Around 20 degrees C or 68 degrees F are optimal temperature for absorption. Do feed your trees during the growing season, usually spring and summer in most temperate climates.

What should I feed my bonsai?

Plant food is broken into two major groups; macro-nutrients and micro-nutrients. Most potting mixes will provide your trees with enough of the micro-nutrients so you shouldn't need to add any. So we will talk about the macro-nutrients. Nitrogen - Plants use nitrogen to grow foliage and it’s used up very quickly by most plants. Lack of nitrogen causes stunting and yellowing of older foliage. This is by no means the only thing that causes yellowing so be careful. Check your plant for other symptoms before feeding. Make sure yellowing is mainly on older growth and not on young shoots. Phosphorus - This helps promote strong root growth and will give your tree a look of maturity. Potassium - This works with the other nutrients as a catalyst. It also inhibits the nitrogen preventing it from producing too much soft wood. Potassium aids in the manufacture of sugars and starches, it also helps produce chlorophyll. It is good for fruit and flowers and builds resistance to cold. Calcium - This is used to build the cell walls it also makes phosphorus and potassium more available to the plant. Don't over use it on acid loving plants like azaleas, it will make the soil alkaline. Sulphur - Sulphur is part of the molecules that make up plant proteins, a deficiency may lead to weak root development and paler foliage.

Ok so; Fertilizer companies are required by law to tell you how much of three of these macro-nutrients are in their products. They are Nitrogen, Phosphorus, and Potassium. They will be marked as a ratio of N.P.K. Some N.P.K. numbers seem low 2-1-3 others high 24-4-16. This is because they are expressed as a total percentage of the packaged weight. The powder nutrients tend to have higher N.P.K. values than liquid as they are not already partially diluted in water. You could give yourself a headache trying to work it out. Don't worry too much; what you need to look for is a good balanced ratio where all three numbers are very close to each other, unless you particularly want to boost one of the characteristics promoted by a particular compound in that ratio.

Organic versus Chemical


Pro - Less chance of burning slower acting, won’t bolt the tree encourages beneficial bacteria

Con - A lot of people are using sterile potting mix, without the bacteria in the soil organic food will not work very well.


Pro - You can buy varying degrees of nutrients You can get slow release (Osmocote etc) Ease of use/no smell

Con - Can burn trees roots Can harm useful bacteria

Organic fertilizer A fish based fertilizer is good watered in thoroughly every 2-3 weeks in spring and autumn. Be sure to water until it reaches the bottom of the pot. If you only have a few trees it may be best to mix up a bucket and dunk your trees to make sure. You don't want to water only a section of the soil or it will draw the roots to the surface.

Chemical There are hundreds of different chemical fertilizers that come in powder liquid and slow release pellets. I cannot recommend any one in particular. I will say be careful with what ever one you choose, it is better to mix under the recommended strength and apply more regularly than give your plants one strong dose.

Here is a recipe I got from Deborah Koreshoff's book "Bonsai Its Art, Science, History and Philosophy" that I managed to pick up on Trademe. (The book is out of print) It is a great book. Ingredients 3 parts soya bean meal 1 part blood and bone 1 part chicken manure 1 part wood ash 1 part fish emulsion

Method Place all the ingredients into a container, 4 times the volume of the ingredients and add water to bring it to half way. Put a lid on and leave it to bubble away for about 3 months. After it is fermented top up the bin with water. Use one part of this mix to five parts water and dunk or water your plants thoroughly. You can also make small fertilizer cakes from the same mixture by adding less water allowing it to evaporate. Once evaporated add plain flour to make it sticky and form it into small biscuits. Let these dry in the sun until hard. Just place these on the soil of your trees they will act as a slow release fertilizer.

Our fertilizer

There is nothing in these tablets that will harm your trees, so they are ideal as a long-term slow release. There are some plants that can be sensitive to chemical fertilizer such as Ulnus Pavaflora Chinese Elm . I have looked up a few recipes for these tablets and most require cottonseed meal, which I am having trouble finding in New Zealand. I do however have a lot of coffee grounds, which I have successfully used instead.

As mentioned above all fertilizer has an N.P.K rating and someone has gone to the trouble of rating several organic materials as well, so cotton seed meal has a rating of 6.2.1 and my coffee grounds are at 2. a little bit lower in the nitrogen stakes. This is rectified by other additives.

Our Recipe


4 parts coffee grinds or cottonseed meal

4 parts Blood and Bone

1 part rice flour

1 packet yeast

3 cups fish emulsion

Water or worm farm juice to mix


It is as simple as mixing the ingredients together, adding enough moisture to allow it to hold together when squeezed and rolling them into suitable sized balls. they should be allowed to dry out before use. The cages are a good idea as they stop them being washed off the soil or being eaten be animals or insects.

I tried these on some elms that needed a boost. They had a prune and a weed. The second photo is taken two months on.

Good luck and all the best Mark

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