Zeolite

Zeolite is currently high on the list of soil amendments I use, following a close behind good old common or garden compost.





Zeolites (from the Greek, Zeo, to boil, and lithos, stone) are a family of hydrated aluminosilicates that swell when subjected to flame.   They have a crystalline structure, which effectively consists of units of Tetrahedra (TO4) molecules, connected by their oxygen atoms.  These then form lattice shapes, meaning that zeolites are full of micropores.

Although zeolites were first identified as naturally occurring minerals over 250 years ago, it's only been over the past 60 years that techniques have existed to allow them to be synthesised.  Naturally occurring zeolites are still obtained through mining (there are more than 50 types), and their high availability and low processing cost make them a fairly cost effective product in comparison to chemically produced fertilisers.

Over a hundred different zeolites have so far been synthesised, to tailor their composition, the size of their micropores and their acidity.  There specific properties have allowed them to be widely used in different commercial applications:

• microporous structure - gives them a huge surface area in comparison to their size;
• hygroscopic - they can attract and retain water;
• adsorption - zeolites can trap other atoms, ions and molecules on their surface;
• filtration - can be used to remove harmful chemicals from water, air and the soil.  Have also been used to trap radiation and are then compressed, preventing the radiation from escaping;
• catalysts to speed up reactions (particularly in the petrochemical industry); 
• stability - they can contract and expand whilst retaining structural integrity and do not break down in the environment.

In the home, you will come across the use of zeolites in cat litter, your pool filter, your aquarium, perhaps even in the concrete you used to construct the foundation of your home.  It's also commonly used as an additive in feed stuff for livestock, as well as occasionally making it's way into food for human consumption.

The highest proportion of zeolite usage is in the production of laundry detergents, where they act as a substitute water-softener (replacing sodium tripolyphosphate or STPP).  Zeolite has no effect on the environment, whereas STPP, which was used for many decades, has been a major contributor to eutrophication of our water systems.

Now, how is this relevant to a website regarding horticulture I hear you ask?

Well, a particular naturally occurring zeolite, named Clinoptilolite, is used as a soil amendment in agriculture and is increasingly making it's way into the domestic horticultural market.

When quite literally chucked around and then dug into the top 10-20cm of your soil, Clinoptilolite can provide a long term source of slowly released potassium.  When loaded with ammonium before use, it can also provide a long term source of nitrogen. The microporous structure and adsorption capabilities of Clinoptilolite allow it create a nutrient reservoir in your soil, releasing other macro & micro nutrients directly to the plant roots.

Perhaps once of the best functions for those of us living in Australia however, is the hygroscopic capabilities.  Clinoptilolite can retain around half of it's weight in water.  So, if you're in a drought situation, your plants will need less watering as their zeolite friends in the soil will keep some handy to deliver directly to their roots when needed.  If on the other hand you're in receipt of a lot of rain, the capture of water by the Clinoptilolite can prevent your plants from having wet feet.

If you're not in the industry, it can be difficult to get hold of a particular zeolite.  However, the properties of most zeolites commonly available to the public will make them a great alternative to Clinoptilolite.

In Australia, you can often purchase zeolite from stores selling swimming pools (such as Clark Rubber - don't try to find it on their website though!  It's $26.95 for a 15kg bag and they don't always have it in) or those providing supplies to the turf industry (e.g. K & B Adams Turf, in Ferntree Gully, Victoria sell 20kg bags for $21).  Also, there's specialist suppliers for organic fertilisers etc that often stock it.  A good example of this is The Green Life Soil Company.

Application rates... well, these depend on your soil type and if you're trying to resolve a particular problem.  However, in general, recommended practice that I've seen quoted in several places is:

• Sand - 250-500g per m2
• Sandy loam - 100-200g per m2
• Poor clay - 50-100g per m2
• Compost - 10kg per m3 (as an alternative to applying it to your garden soil)
• Liquid manure / compost teas - a handful for every 10l
• In potting mix - 2-10% of volume 

Note however that you can't overdose your soil, unless of course you get so carried away that you end up with just zeolite and no soil....  but stuff will still grow in it!!

Where to use it... that's up to you!

I use it in potting mixes for hanging baskets, raised veggie beds, free draining areas where we have indigenous plants, where we have hedges, fruit trees,  in areas where the soil is water repellent, where the soil is heavy clay and of course on the lawn areas.

Give it a try and see what it does for your garden.  You won't regret it :)

Main Sources:
Zeolite Australia
Methods and Reagants for Green Chemistry: An Introduction, by Pietro Tundo, Alvise Perosa and Fulvio Zecchini