My last post covered Cation Exchange Capacity or CEC. Research for that article surfaced zeolite as the highest cation exchange capacity of any common bonsai substrate by a wide margin. So what is zeolite, and is it worth adding to your mix?

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What is zeolite?

Zeolite is a microporous aluminosilicate mineral, which means that its crystal structure is built from a 3D framework of silicon-oxygen and aluminium-oxygen tetrahedra (the same basic units that make up most rock-forming minerals), but with tiny channels running through it. The channels are around 3–10 angstroms across — small enough that the mineral effectively acts as a molecular sieve, with water and dissolved ions able to enter and exit through the structure rather than just interacting with the outside surface.^ref

Zeolite forms naturally when volcanic ash settles into alkaline lakes and slowly recrystallises over millions of years. Most commercial deposits being mined today are 10-30 million years old, with significant sources in the western United States, Cuba, Turkey, Slovakia, Bulgaria and Australia.^ref

There are over 40 naturally occurring zeolites and ~150 synthetic ones. The one used horticulturally is almost always clinoptilolite, the most abundant and chemically stable natural zeolite.^ref

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Why does clinoptilolite have such a high CEC?

The high CEC comes from two characteristics of the crystalline structure of zeolite. The first is that the presence of Aluminium creates a persistent negative charge in the crystal which isn’t affected by pH. This is in contrast to akadama which has lower CEC as pH goes lower.

The second is that the tiny channels mentioned above massively increase the available surface area available for cation exchange since it can take place within the particle itself. In mediums without these channels, only the surface of the particle is available.

The theoretical CEC of zeolite is 220–260 meq/100g, which real-world samples typically come out at 75–94% of this or 150–220 meq/100g.^ref This is unmatched in any other commonly used bonsai medium component.

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What does clinoptilolite (zeolite) do that other mediums don’t?

1. Pre-charging with ammonium for slow-release nitrogen

Clinoptilolite happens to be very good at holding onto the key macronutrient of Nitrogen, in the form of Ammonium (NH4+). This is because the tiny channels running through the crystal are around 3 angstroms across which is almost exactly the same size as a hydrated NH4+ ion. This slips into the ‘cages’ within the clinoptilolite and is held by hydrogen bonds to the surrounding framework oxygens, like a puzzle piece fitting into place.^ref

If the zeolite is pre-soaked in an ammonium-rich solution before potting, nitrogen is absorbed and then released slowly to roots over weeks to months as it’s displaced. This is important for bonsai because nitrogen is usually quickly leached out of soils by watering. Some data: in sand-based container substrates, pre-charged clinoptilolite at 5–10% reduced nitrogen leaching by 77–89% and phosphorus leaching by 90–96% without compromising plant growth.^ref

The original trials use ammonium sulphate solution at a few grams per litre, soaked overnight. Ammonium sulphate is available as a horticultural fertiliser but a practical alternative more in keeping with an organic feeding regime is a compost or comfrey tea. Both are NH4+-rich leachates and would charge the exchange sites with the same ion you’d be supplying through normal fertilisation. Including some organic matter in the substrate would also then resupply the zeolite as it broke down.

2. Urease inhibition – extending nitrogen availability

Urease is the enzyme that breaks urea down into ammonium, and it’s one of the fastest-acting enzymes known. It can break urea down into ammonium very rapidly and typically within a few days. The problem with this fast action is that the resulting ammonium either gasses off (10–40% of applied nitrogen is lost this way in agricultural studies), or gets converted by soil bacteria to nitrate which then leaches out of the pot with the next watering.

Zeolite slows this whole cascade down by trapping ammonium on its exchange sites as fast as urease produces it. The ammonium isn’t lost to the atmosphere because it’s pulled out of solution and it isn’t converted to nitrate because the nitrifying bacteria are starved of substrate. It stays plant-available for weeks rather than days.^ref This matters for bonsai because urea (or urea-like intermediates) is the major nitrogen pathway in every animal-derived organic fertiliser (manure, blood meal, fish emulsion) so the addition of zeolite will assist with retaining more of the nitrogen in the fertiliser for your tree.

