All The Things You Need to Know About Bonsai Fertilizer

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To grow a vigorous and healthy bonsai tree, you’d have to supplement it with the right nutrients—and the best way to do that is by applying bonsai plant food, also known as fertilizer, to your tree.

The problem is that there’s no one exact way of properly fertilizing a bonsai, so first-timers can find it a bit tricky and confusing…but not anymore!

To help you understand the importance of fertilizing bonsai trees and how to properly do it, check out the comprehensive guide below.

Read on to discover:

• What NPK in bonsai fertilizers stands for.
• How much fertilizer pellets to apply on each tree.
• When and how to properly fertilize bonsai plants.
• The difference between granular and liquid bonsai fertilizers.
• The difference between organic and inorganic bonsai fertilizers.
• And that’s only the beginning…

Why You Should Fertilize Your Bonsai Tree

Unlike normal trees planted in the ground, bonsai plants have limited access to some natural-occurring nutrients due to being potted in a small container and coarse bonsai soil. To compensate for those lost or insufficient nutrients, you’d have to fertilize your bonsai tree. Doing this helps promote strong and healthy tree growth.

Also, fertilizers are considered bonsai plant food that provides them with necessary vitamins. They significantly enhance the tree’s photosynthesis and cell division processes.

What is NPK in a Bonsai Fertilizer?

Bonsai fertilizers always have a three-digit number (appears as #-#-#) on their packaging, called NPK. Sometimes it may have an “NPK” label next to it; other times, it does not.

This NPK number represents the ratio of the macronutrients—particularly nitrogen (N), phosphorus (P), and potassium (K)—that a particular fertilizer contains.

• N represents the percentage (%) of nitrogen.
• P represents the percentage (%) of phosphorus pentoxide (P₂O₅ = 43.6% phosphorus by weight + 54.6% oxygen).
• K represents the percentage (%) of potassium oxide (K₂O = 83% potassium by weight + 17% oxygen).

For you to understand the importance of these elements in bonsai gardening, let’s discuss the plant’s important macronutrients and micronutrients:

Macronutrients

Every plant needs these three supplemental primary nutrients to grow and thrive:

• Nitrogen
• Phosphorus
• Potassium

These macronutrients are naturally present in healthy, organic bonsai soil. However, since bonsai trees are planted in coarse, inorganic soil, naturally acquiring these macronutrients can be difficult for the trees.

Nitrogen (N)

Nitrogen affects the elongation and expansion of new growths and the general development of the tree above ground. Specifically, it increases the leaves’ size and the stems’ length. It also contributes to the green coloration of the leaves and needles—the higher the nitrogen content, the greener the shoots will be.

Apart from those, nitrogen assists in photosynthesis and the plant’s storage & use of energy.

Common organic sources:

• Manure
• Compost
• Blood meal
• Feather meal
• Cottonseed meal

Common inorganic sources:

• Urea
• Anhydrous ammonia
• Urea ammonium nitrate

What does it mean in bonsai growing: Bonsai fertilizers with high nitrogen content help young, developing trees to thrive more.

Phosphorus (P)

Phosphorus encourages healthy root growth, helping the bonsai plant have a strong and good foundation. It also helps promote good fruit and flower production in fruit-bearing and flowering bonsai tree species.

Common organic sources:

• Manure
• Biosolids
• Compost
• Bone meal
• Blood meal

Common inorganic source: 

• Crushed phosphate rock

What does it mean in bonsai growing: To encourage fruit, root, and flower production, get a bonsai fertilizer with a high phosphorus content.

Potassium (K)

Potassium is a macronutrient that enhances the tree’s overall health. It affects the quality of the tree, including its size, shape, and color. It also plays an important role in photosynthesis since potassium controls the opening and closing of the stomata.

Trivia: Potassium’s Latin name is “Kalium,” hence its element symbol, “K.”

Common organic sources:

• Manure
• Compost
• Wood ash

Common inorganic sources:

• Potash
• Langbeinite
• Granite dust
• Potassium sulfate

What does it mean in bonsai growing: Bonsai fertilizer with high potassium content can help boost a tree’s overall health and growth.

Micronutrients

Apart from the NPK elements, your bonsai tree needs other micronutrients to grow and function properly.

Bonsai plant food or fertilizers contain several of these micronutrients or trace elements. However, they aren’t emphasized the same as NPK elements since the plants only consume a small quantity of them.

