Once in the pot, the nutritional content of the soil on its own is often not able to satisfy bonsai requirements to grow strong and healthy trees. For this very reason, bonsai enthusiasts regularly use organic or inorganic fertilisers, liquid or solid, pre-made or personally mixed depending on their preferences.
Your bonsai will need seventeen very important nutrients, which are divided into macronutrients and micronutrients. The nutrients include calcium, magnesium, zinc, and iron, to name a few, and they will be absorbed by the tree through the soil.
While the micronutrients are required in very small quantities (one part per million), and trace elements of the most important are usually contained in fertiliser products, the macronutrients are the ones that we are after. Specifically, we’re looking for nitrogen, phosphorous, and potassium. These are often highlighted on fertiliser packaging as NPK with their quantity (for example, N-P-K 4-5-4). These three macronutrients are responsible for the vigour of the tree, the colour of the leaves, the health of the roots, and the fruit and flower production, along with resistance to diseases.
It is important to choose a well-balanced fertiliser that is not too high or too low in these nutrients. Overfeeding may burn the root system or unbalance the growth of the tree and leaves, which we aim to keep at a certain size. Alternatively, there may be enough to keep the tree healthy because it will not be able to withstand the poor nutritional environment.
I personally use liquid fertiliser from spring until the end of summer and then apply solid pellets in autumn and, in some cases, winter. I look for the following proportions depending on my trees’ needs, the season, and the specimen:
- N-P-K 6-3-6
- N-P-K 4-5-4
- N-P-K 5-3-5.
The feeding period commences with the start of the growing season (the end of March), and I feed at regular intervals of at least once every two weeks. The gap increases to once a month in autumn, and then I stop feeding in the winter when the tree starts its dormancy phase (except for indoor bonsai, which I continue to feed).
I hope that this guide for feeding frequency, timing, and portion gives you a starting point to approach this task. Along the way, you will make your own adjustments and improvements to suit your trees.