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.