Zeolite for Bonsai Mediums

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?

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

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.

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

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.

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!

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/