This is what I have learnt so far about Laterite (btw, I use it in my tank with just gravel & EI(ish) dosing).
Laterite
I believe the German company Dupla was the first to market Laterite for the aquarium. Since it is found in the same tropical regions as the plants we use come from, and Laterite run off is found in the waters of the tropics, it seems a logical conclusion that Laterite is part of tropical plants natural element.
Laterite is not really a substrate but more of an additive to substrate. It is a form of clay that contains iron and acts as a sponge to store nutrients, but is meant to be mixed into the existing substrate (usually inert gravel). However, it is clay, and thus gradually softens into a mud-like substance that if disturbed, will cloud your tank dramatically. If, like me, you move plants around frequently, this can mean a cloudy tank all the time. It does not last forever and may have to be replaced every 1 to 2 years.
The high concentration of iron provides an unlimited source of the mineral for the plants, it is however void of any other minerals and contains no organic material as a source of nitrogen. The oxidized iron (Fe3) can be turned into Fe2, (the form of iron most easily used by plants) by organic acids in the aquarium. This happens naturally as the aquarium ages, but can be sped up by adding a small amount of peat (any plain, 100% sphagnum ground peat will do ) to the Laterite.
It is also believed that the soft texture of the lateric soil is conducive to plant root hairs and may be able to absorb other nutrients from the water.
To use Laterite, wash it thoroughly first (make sure you do this!) & then mix a small amount with gravel or course sand and add ground peat if desired, to the bottom layer of the substrate and then cover it with at least two inches of a top layer of gravel or sand. Using Laterite instead of a clay gravel enables you to use any type of decorative gravel you choose. It also does not affect the pH or add silica to the water. Many people have sworn that they get tremendous growth with plants such as Echinodorus and Cryptocorynes.
A mix of Laterite and potting soil (Get some from a nursery & boil it first for 10mins) for the lower layer (2-3 inches) will ensure a reservoir of nutrients (apparently without some humic matter, and a certain level of anaerobic activity the minerals in Laterite could remain locked away). . The top layer can consist of sand or gravel (1-2 Inches). Waste particles are easier to separate from gravel when siphoning the substrate.
Adding a bit of Laterite to other substrates will add a bit of Iron and the plants will look and do better.
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I really like the idea of soil sprinkled with a bit of peat mixed with laterite - oh and for that ultimate kick start, also mix in some mulm. There are more good reasons for doing this that I have read in Diana Walstad's book. I am going to totally rip my tank apart soon and give the 'soil' idea a try.
Andy
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