Zeolite
Also known as Clinoptilolite
In short
Zeolites are porous aluminosilicate minerals; the natural form used in horticulture is mostly clinoptilolite. Added to potting mixes they hold and slowly release nutrient ions such as ammonium and potassium and some water, while adding a stable mineral grit.
How it looks
Pale granules, typically white, cream, greenish or pinkish-grey, hard and gritty with a dull or glassy surface. Clinoptilolite crystals are white to green or reddish.
Signs
A zeolite mix behaves like a mineral grit mix: it drains well and does not decompose.
What favours it
Zeolites act as molecular sieves with strong cation exchange; clinoptilolite has a particular affinity for ammonium. They can absorb up to about 55% of their weight in water and release it slowly, and can be a source of slowly released potassium.
Prevention
- Use zeolite as a partial additive rather than the whole mix.
- Rinse dust from granules before use.
Treatment
Start with the first step and move down only if the problem persists.
- Treat it like other mineral grit: flush pots periodically if fertiliser salts build up.
Pros and cons
Pros
- High cation-exchange capacity holds nutrients against leaching.
- Holds some water while staying free-draining.
- Does not decompose.
- Also used to adsorb ammonia in aquarium filters and cat litter.
Cons
- Mined, non-renewable material.
- Holds and slowly releases nutrients, but supplies only some (such as potassium) and is not a complete feed.
Look-alikes
- Pumice
- Pumice is visibly bubbly and much lighter.
- Horticultural sand
- Sand grains are smaller and hold no water or nutrients.
Typical uses
Succulents, bonsai and general container mixes as a mineral additive.
Related
Sources
Written in our own words from these sources. Check a product label before using any treatment.