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Bonsai Fertiliser NPK Explained

Updated: Jan 7, 2023

Bonsai Fertiliser can be a tricky subject as there isn't a 1 stop solution for everything, We have to be calculated about what we use, how much we use and at what ratio we have our ingredients at. So how to we work this out? lets take a look.

Bonsai Fertilizer Explained

To start out with lets look at the basic ingredients that you will see in most fertilisers that is marked on the bag as NPK followed by a ratio like 8-12-5.

(N) Nitrogen : Nitrogen is the first ingredient you will see and can be your best friend or your enemy depending on what you are trying to achieve. Nitrogen is responsible for not only keeping your tree nice and green but it also helps the tree grow. In development we use a high nitrogen fertiliser on our trees because we want to put on as much growth as possible and the larger that growth the more photosynthetic it will be meaning more energy for the tree. This sometimes confuses beginners because we always hear in Bonsai that we want really small growth and it is believed that we try to achieve this all the time, This is where learning that development and refinement are 2 different beasts will greatly benefit your Bonsai skills. So keep in mind that when developing Bonsai as nursery stock we want to get as much growth as quick as possible so the tree can thicken quicker, Before we go into refinement we will cut most of the larger growth back so having growth not useable for bonsai is no issue for now. this is where nitrogen will be your best friend. But why can it also be your enemy?

In refinement this is the stage where we have now thickened our tree to our desired thickness and have cut it back to the rough size and branching we want for our final Bonsai. When in refinement our goal is now the opposite of development, now we want those super small leaves and short internodes ( the distance between the leaves ). If we were to fertilise with a high nitrogen now we wont achieve this because we now know that nitrogen causes elongation ( longer internodes ) and larger growth. Nitrogen is still important in the refinement stage to keep your tree nice and green and healthy but it is applied at much smaller rates then in development. In development we also use In Organic soil mediums which allows us to control how much fertiliser is in our Bonsai Pot at any one time as most in organic soils have little to no Cation Exchange Capacity ( nutrient holding capacity ). This means that what ever you put on the tree can access but in the case of nitrogen it will be completely gone within 4 weeks so if you need more nitrogen you will need to re apply. Organic soils in development hold on to way more nutrients within the soil.

(P) Phosphorus : Phosphorus is the ingredient that helps us grow a nice strong root system as well as helping with strong healthy flowers and fruits. If you have a tree that is struggling with root growth then a higher phosphorus fertiliser might be the trick to give it the boost it needs. Phosphorus is also helpful to younger trees trying to establish a healthy root system.

Phosphorus also helps your trees set new buds so this can be important through the autumn season to help your tree get its buds set for winter dormancy and the spring flush.

(K) Potassium : Potassium some times refereed to as potash while also helping establish healthy fruits and flowers is also important for protecting the tree against stress and helping with winter hardiness and fighting off diseases. Potassium is a vital component of helping the tree function properly.

Ok so now we know what the NPK stands for and what each component does best, how about some general tips?


So the NPK is what we refer to as our Macro Nutrients, But we also have micronutrients that can either be found in organic matter or added to in organic fertilisers. There are a few extra nutrients that can help your tree thrive in smaller amounts which are

boron, copper, iron, molybdenum, zinc, chlorine and manganese.

these micro nutrients are not needed in the higher amounts like the Macro nutrients but it is beneficial to your soil health and tree health to have them present as they do play a role.

These micro nutrients are more important in the refinement stage due to using in organic soils which have a harder time creating a beneficial soil system, In development we use organic soils which have a decent amount of Micronutrients already available.