There are numerous micronutrients that benefit the tree’s growth and health, but here are just some of the top essential ones:

• Iron (Fe)
• Zinc (Zn)
• Boron (B)
• Copper (Cu)
• Manganese (Mn)
• Molybdenum (Mo)

You can find different bonsai fertilizers on the market with different NPK ratios. That’s because each bonsai fertilizer serves different purposes. Those with high nitrogen content are often marketed for young, developing trees. Then, those with high phosphorus are aimed at fruit-bearing or flowering specimens.

Two Types of Bonsai Fertilizer to Use

Before learning how to fertilize your bonsai tree, let’s first discuss which type of bonsai fertilizer you should use.

There are two types of bonsai fertilizers on the market: granular and liquid concentrate. To further understand these two, let’s talk about them individually:

Granular

Granular bonsai fertilizers come in various sizes and shapes. It can be in the form of pellets, cakes, or spikes. Small, rounded ones are best for mixing in with the soil surface or shohin bonsai, while triangular-shaped ones are best for placing in sloping nebari or topsoil.

Although this type of bonsai food is heavily concentrated, it releases the nutrients very slowly and consistently—only whenever you water your tree. It’s also easier to control, monitor, and store.

Granular bonsai fertilizers sold on the market have different NPK ratios. Always choose one with well-balanced nutrients, such as a pellet-type bonsai fertilizer with an 18-6-8 NPK ratio.

How to use it: Just place a certain amount of granular bonsai fertilizers on the top layer of the soil. You can use fertilizer baskets, tea bags, or bonsai wires to protect them from birds and keep them in place. 

Here’s a video tutorial on how to pack bonsai fertilizers using tea bags:

Liquid concentrate

Liquid fertilizers are fast-acting and can immediately deliver nutrients to the tree’s roots. Bonsai gardeners also use this type of plant food to promote auxin translocation from the tree’s shoots to its roots. This type of fertilization is called “foliar feeding.”

Excess fertilizer in the soil and the foliage are washed away whenever you water your bonsai. That said, you’d have to apply liquid fertilizers more frequently than granular ones. Make sure to follow the measurement in the packaging; otherwise, your tree will suffer from root burn due to over-fertilizing.

Tip: Aside from products specifically marketed as “liquid fertilizer,” you can also use fish emulsion as a liquid fertilizer. 

For balanced nutrition, apply liquid fertilizer with a 3-1-2 NPK ratio to your bonsai tree.

How to use it: Pour a certain amount of liquid fertilizer into your watering can. Then, water your bonsai plant using that mixture as usual. 

For best results, use both types of fertilizers when feeding your bonsai tree, especially on young, developing trees.

Organic vs. Inorganic Bonsai Fertilizer: Which One to Buy?

Bonsai fertilizers can be made from organic or inorganic sources. Continue reading this section to learn about their differences.

Organic Bonsai Fertilizer

This type of fertilizer is made from animal or plant waste. Some of its commonly used natural sources are the following:

• Manure
• Seaweed
• Bone meal
• Blood meal
• Feather meal 
• Fish emulsion
• Cottonseed meal

Friendly reminder: Some manure or fish emulsion products have a distinct odor; you may want to avoid using them on your indoor bonsai plant.

Raw organic fertilizers must undergo a few weeks of fermentation before they can provide nutrients to the tree. However, many organic bonsai fertilizers today are manufactured as ready-to-use products. Most of them come in the form of pellets or cakes that you can easily place on the soil surface of your tree.

Since it releases nutrients slowly, organic fertilizer is less likely to cause root burns on delicate bonsai trees. Some also believe that they are chemically softer in the roots.

The problem with raw organic fertilizers is that they don’t always contain well-balanced NPK and other trace elements. Therefore, you’d have to apply several other fertilizers or use what you have more frequently.

Inorganic Bonsai Fertilizer

Inorganic bonsai plant food is also referred to as “synthetic” or “chemical” fertilizers. That’s because they’re chemically formulated.

Some of its non-organic sources include:

• Urea
• Granite dust
• Phosphate rock
• Potassium sulfate
• Anhydrous ammonia

Since inorganic fertilizers are manufactured “ready-to-use,” the roots can quickly absorb nutrients from them, giving trees a boost in growth. However, you must be cautious in using inorganic bonsai fertilizers as too much of it can lead to root burn. To avoid that, strictly follow the measurements indicated on the fertilizer packaging.

Important note: Some bonsai gardeners advise beginners to avoid using inorganic bonsai fertilizers that contain urea. The reason is that it contains extremely high nitrogen content, which can burn delicate bonsai tree species.

Here’s an example of a urea fertilizer:

A Comprehensive Guide on Fertilizing Bonsai Trees

Fertilizing bonsai plants can be tricky for anyone, as there’s no one correct way of doing it. 