3. Particle stability

Unlike akadama, clinoptilolite is a crystalline silicate and it does not break down under environmental stresses. The particles are resistant to damage by freeze-thaw cycling, repeated wetting/drying, or root pressure and last a lot longer than akadama whilst retaining their beneficial properties. Industrial uses for water treatment and radionuclide capture have demonstrated the same particles functioning for decades.^ref

4. Water retention via internal porosity

The channels inside clinoptilolite particles help hold onto water, so zeolite can improve the water retention qualities of your substrate. Horticultural studies have reported water-holding up to ~60% by weight, and substituting 30% clinoptilolite into peat-based potting substrate increased water-holding capacity 2.6× and total porosity 8%.^ref

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What does zeolite NOT do well?

pH. For various reasons, zeolite holds the substrate close to neutral. For ericaceous species being deliberately kept acidic (such as those potted in kanuma at pH 5.5), zeolite would slowly push pH up toward 6–7, against the goal for those trees. So zeolite is not recommended for acid-requiring species.

Variable product quality. Commercial clinoptilolite ranges from ~50% pure (mixed with feldspars, smectites, opaline silica) to >90% pure and bag labelling rarely specifies the deposit source, purity or measured CEC. The CEC of what you actually buy could be anywhere from 80 to 200 meq/100g.^ref Probably it’s best to source from a bonsai supplier who will be motivated to find a product which works for bonsai.

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How to use zeolite in a bonsai mix

I personally add some zeolite to all my substrates except for azaleas and ericaceous species (as well as biochar). If you want to do this too, here are the pointers:

• Buy clinoptilolite specifically — other types of zeolite don’t have the same evidence base.
• Source from a bonsai supplier where possible — they’re motivated to stock a product that works for bonsai.
• Match particle size to your substrate — 2-5mm for typical mixes, larger only for very coarse mixes.
• Mix at 10-20% by volume of total substrate — higher proportions don’t add proportional benefit.
• Pair with organic fertiliser — these supply nitrogen as ammonium, which zeolite holds strongly.
• Pre-charge (optional) — soak in dilute ammonium sulphate solution or a strong organic fertiliser leachate for 24 hours before potting to load the exchange sites with ammonium for slow release.

I hope this is useful and happy soil mixing!

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Full reference list for this article

1. Mumpton, 1999 — La roca magica: uses of natural zeolites in agriculture and industry. PNAS. https://doi.org/10.1073/pnas.96.7.3463
2. Inglezakis et al., 2022 — Ion Exchange in Natural Clinoptilolite. Minerals. https://doi.org/10.3390/min12121628
3. Cappelletti et al., 2002 — Methods of determining cation exchange capacities for clinoptilolite-rich rocks. Clays and Clay Minerals. https://doi.org/10.1346/000986002761002739
4. Eroglu et al., 2017 — Applications of Natural Zeolites on Agriculture and Food Production. Agronomy. https://www.mdpi.com/2073-4395/11/3/448
5. Williams & Nelson, 1997 — Nitrogen leaching reduction in container substrates amended with clinoptilolite. HortScience. https://journals.ashs.org/hortsci/view/journals/hortsci/32/4/article-p615.xml
6. Tzanakakis et al., 2021 — Clinoptilolite zeolite influence on nitrogen dynamics. Journal of Soil Science and Plant Nutrition. https://doi.org/10.1007/s42729-021-00566-1
7. Ostrowska & Porębska, 2021 — Long-term stability of zeolite in soil. Minerals. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8156034/
8. Gholamhoseini et al., 2018 — Effect of clinoptilolite zeolite on water-holding and porosity of potting substrates. Acta Universitaria. https://www.redalyc.org/journal/2033/203359541002/html/

Bonsai trees take up nutrients through their roots from the ‘soil solution’ within the pot. The soil solution is the water held in the spaces between substrate particles, together with everything dissolved in it, like dissolved ions (the nutrients themselves), along with gases and organic compounds. Given that bonsai are usually kept well-drained and water generally runs through in a few minutes, any nutrients dissolved in that water leave the pot with it so there’s a limited window for roots to take nutrients up directly from solution. This is where the Cation Exchange Capacity (“CEC”) of the substrate in the pot becomes very important.

What is CEC?

