Training bonsai is a stressful process for the tree and without proper fertilization, results will be disappointing. Normal soils that we see in our backyard usually have a small amount of organic matter already present. This organic matter provides nutrients for growth, but in the bonsai medium no organic matter is present since it consists of akadama, scoria and pumice. Since we are growing in a non-organic medium, we must add nutrients to our bonsai mix or the tree will become nutrient deficient. There are many subtleties when it comes to fertilizing trees, but in this segment, we will focus on the basics. There are 17 elements that are necessary for plant growth. Below there is a table taken from a textbook Soil Fertility and Fertilizers for referene (Havlin et al., 2013).
Visualizing the relative concentrations of these elements in plants is useful for conceptualizing fertilization. When looking at the chart you can see high concentrations of hydrogen, carbon and oxygen. Luckily these concentrations can be captured from water (H2O) and the air (CO2 and O2). The remaining elements must be acquired from the soil. In terms of macronutrients, we have notable concentrations of nitrogen, potassium, calcium, magnesium, phosphorus, and sulfur. We will be focusing on the most important big three, nitrogen, phosphorus, and potassium. When reading nutrients labels you may have seen these elements displayed as three numbers (example 5-5-5) that represent NPK.
Nitrogen is the most essential plant nutrient needed for proper growth. Nitrogen is used to make proteins and nucleic acids in plant cells, making it vital for form and photosynthetic function. Without this necessary element the plant will not be able to continue homeostasis. Additionally, ATP which can be viewed as chemical energy also utilizes nitrogen. There are many other molecules and proteins that utilize nitrogen which makes it incredibly important to incorporate into your nutrient regime.
When nitrogen is abundant your plant will have dark green foliage, vigorous vegetative growth, high photosynthetic activity, and delayed flower/fruit maturity (Havlin et al., 2013). When nitrogen is deficient the plant will exhibit a yellowing of the foliage and sometimes chlorosis (yellow veining of the leaf). Nitrogen is a mobile nutrient which means older leaves will become yellow first because newer leaves will take available nitrogen from older areas of the plant. Below is a photo of nitrogen deficiency in a plant.
When fertilizing your bonsai with nitrogen you most be careful not to overfertilize. Too much nitrogen will create long internodes which are the enemy of refined bonsai. Refined bonsai refers to trees that have been trained to a mature stage of design where branching is often fine and compact with small internodes. Another factor to consider is how nitrogen and carbohydrates interact. High levels of nitrogen cause rapid protein formation which translates to immense vegetative growth with long internodes. If there are not enough stored carbohydrates to match the growth catalyzed by the excess supply of nitrogen, then protoplasm will form creating succulent foliage. This watery foliage is tender and prone to pests and disease. Lower amounts of nitrogen and high amounts of stored carbohydrates in a plant will cause thickening rather than leggy elongation (Havlin et al., 2013). With this said nitrogen is a double-edged sword when cultivating bonsai and you must be careful to minimize over application, especially on refined trees.
Phosphorus has many functions for plants, but the most essential is energy storage and transfer (Havlin et al., 2013). Phosphorus is a component in ATP, an energy rich molecule that is required for most metabolic functions in a plant. Accordingly, when phosphorus is low, growth often becomes retarded. Phosphorus is also part of the structure of DNA, the genetic data of the plant. Adequate phosphorus application will often help facilitate healthy and vigorous root growth and hastens the maturity of flowers, fruits, and the entire plant.
Deficiency in phosphorus is apparent when overall plant growth is slowed, foliage appears darker green and sometimes exhibiting purple coloration. Phosphorus is a mobile nutrient so older leaves will first show signs of deficiency. Below is a photo of phosphorus deficiency.
Potassium is necessary for ATP creation, production of several important enzymes, the absorption of carbon dioxide through the stoma, helps pull water via the roots and maintains electrical balance in the chloroplast. In areas like California where transpiration levels are high, potassium is especially important since it helps the stoma open and close properly. Adequate potassium increases the efficiency of water consumption and prevents plant stress.
When potassium is deficient you will often see necrosis and chlorosis of the foliage. Potassium is a mobile nutrients so older leaves will be first to exhibit symptoms. Below you can see a photo of potassium deficiency.
Organic or Synthetic Fertilizer
When cultivating refined bonsai, we choose organically derived forms of fertilizer because it has a dual function of nurturing the soils microorganisms and providing proper nutrients to the plant. Most synthetic fertilizers are effective and fast acting which is great for developing bonsai in early stages of development, but potentially dangerous for refined bonsai. When growing refined bonsai trees, we want to grow the tree slowly, so internodes are tight, and growth is controllable. Fast acting synthetic fertilizer is easier to overapply which leads to longer internodal growth and potential burning of the tree’s foliage. Additionally, synthetic fertilizers do not provide benefits to soil organisms which are essential to long term cultivation of trees. Below is a picture of a commonly used bonsai fertilizers named biogold.
For conifers and broadleaf evergreens, you can fertilizer for most parts of the year. You want to focus fertilization applications during spring and summer for most bonsai trees. Colder months of the year you can limit fertilization since growth is slowed. Deciduous trees should not be fertilized when they are dormant or entering dormancy since they are not growing vigorously at that time.
Havlin, J., Tisdale, S., Nelson, W., & Beaton, J. (2013). Soil Fertility and Fertilizers (8th ed.). Pearson.