Growing a bonsai is not just about trimming, pruning, and perfect placement — that’s only half the battle. Knowing about the best bonsai soil and fertilizer will give your trees the best possible environment to grow strong and healthy; simply put, you can’t go wrong if you use this guide.
Here are the essential guidelines to find or get the best bonsai soil or fertilizer:
The ideal bonsai soil mix needs to have adequate water retention and good drainage properties. Also, the soil should allow proper aeration so the air can flow properly between soil particles.
When it comes to fertilizers, there are organic or chemical products. Organic products can be more gentle but slower to act, while chemical products can act faster but they will tend to be abrasive.
Does it seem like too much? Don’t worry, it’s actually quite simple once you get into it; with these bonsai soil and fertilizer tips, there’s no way you’ll have anything but strong, healthy trees that’ll be your pride and joy for many years to come.
Bonsai Tree Soil Guidelines
Bonsai Soil Requirements
With all the excitement about what tree to bonsai, we may tend to overlook the obvious. Before we get into obscure tree varieties, techniques, the best latitudes to grow in, etc., we need to think about the life-giving soil our tree will grow in.
Some people even like to call their soil mixes “substrates”, as technically there’s no naturally-occurring soil there. What we try to do with soil mixes is to imitate the ideal natural conditions where bonsai trees grow.
To achieve that end, there are three conditions to keep in mind.
The first is water retention. Your soil mix needs to hold enough water so that your tree stays strong and healthy.
Air is also important, as you need air pockets between soil particles. This enables roots to have enough oxygen, and for bacteria necessary for photosynthesis to proliferate.
The last, and most important bonsai tree soil condition is drainage, which merits its own section.
Drainage: Why is It So Important?
Drainage serves a twofold purpose in the health of your bonsai tree.
- First, it ensures that the amount of water your tree’s receiving is just right. You may think that you could deal with any excess water by having holes at the bottom of your pot. And sure, that’s necessary too, but it’s not an absolute solution.
As time goes on, soil tends to become more compact. If that happens, both water and air may not have enough space to flow efficiently. If you’ve got excess water in that system, it could end up overwhelming your bonsai’s roots, leading to what’s known as “root rot.”
- Second, is that it provides good aeration to the roots. Oxygen is just as important for strong roots as water. It may not kill the tree to have a little less air, but it’ll grow weaker. That virtually defeats the purpose of growing a bonsai, as the limbs will be too frail or brittle to wire into shape.
Do I Need Organic or Inorganic Bonsai Soil Components?
This is going to be a matter of preference. Both have their advantages and disadvantages.
Organic bonsai soil components are made of dead (decaying) plant matter. Think leaf-litter, peat, etc. They tend to be what’s more closely associated with “soil”, and what’s in most ready-made, cheap, store-bought bonsai pots.
The main disadvantage of organic components is that they break down over time and become too compact, which tends to reduce (or eliminate) water drainage. Their main advantage is higher nutrient absorption.
Inorganic bonsai soil components are made of no organic matter. Volcanic lava and baked clay are very popular choices. These soil types are very well spaced-out, which tends to lead to much better drainage. They absorb fewer nutrients, though.
In current times, there’s a preference for inorganic components in general, but it’s your choice if you know what you’re doing.
Bonsai Soil Substrates
The most common components that makeup bonsai soil mixtures are:
- Akadama (baked Japanese Clay)
- Lava (retains water, adds structure, not suitable for root growth)
- Pumice (it absorbs nutrients and helps root ramification)
- Organic Potting Compost (retains plenty of water, but it’s not great at drainage and aeration, it’s recommended as part of a mix, not used as the only substrate)
- Gravel (it provides grit and space, and is great for aeration and drainage; it used to be the bottom layer in any bonsai pot, but has now been phased out by the first three on this list)
You can mix and match these bonsai soil substrates to make your own soil mixture. Of course, you’ll need to look up which mix goes well with each tree, as they’re not all the same.
How to Make Your Own Mixes
For reference, the most common mix for deciduous trees is:
- 50% Akadama
- 25% Pumice
- 25% Lava rock
And for coniferous trees:
- 33% Akadama
- 33% Pumice
- 33% Lava rock
As you can see, we’re going with inorganic ingredients in this case. They’re the same for both bonsai soil mixes, but the end result is ideal for different types of trees, so mind your proportions!
The basic steps are:
- Sift out the dust from medium-grain Akadama.
- Add the needed quantity of pumice
- Finally, add lava rock. This might need some additional sifting.
- If you need extra water retention, you may add some organic soil ingredients.
Why is Bonsai Fertilizer Important?
You need to provide your tree with maximum nutrition for it to grow healthy and strong. Keeping it well-fed is a crucial part of your strategy to make your bonsai a long-lasting companion.
The basic fertilizer elements are Nitrogen (N), Phosphorous (P), and Potassium (K). Nitrogen helps leaves and stems grow; Phosphorous helps the roots, flowers, and fruits; Potassium is an all-around health promoter.
You should fertilize your tree throughout its growth season (early spring to mid-fall). As a rule of thumb, older trees require less fertilization. Remember: each tree has different requirements, so you need to go on a case-by-case basis.
Types of Fertilizer
You’re going to need different Nitrogen, Phosphorous, and Potassium (NPK) proportions for different periods in the tree’s growth cycle. Many experts apply Nitrogen-rich mixtures (NPK 10:6:6) during the spring, and generally decrease the Nitrogen content towards a more balanced NPK 6:6:6 for summertime.
Nowadays, most experts use NPK 6:6:6 (balanced) mixes throughout, and decrease the amount for the slower months at the tail end of the growth season.
Fertilizers can be classified as organic or chemical.
Chemical fertilizers are fast-acting, but they tend to be abrasive. You may use them if your soil or tree needs a rapid boost, but continued use will inexorably lead to a burnt-out bonsai soil mix.
Organic fertilizers have to be broken down by microbes first. Thus, their action is slower, gentler, but equally durable and effective.
We recommend you go organic when you can.
How to Fertilize your Tree
If you use a solid fertilizer:
- Place it in a small basket to avoid washing it away when watering your tree, or eaten away by birds, bugs, or other meddlers.
- Place the basket face-down on the surface of the bonsai soil.
- Around three baskets should suffice. Be mindful of the fertilizer’s strength and the NPK proportions!
If you use a liquid fertilizer:
- First, pay special attention to proportions. It’s easy to overdo it with liquid fertilizers because you don’t have such clear visual cues as with solid types. Follow the instructions, but in some cases, you’re going to want to dilute your fertilizer a bit, especially after the growth season’s over.
- You’re going to want to apply your liquid fertilizer during your tree’s watering cycles.
Can You Grow Bonsai in Normal Soil?
You can, but it’s not recommended. Generally, you’re going to find pre-potted bonsai in normal soil, but this is not ideal due to a variety of reasons. It mostly comes down to poor drainage and aeration. Normal bonsai soil tends to harden when it dries out, and could potentially damage the tree, or even prevent it from growing outright.
What is the Best Fertilizer for Bonsai Trees?
We recommend you go organic. Chemical fertilizers can damage the soil in the long term and could lead to you having to repot the tree before its time if you overuse it.
As we recommended above, your fertilizer is going to need to have the required mix of Nitrogen, Phosphorous, and Potassium (NPK). Brands like Biogold or Tamahi make great fertilizers. You only have to worry about the adequate proportion for different times of the year, and you’re golden!