Comprehensive Guide to Bonsai Fertilizer｜How to Use Organic and Chemical Fertilizers
A detailed explanation of fertilizer types and fertilization schedules suitable for bonsai. Introduction to how to use organic fertilizers such as oil cakes and bone meal versus liquid chemical fertilizers, recommended fertilizers by tree species, and fertilization techniques to improve flowering in flowering bonsai.
Key Takeaways
A detailed explanation of fertilizer types and fertilization schedules suitable for bonsai. Introduction to how to use organic fertilizers such as oil cakes and bone meal versus liquid chemical fertilizers, recommended fertilizers by tree species, and fertilization techniques to improve flowering in flowering bonsai.
Related Species
Why Bonsai Needs Fertilizer
Bonsai grow in small pots and require fundamentally different nutrient management compared to trees planted in open ground. In a landscape planting, roots can spread widely and absorb nutrients from the soil independently, but bonsai have access to only a limited volume of soil, and fertilizer components are lost with each watering. Additionally, red volcanic soil and pumice soil—the standard bonsai media—have low nutrient-retention capacity and contain almost no nutrients. After mastering the fundamentals of watering, establishing a consistent fertilization rhythm becomes the foundation for healthy growth.
Proper fertilization improves leaf color and luster, strengthens branch development, enhances root vigor, and promotes flowering and fruiting in species that produce flowers and fruit. Conversely, prolonged fertilizer deficiency causes yellowing leaves, weak branches, and eventually a severe decline in overall tree vigor.
To understand bonsai fertilization, begin with the three primary nutrients:
| Nutrient | Primary Function |
|---|---|
| Nitrogen (N) | Promotes growth of leaves and stems |
| Phosphorus (P) | Contributes to root development and flower/fruit formation |
| Potassium (K) | Strengthens roots and improves disease and cold resistance |
The balance of these three elements determines the healthy growth of your bonsai.
Characteristics and Types of Organic Fertilizer
Organic fertilizers are made from fermented and processed plant and animal materials, and have long been the standard choice among bonsai enthusiasts. Representative examples include the following:
- Oil cakes (rapeseed, soybean)
- Bone meal
- Fish meal
- Chicken manure
- Leaf mold
The greatest advantage of organic fertilizers is their "slow-release" nature—they gradually decompose while supplying nutrients. As soil microorganisms break down organic matter, nutrients are released gradually, avoiding sudden concentration changes and minimizing the risk of root damage. After decomposition, they become humus, which improves soil structure and enhances microbial activity.
| Fertilizer | Characteristics |
|---|---|
| Oil cake | Rich in nitrogen; typically used as a "placement fertilizer" set at the pot's edge in solid form. Fermented oil cake has less odor and is easier to handle, while unfermented types are prone to mold and require careful attention |
| Bone meal | Contains high levels of phosphorus; valuable for flowering and fruiting bonsai before and after blooming |
| Fish meal | Contains all three primary nutrients in balanced proportions; has intermediate characteristics between quick-acting and long-lasting effects |
Drawbacks include placement fertilizer becoming moldy during the rainy season, and fertilizer effectiveness depending on air temperature (soil temperature), becoming less effective in cold periods.
Characteristics and Applications of Chemical Fertilizer
Chemical fertilizers are synthetically produced from inorganic salts, with clear and stable composition and fast-acting properties. They come in liquid form (liquid fertilizer) and granular form; liquid fertilizer is widely used for bonsai.
Liquid fertilizer displays an N-P-K ratio and can be selected according to your purpose.
| Season | N-P-K Ratio Target for Liquid Fertilizer |
|---|---|
| Spring budbreak | High nitrogen ratios such as 6-10-5 |
| Fall, after summer heat subsides | Ratios high in potassium such as 5-10-8 to strengthen roots |
Because commercial products are used diluted, concentration management is straightforward even for beginners.
Another advantage of chemical fertilizers is minimal odor. They are especially valuable for bonsai grown on balconies or indoors. They are also free from rot concerns during the rainy season and maintain stable effectiveness.
However, overuse easily causes "fertilizer burn," which damages roots, and inhibits soil microbial activity. Compared to organic fertilizers, chemical fertilizers offer no soil improvement benefits, and over the long term soil conditions may deteriorate.
How to Use Organic and Chemical Fertilizers Together
Many experienced bonsai growers use a "combination" approach, using organic fertilizers as the foundation while supplementing with chemical fertilizer.