To properly fertilize your bonsai, you’d have to consider several things, including the following:

• Season of the year
• Bonsai tree species
• Quality of your bonsai soil
• Age or state of your bonsai tree
• Type and NPK ratio of your fertilizer 

To help you figure out when and how to fertilize your bonsai tree, check out this comprehensive guide:

When and when not to fertilize bonsai?

The best time to fertilize bonsai trees is during the growing season—early spring to mid-autumn. This is when the trees are actively growing, so they take up more nutrients. You should start fertilizing in early spring when new bud growths start appearing. Then, reduce the bonsai tree food quantity as the tree’s growth slows down mid-autumn.

Now, here’s when you should NOT fertilize bonsai trees:

• When the tree is dormant
• When the tree is under stress
• When the tree is sick or unhealthy

The reason is that sick or dormant trees absorb less or no nutrients, leaving the excess fertilizers to waste. Excess fertilizers can also cause salt buildup on the bonsai soil surface, which can negatively damage the quality of the bonsai soil.

Fertilizing bonsai trees based on their tree type

• Coniferous evergreen trees (e.g., Juniper, Hinoki Cypress) Fertilize during the growing season, from early spring to mid-autumn. 
• Broadleaf & deciduous bonsai trees (e.g., Maple) Start fertilizing in late spring, just after the first leaves harden off. Then, continue until late autumn. If you’re planning to defoliate in May, wait until the second flush of leaves hardens off.

How much fertilizer to use?

The amount of bonsai plant food you should apply would vary depending on your product. Make sure to follow the exact measurement indicated on the product packaging.

If you’re using granular bonsai fertilizers, measure your bonsai pot beforehand. That’s because the size of the pot would determine how many bonsai fertilizer pellets or cakes you should use for your bonsai tree.

Some helpful tips when applying bonsai fertilizers:

1. Always use well-draining bonsai soil to prevent salt buildup on the soil surface.
2. Strong fertilize your trees after decandling or doing a major cutback on deciduous bonsai trees to promote strong and fast growth.
3. Place new pellets or cakes in another area when replacing fertilizers to ensure all parts of the bonsai soil are nourished.

Fertilizing bonsai trees based on their growth stage

• In early development: (Prioritize plant health and growth) Fertilize stronger and more frequently. Using a bonsai fertilizer with higher nitrogen content can help promote fast and vigorous growth.
• In late development: (Prioritize balanced growth) Slightly reduce the amount of fertilizer. Then, use a bonsai fertilizer with a well-balanced NPK ratio.

How often should I fertilize my bonsai?

The frequency of applying fertilizer would depend on the product you use. 

Generally, you should apply liquid fertilizer weekly, bi-weekly, or monthly, depending on the condition of your tree. Slow-release granular bonsai fertilizers can last for several weeks or months, depending on the product and frequency of your watering.

Fertilizing bonsai trees based on certain circumstances

• Older and mature specimens: Fertilize less frequently. You can also cut back on foliar feeding. 
• Coniferous trees: Can be fertilized during the winter season as they’re not fully dormant, so they continue to take up nutrients.
• Tropical and subtropical trees: Can be fertilized year-round as they continue to grow throughout the year and don’t experience seasonal changes.

Step-by-step guide on applying bonsai fertilizer

Now that you know when and how often to fertilize bonsai trees, here’s how you should apply bonsai fertilizers on your tree:

How to apply liquid bonsai fertilizer

1. Prepare the necessary equipment. That includes the watering can or spray bottle, supply of water, liquid fertilizer, and any tool that can be used as a mixer.
2. Read the instructions in the product packaging. Then, look for the fertilizer-to-water ratio.
3. Pour the liquid concentrate into the water.
4. Mix it properly using any tool.
5. Then, water your bonsai tree using the mixture as usual—until the water runs through the bottom of the pot.

How to apply pellet or cake bonsai fertilizer

1. Measure the dimensions of your bonsai pot.
2. Read the per-inch-square measurement indicated on the product packaging.
3. Then, place the bonsai fertilizer pellets or cakes around the soil surface. You can also strategically place them on certain spots or use fertilizer baskets to keep them in place.

Conclusion

There’s no one ultimate rule book for bonsai plant food and its use. To properly use it, you’d need to consider several things, including the condition of your bonsai tree, the time of the year, and the type of bonsai fertilizer you have.

On top of that, you’d have to understand what NPK is and how these macronutrients affect your bonsai. That way, you can achieve the look and growth you want on your tree.