Fertilizing bonsai: when, how and which product to use ?

Bonsai Fertilization: The Key to a Healthy Tree

Fertilization is a fundamental element in ensuring the vigor, balance, and beauty of a bonsai. A well-draining substrate low in natural nutrients, regular pruning cycles, and often cramped pots make nutrient inputs essential. But is it absolutely necessary to choose a "special bonsai" fertilizer ? Not necessarily.

At the nursery, we chose a universal mineral fertilizer NPK 12-12-17 (+2MgO), originally intended for tomatoes. Simple, complete, and effective, it has proven to be perfectly suited to our trees, regardless of their species.

Which fertilizer should you choose for a bonsai? Comparison of the main categories

1. Organic fertilizers (solid)

Examples: manure pellets, compost, vegetable oil cakes, Biogold

• ✅ Advantages : slow-release, enriches soil microbial life, very low risk of burns
• ❌ Disadvantages : sometimes strong odor, slow effect, reduced effectiveness in cold weather

2. Mineral fertilizers (solid or granular)

Examples: NPK 12-12-17 granules

• ✅ Advantages : fast and measurable action, precise composition, long shelf life, easy to dose
• ❌ Disadvantages : no biological effect on the soil, requires careful dosing

3. Liquid fertilizers

Examples: bonsai fertilizer to be diluted in water, growth promoters

• ✅ Advantages : immediate action, useful in case of temporary deficiency
• ❌ Disadvantages : effect too short, dissolves too quickly, risk root burns, requires frequent feeding

❌ Why we don't recommend liquid fertilizers

Liquid fertilizer is often tempting for its ease of use, but we strongly advise against it for regular use on trees.

• It dissolves immediately in the irrigation water, making its action very short-lived.
• There is a real risk of root burn, especially if the tree is underwatered or during periods of extreme heat.
• It is difficult to dose precisely and can quickly lead to imbalance or deficiencies if used incorrectly.

In summary : it is suitable for occasional applications, but not as a primary fertilization.

✅ Why we use a universal mineral fertilizer in granules

The NPK 12-12-17 (+2MgO) fertilizer we use is not originally specific to bonsai—it is a universal fertilizer, designed for vegetable crops (especially tomatoes). But it's precisely its simplicity, consistency, and balanced composition that make it an excellent choice for growing in pots. 

Composition :

• 12% Nitrogen (N) : for stem and leaf growth
• 12% Phosphorus (P) : for root development
• 17% Potassium (K) : for flowering, general health, and resistance
• 2% Magnesium (MgO) : to promote nutrient uptake and intensify foliage color
• 22% Sulfuric Anhydride (SO₃) : soluble and assimilable
• Trace elements : Boron, Copper, Iron, Manganese, Zinc, etc., essential for plant health

How to use

Apply every 2 months, from approximately May to October

Recommended dosage :

Spread over the surface of the substrate, then water lightly.

⚠️ Be careful with fertilizer dosage : excessive application can weaken your bonsai and, in severe cases, lead to its decline.

During periods of intense heat, be particularly vigilant. Nitrogen runoff can cause leaf burn on deciduous species. Ensure that the fertilizer granules are evenly distributed on the surface of the substrate, then water generously.

After fertilizing during hot weather, it is advisable to place the bonsai in the shade for one to two weeks to mitigate this effect. If possible, avoid fertilizing during heat waves.

Can you alternate with an organic fertilizer?

Yes! We even recommend alternating with an organic fertilizer, such as well-rotted horse manure, to sustainably enrich the soil and stimulate microbial life. This is a good way to balance rapid application with a more natural nutrient base. In this case, you can alternate monthly between mineral and organic fertilizer.

In summary

There are a wide variety of bonsai fertilizers, and each can be suitable for a given context or method. For our part, we have chosen to keep it simple, effective, and stable: a solid, universal mineral fertilizer that is easy to manage and perfectly suited to the growing conditions of our nursery.

This is not an absolute rule : other formulas may work, but this one suits us very well because it is a reliable and accessible solution.