The basic strategy is to use organic fertilizer primarily during growth periods (spring through summer) and chemical fertilizer as a supplement during periods of tree recovery and development (fall). Seasonal fertilization guidelines are as follows:
| Season | Fertilization Guidelines |
|---|---|
| Spring (March–May) | Trees become active during budbreak; place fermented oil cake solids at 3–4 locations around the pot's edge and supplement with liquid fertilizer 1–2 times monthly |
| Rainy season (June–July) | High temperatures and moisture promote rot; remove placement fertilizer temporarily and switch to liquid fertilizer only |
| Mid-summer (August) | Trees enter semi-dormancy; apply no fertilizer as a rule. Excess fertilizer damages roots |
| Fall (September–October) | A critical season for recovery from summer stress. Applying liquid fertilizer high in phosphorus and potassium about once weekly promotes next year's flower bud formation and cold hardiness |
| Winter (November–February) | Deciduous trees are dormant and need no fertilizer. Evergreens and conifers can be maintained with diluted liquid fertilizer applied about once monthly to sustain root activity |
Consulting the annual management calendar for conifer bonsai also helps align fertilization timing with other maintenance tasks.
Fertilizer Tips by Tree Species
| Tree Species | Fertilizer Tips |
|---|---|
| Conifers (black pine, red pine, five-needle pine, etc.) | The presence of mycorrhizal fungi symbiotic with roots is important; avoid relying too heavily on chemical fertilizers, which harm soil microorganisms. Use organic fertilizers as the primary base and apply liquid fertilizer diluted. Avoid excess nitrogen, which promotes undesirable long, thin growth; select formulations low in N content. See pine bonsai candle pruning manual for the relationship between pruning and fertilization |
| Flowering and fruiting types (plum, cherry, apple, crabapple, etc.) | Phosphorus supplementation before and after flowering is critical to fruiting. Deliberately applying fertilizer high in phosphorus and potassium during flower bud formation (summer through fall) significantly improves next year's flower and fruit set |
| Mixed deciduous trees (zelkova, maple, Japanese maple, etc.) | Nitrogen supplementation in early spring when new leaves emerge affects leaf color. However, excess nitrogen causes leaves to grow too large, diminishing the delicate beauty of the bonsai form; careful adjustment of fertilizer amounts is essential |
Guideline for Placement Fertilizer Amount by Pot Size
The amount of placement fertilizer is basically determined by "pot size." Since product granule sizes vary, the following guidelines use commercial solid oil cake (spheres approximately 2 cm in diameter) as a standard:
| Pot Diameter | Placement Fertilizer Guideline (spheres) | Application Method |
|---|---|---|
| Up to 10 cm (miniature/small) | 1–2 spheres | One location at the pot edge; next application on the opposite side |
| 10–20 cm (medium) | 3–4 spheres | Evenly spaced around pot edge |
| 20–30 cm | 5–6 spheres | Evenly spaced around pot edge |
| 30 cm and larger (large) | 7–8 spheres | Evenly spaced around pot edge; do not place at the trunk base |
- The amounts above are near the upper limit for trees with normal vigor. Beginners should start with half and increase based on leaf color
- Liquid fertilizer is managed by dilution ratio regardless of pot size (according to product instructions or more dilute); pot size only changes the volume applied per application, not the concentration
- Replace placement fertilizer approximately every month. If placement fertilizer crumbles and white mold appears, this indicates rainy-season rot rather than the fertilizer being exhausted; replace it early
- For trees where vigor should be reduced (deciduous trees where leaf size is to be minimized, conifers where elongation is to be prevented), intentionally limit fertilizer to half the guideline amount or less
How to Apply Fertilizer and Precautions
Place solid fertilizer at the pot's edge (where the absorptive roots are concentrated) in an even distribution. Direct contact with the trunk causes root rot, so position it 3–5 cm away from the trunk base. Apply liquid fertilizer as a substitute for regular watering, distributing it evenly across the soil mass, and give plenty until the solution drains from the pot bottom.
Avoid fertilizing during these times:
- Immediately after repotting (roots are damaged; refrain for one month)
- In direct summer sun (fertilizer burn easily occurs)
- When tree vigor is severely weakened (further root damage is possible)
Fertilizer has the property that "too little is ineffective, too much is lethal." Beginners should start with half the recommended amount and gradually adjust while observing the tree's response. Regularly observe leaf color, vigor of new growth, and root development, confirming the fertilizer's effects with your own eyes—this habit is the shortcut to bonsai mastery.