Cation Exchange Capacity is literally the capacity of a substrate to hold positively charged ions (cations) which can be accessed by roots. Why do we care about positively charged ions? Because this is the form taken by many plant nutrients, including four of the six macronutrients — ammonium (NH4+, one of the two nitrogen sources), potassium (K+), calcium (Ca2+) and magnesium (Mg2+) — and five micronutrients: iron (Fe2+/Fe3+), manganese (Mn2+), zinc (Zn2+), copper (Cu2+) and nickel (Ni2+).

The other macronutrients — phosphorus, sulphur and the nitrate form of nitrogen — exist as negatively charged anions in soil solution and aren’t held by CEC at all. They have to be supplied by regular feeding because the substrate can’t store them between waterings.

The reason that substrate particles are able to attract and hold cations is because they themselves are negatively charged. This arises due to imperfections in their mineral structure (where a lower-charged atom sits where a higher-charged one should) and because of acidic hydroxyl (-OH) groups on their surfaces that release their H+ at typical soil pH, leaving the surface negatively charged. The strength of the cation attraction for a particular substrate is its CEC measurement.^ref

How do roots get the ions back from the substrate?

Root cells actively pump out Hydrogen ions (H+) using membrane-bound proton pumps which are powered from cell respiration. You can learn more about these proton pumps and their role in plants in this video. When these ions exit the roots they create a space for another positively charged ion to enter – in this case the nutrient cation. So the plant swaps the hydrogen ion for the cation, bringing that nutrient in.^ref

It should be noted that different substrates bind cations in different priorities. For clinoptilolite (zeolite): Potassium (K+) > Ammonium (NH4+) > Sodium (Na+) > Calcium (Ca2+) > Magnesium (Mg2+) which means a potassium-heavy fertiliser will outcompete ammonium for available sites, effectively reducing the substrate’s nitrogen-holding capacity.^ref

So What?

Hopefully you can see that a substrate with higher CEC will attract some nutrients as they flow through during the watering process and hold onto them so they are available to roots over a period of time. Let’s take a look at the evidence available for CEC levels in different bonsai substrates, where we are looking for the highest numbers:

Component	CEC (meq/100g)	Source
Clinoptilolite (zeolite)	150–220 (theoretical 220–260)	Peer-reviewed [Cappelletti et al., 2002]; [Inglezakis et al., 2022]
Sphagnum peat	100–180	Standard horticultural
Composted pine bark	50–100	Peer-reviewed [Jackson et al., 2014]
Kanuma	62	Hobbyist journal [J. American Bonsai Society Vol 43 #4, 2009]
Turface	33	Hobbyist journal [J. American Bonsai Society Vol 43 #4, 2009]
Akadama	21–31	Hobbyist journal [J. American Bonsai Society Vol 43 #4, 2009]; mineralogy peer-reviewed [Asaoka & Aono, 2006]
Allophanic andisols (akadama parent soil)	10–60, pH-dependent	Peer-reviewed [Harsh et al., 2002]
Kaolin clay	~28	Peer-reviewed [Ma & Eggleton, 1999]
Pumice	~10 (variable: 9.9–73 depending on source)	Peer-reviewed [Guler & Sarioglu, 2014]; higher-end values for zeolitically altered pumice [Kantiranis et al., 2011]
Kiryu (river sand)	11.7	Hobbyist journal [J. American Bonsai Society Vol 43 #4, 2009]
Perlite	3–4	Peer-reviewed [Kantiranis et al., 2011]
Coarse sand	~0	Standard mineralogical

You can see straight away that zeolite has by far the highest CEC of any bonsai medium. In fact its CEC properties mean it is also used in the nuclear energy industry as a way of capturing dangerous radioactive ions such as Cesium-137.^ref

Caveats – pH

Although CEC measures how many cations a substrate can hold, it doesn’t directly translate to the cations accessible to the plant, because pH also affects soil solution properties. Above roughly pH 7 (neutral), several essential nutrients precipitate out of solution and become unavailable regardless of how much exchange capacity is present, including iron, manganese, zinc, copper, boron and phosphorus.^ref

In the UK, tap water in southern and eastern England is ‘hard’ — rich in Ca2+ and bicarbonate (HCO3-) (see my post on Water hardness, pH and bonsai). Over time this causes two related problems in a bonsai pot: calcium builds up on CEC sites in preference to other cations (displacing the K+, Mg2+ and NH4+ you actually fertilised with), and the bicarbonate gradually raises substrate pH into the range where micronutrient availability drops. A high-CEC substrate watered with hard tap water therefore slowly alkalinises and Ca-saturates over months, even if it started at a sensible pH.