When choosing fertiliser you will often be faced with organic fertilisers ( made from organic matter such as bone or plant meal or manures ), or In Organic fertilisers which are chemical fertilisers.

organic fertilisers are often the preferred method as they have much more beneficial matter in them and they promote a healthy soil system overall rather then just a 1 time boost of nutrients. Organic fertilisers create microorganisms which improves your soil make up, Microbes help organic fertilisers be soluble to your plants so they can take up the nutrients and most organic fertilisers have all the micro nutrients you need. Organic fertilisers are not water soluble and will need to break down to begin feeding the plant so there is usually a pre application period of around 4 weeks. If you are planning on fertilising closer to the late spring early summer period after your growth has hardened off then you will need to apply organics 4 weeks in advance then every 4 weeks there after at the desired rate for what you are trying to achieve. Be warned also that wildlife is drawn to organic fertilisers and you will often have birds stealing your tea bags or they will pick the fertiliser directly off the soil surface of your tree, that's where fertiliser baskets come in handy to protect your fertiliser.

In Organic fertilisers are usually a powered form, or a liquid that is concentrated and mixed in with water to give your trees an instant boost that it can take up instantly as these fertilisers are water soluble. In organic fertilisers will need to be applied either every 2 weeks or 4 weeks depending on what you are trying to achieve and what type of fertiliser you are applying. As a word of caution though liquid fertilisers do have the ability to burn your root tips if applied to heavy, un like organic fertilisers which are released slowly to the tree as it breaks down in organic fertilisers are available instantly which can cause you to provide too much instantly.

There is 1 more thing i would like to mention which is health tonics such as seasol sold here in Australia. Although seasol do have a range of fertiliser i often see people buy their health tonics as a replacement for fertiliser or buying it mistaking it for a fertiliser. think of these like a human taking vitamins, it gives you a top up in essential things your body needs to thrive but it doesn't actually replace what you get from food and a healthy balanced diet. Imagine how you would feel if you stopped eating food and just ate vitamins everyday.

Tonics are good for helping a tree after a stressful period such as re potting or a decline in health overall, most tonics can be applied as a foliar feed which is helpful as nutrients that are applied via foliar feeding can be taken up around 10x quicker by the tree then if applied through the root system, this can see an increase in health fairly quickly. This needs to be applied during the morning time when the Stamata on the leaves are fully opened up, if they are applied as the day gets warmer the stamata begins to close as the weather warms up which will lower the amount the leaf can take in.


If the Soils PH is too low or high it can be harder for a tree to take up nutrients, this can be caused either by the type of soil you are using or by the water you are using. You can buy cheap test kits which can test both your soils PH and your Water PH and you can also purchase products which will help raise or lower your PH in either your soil or in your water.

For most trees a healthy soil PH will be between 6.0 and 7.0. For things like azaleas the lower the Ph the better. This is why Kanuma is the preferred soil medium for Azalea Bonsai.


This is probably just as important as the type of fertiliser you use, when fertilising the first thing you need to do before applying any type of fertiliser is ask your self " What am i trying to achieve", If you cant answer that question try and figure that out first as that will help you understand what you need to use, and how much you need to use. For example if you are developing a tree and trying to grow it fast then a high nitrogen organic fertiliser will work wonders all through out the year, but if you are in refinement and want small growth then on a deciduous tree for example you wouldn't be applying a high nitrogen fertiliser just before the first flush of growth in spring as this will cause all your branches to elongate which will ruin your design and internodes.

If you are preparing a flowering tree for show then something a little higher is Phosphorus and Potassium will yield you better results for your flowers

If you have a cutting you are trying to establish and organic fertiliser with a higher phosphorus amount will help establish the root system of that tree.

i will give you 1 last example before i go, with our black pines just before we de candle in summer we remove all nitrogen from the tree because we want smaller needles after de candling. if we still had nitrogen during and after our de candling chances are we will get long needles due to the nitrogen.

I hope this article has helped you somewhat with understanding fertiliser a little better.

Although this isn't a scientific article on exactly how fertilisers and plant growth works i have tried to put it as straight forward as possible, as long as you understand the basics there is no real need to go too deep.

Until Next Time, Enjoy Your Bonsai Journey

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