Vermiculite

Vermiculite. Vermiculite is sometimes recommended as part of a substrate mix, though not by me. What is vermiculite? The Vermiculite Association gives this description:
"Vermiculite is the mineralogical name given to hydrated laminar magnesium-aluminum-ironsilicate, which resembles mica in appearance. Vermiculite is found in various parts of the world. Locations of the predominant commercial mines are in Australia, Brazil, China, Kenya, South Africa, USA and Zimbabwe. Vermiculite mines are surface operations where ore is separated from other minerals, and then screened or classified into several basic particle sizes. When subjected to heat vermiculite has the unusual property of exfoliating or expanding into worm-like pieces (the name vermiculite is derived from the Latin 'vermiculare'— to breed worms). This characteristic of exfoliation, the basis for commercial use of the mineral, is the result of the mechanical separation of the layers by the rapid conversion of contained water to steam. The increase in bulk volume of commercial grades is 8 to 12 times, but individual flakes may exfoliate as many as 30 times. There is a color change during expansion that is dependent upon the composition of the vermiculite and furnace temperature."
 
There you are. Don't confuse vermiculite with laterite, or with the zeolites that are used in chemical filtration.
 
Jim Kelly, one of the early Aquatic-Plants Digest subscribers, uses an enriched bottom layer composed of loamy soil and vermiculite. He likes the loam because it is fine-grained, for intimate contact between plants' root hairs and soil particles, and because it has a high cation-exchange capacity (CEC). I like cat litter for similar reasons. However, Jim quotes Diana Walstad's advice not to use potting soil or compost, in order to to avoid decaying organic matter. This suggests to me that as far as the loam is concerned, it's only the silt and clay content in the loam that he really requires. According to Jim, the vermiculite has to be kneaded and squeezed in water to get out the air that would make it float and to separate the layers, getting as fine a mixture as possible. I feel that all Jim's handiwork renders the vermiculite as much like unbaked "cat litter" laterite as possible. You ought to check out Jim's useful article, "Aquarium technology for the very, very budget minded".