There are a few ways to mitigate this. The simplest is to use rainwater where possible, particularly for ericaceous species like azaleas and camellias which need sustained low pH. Reverse osmosis (RO) systems also produce near-neutral water with effectively zero buffering, but they’re a more involved option costing hundreds of pounds with annual filter replacements and a membrane change every couple of years, plus they waste 2–4 litres of mains water per litre of pure water produced. RO probably only earns its place if you already have a system for drinking water or have a small collection of acid-loving species that genuinely warrants the investment.

Where neither rainwater nor RO is practical, some growers acidify hard tap water before feeding using household vinegar (5% acetic acid) at roughly 1 tablespoon per 4.5 litres, which drops typical UK hard tap water from around pH 7.5 to pH 6. This works but with caveats: acetic acid is a weak acid so the effect is short-lived once the water hits the substrate’s buffering capacity, and any sustained pH shift requires consistent dosing at every watering. Commercial nurseries use stronger acids (sulphuric, phosphoric) for the same purpose, but these require careful handling and aren’t really hobbyist-appropriate.^ref

A more sustained option is to add elemental sulfur chips to the substrate. Sulfur isn’t itself acidic — it’s an inert yellow solid — but soil bacteria (primarily Thiobacillus and Acidithiobacillus species) oxidise it to sulfuric acid over weeks to months, releasing H+ ions that lower substrate pH.^ref This is the standard commercial method for acidifying soil around ericaceous shrubs and works in bonsai pots too. The trade-off is that it’s a biological process, so it stalls below about 10°C and finer particle sizes oxidise faster than chips.^ref For a bonsai, chips give a slow, controlled effect that doesn’t require daily dosing, with the bonus of supplying sulfur as a macronutrient.

What does this mean in a bonsai pot?

The implication from this post is that to maximise your fertiliser efficiency and tree health you should probably add a high CEC component to your substrate whilst also taking steps to manage pH if you’re in a hard water area. Practically this means adding zeolite to your mix at around 10–20% by volume, in 2–5mm particle size to match the rest of the substrate.^ref If you don’t have access to rain or RO water, also mix elemental sulfur chips into the substrate at repotting — chip-grade sulfur (not powder) works best as it oxidises slowly over the growing season. Also recognise that a substrate with entirely inorganic components such as pumice, sand or perlite will dramatically reduce how much of your fertiliser makes it into your trees.^ref

You should also aim to use an organic fertiliser because in high CEC substrates ammonium is retained in favour of nitrate. Organic fertilisers tend to provide their nitrogen sources as ammonium whereas chemical fertilisers provide it as nitrates.^ref If you’re growing in a low-CEC mix (pure pumice/lava/perlite), it makes little difference which form you use as both wash through, so frequent weak feeding is the more effective strategy regardless of feed type.

CEC matters more for trees in refinement than in development. A development tree in a large training pot has plenty of substrate volume and is usually being fed hard anyway — the substrate’s ability to retain nutrients between feeds matters less when you’re delivering nutrients faster than they leach. For a refined tree in a shallow pot being fed carefully to control vigour, CEC is more relevant since there is less substrate.

For acid-loving species such as azaleas and camellias which are in low pH substrates such as kanuma, CEC is reduced versus the CEC which would be in place at neutral pH. So ericaceous species need feeding little and often rather than relying on the substrate to buffer between feeds.

Now here’s a topic to generate some internet debate! This is really a subject that every bonsai enthusiast has an opinion about – whether akadama is worth the money, whether cat litter is a legitimate medium, whether to add organic material, there is a ton of disagreement on this subject. So how might we take a scientific approach?

Well the starting point is that the growing medium needs to enable the supply of everything that the tree via its roots requires – specifically water, oxygen (for respiration) and nutrients^ref. Now, you may add nutrients via fertiliser, but the medium needs to catch those nutrients so that the roots (or symbiotic bacteria) can absorb them, similarly with water – so one important characteristic is that the medium must hold water in a form which is accessible to roots.

Another super-important attribute of the medium should be that it helps establish and nourish a thriving rhizospere. This means providing a home for beneficial bacteria and fungi, enabling the roots to come into contact and to interact with them and for the roots to generate their exudates. The medium needs to hold and release the substances which are important to these microorganisms, and it needs to allow them to breathe.

