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20 January 2014

What are Perlite and Vermiculite?

You may have seen perlite and vermiculite mentioned in the gardening magazines or seen bags of them at the garden centre and wondered what they are. The following is a brief description of how they are formed and their main gardening uses.

Perlite is a unique volcanic mineral which expands by up to twenty times its original volume when it is quickly heated to a temperature of approximately 1600 F. This expansion is due to the presence of two to six percent combined water in the crude perlite rock.
When expanded, each granular particle of perlite is sterile with a neutral pH and contains many tiny, closed cells or bubbles. The surface of each particle is covered with tiny cavities which provide an extremely large surface area, hold moisture and nutrients and make them available to plant roots. In addition, because of the physical shape of each particle, air passages are formed which provide optimum aeration and drainage.
The USA is estimated to be the largest consumer and producer of crude and expanded perlite, with China, Greece, Japan and Mexico among the other main producers.

Vermiculite is the geological name given to a group of hydrated laminar minerals which are aluminium-iron-magnesium silicates, resembling mica in appearance. Rock and other impurities are removed from the crude ore which is then crushed and sorted into sizes.
As with perlite, heating to a high temperature changes the contained water to steam, causing it to expand (exfoliate) up to 30 times its original volume, converting the dense flakes of ore into lightweight porous granules containing innumerable minute air layers.
Exfoliated vermiculite is light and clean to handle, and will absorb and hold a wide range of liquids. It possesses cation exchange properties, allowing it to hold and make available to the growing plant ammonium, potassium, calcium and magnesium.
The main commercial mines are found in Australia, Brazil, China, Kenya, South Africa, USA and Zimbabwe.

Essentially perlite and vermiculite are used in the horticultural industry because they both provide aeration and drainage, they can retain and hold substantial amounts of water and later release it as needed, and are sterile and free from diseases. They have a fairly neutral pH (perlite is neutral), and they are readily available, non-toxic, safe to use, and relatively inexpensive. They are both very light and are ideal for providing a thin covering over seeds.

Both perlite and vermiculite are excellent for seed germination, generally mixed with peat or potting compost, though both can be used without compost provided that they are kept wet.

They are both ideal for rooting cuttings, generally mixed 1:1 with peat, but for harder cuttings and fragile plants use 4 parts to 1 part peat.

Both vermiculite and perlite will give a very light, open compost, holding more water and facilitating re-wetting, thereby lengthening the time between watering.
A 50/50 mix of vermiculite and sphagnum moss peat is widely used for greenhouse pot plants and hanging baskets, while a 25/75 mix is generally suitable for for bedding plants, nursery stock, etc.
To improve an existing compost add 20-25% by volume of Vermiculite and mix thoroughly.

Perlite is used wholly or partly in place of sand or grit in potting compost and can be used to open up ready-mixed loam or peat-based composts. For soil-less composts mix 3 or 4 parts of Sphagnum Moss Peat with 1 part of perlite (80/20).
For loam based compost mix equal parts of sterilised loam, peat and perlite (1:1:1) plus limestone and nutrients. Alternatively a 1:2:1 mix may be used. Mix thoroughly, then water well after planting, and feed as appropriate.

I will be using vermiculite in my seed trays again this year. I suggest you give it a try and see the benefits for yourself.

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