We also want to have a medium in which roots grow freely, and ramify, to better support the tree in the pot and provide more surface area for nutrient and water absorption.

Wouldn’t it be amazing if there was such a medium out there? Oh, well actually there is – soil! The world over, the nutrient, rhizosphere and root growth requirements of trees are supplied by soil. According to the Royal Society, “‘well-structured soil’ will have a continuous network of pore spaces to allow drainage of water, free movement of air and unrestricted growth of roots…typically, a ‘good’ agricultural soil is thought to consist of around 50% solids, 25% air and 25% water,”^ref

They also say that “bacterial diversity is affected by soil particle size, with a higher percentage of larger sand particles (ie coarser soil) causing a significant increase in bacterial species richness” and “the ability of soil structure to hold moisture is linked to a high microbial diversity and more robust populations of soil mesofauna and macrofauna”^ref

This study found that bacterial and fungal abundance was positively associated with high phosphate, high pH, a lower Carbon:Nitrogen ratio, sandiness of soil texture and soil moisture. It was negatively associated with the presence of Chromium, Zinc, silt, a high Carbon:Nitrogen ratio or clay soils.^ref

So what can we conclude from all of this? In terms of structure we want the right ratios of soil/water/air (50% soil particles, 25% water, 25% air) and the soil to have a higher percentage of larger, sandy particles (not clay or silt). The question for bonsai comes down to water retention since a pot with a hole is much more draining than soil. Options for water retaining elements in bonsai medium include bark, compost, biochar, perlite or vermiculite. Clay also has high water retention but perhaps too much, as it can cause anaerobic conditions which results in nasty gases being produced by bacteria. Different components such as akadama, lava rock, pumice and so on can provide the structural part of the mix which create air spaces.

Some media have so-called pores – tiny holes which hold water which is accessible by roots. “The higher the large pore (macropore) density, the more the soil can be exploitable by plant roots… the presence of continuous macropores significantly benefits root growth.”^ref An example would be biochar which has a huge surface area thanks to many tiny tubes and pores throughout its structure.

What you want to avoid in your bonsai medium is anything which is too acidic (except if you have an acid-preferring tree) as this would reduce the microbes, or anything with anti-fungal or anti-bacterial properties (such as – ahem – cat litter or diatomaceous earth). You also want to avoid (per the above) anything which reduces the roots’ access to air & water by getting overly compacted or wet, or having overly draining components which don’t hold water.

Bonsai wisdom says that adding ‘organic’ components such as compost or leaf litter is bad for various reasons – they break down and reduce drainage, they run out of nutrients too quickly, they aren’t controllable. But personally I think adding organic matter of some kind is a good thing, as it mimics the natural world, has all sorts of beneficial compounds (such as those included in some non-nutrient additives) and provides some small particle sizes as part of an overall mix.

As it happens, I finally found a bonsai-specific research study! These are extremely rare. In the Journal of American Bonsai Society this article showed the results of an experiment measuring the water retention of different bonsai soil components. See below:

Based on this, if you were using the 25% air 25% water rule of thumb, most of these would be fine as bonsai soil with just a bit of added water retention. Interesting that pine bark is actually quite similar to akadama – I have recently been wondering whether you could grow trees entirely in bark if it was the right size. Maybe it’s time to try!

Another study looked at particle size, finding that “media components that differ significantly in particle size have lower total porosity, water-holding capacity and air-filled porosity than media composed of similar particle sizes.”^ref

One final word on different mediums for different trees. Obviously, different trees come from different habitats and happily grow on soils native to that habitat. I have a tiny olive in a tiny pot with extremely coarse medium that dries out easily and it’s thriving (albeit, I live in London). Angiosperms transpire more than gymnosperms so in theory need more a more moisture-retaining medium. A tree with a very high foliage ratio relative to the size of the tree will also need a lot of moisture. So think about the ‘natural’ habitat of your tree and what the soil conditions likely are, and try to adjust accordingly.

The nice thing about the scientific method is that it’s not all theory – observation and experiment is an integral part. If you start with a general medium, you can adjust it to be more water-retaining by adding compost or bark, or less by adding more akadama/pumice or increasing the particle size. See how things go and adjust when you repot.