Clay and soil

In setting up a new planted aquarium, modest enrichment of the bottom layer of the substrate provides initial cation exchange capacity for plant roots during the first months of the system's settling-in. There are some alternatives to laterite you might consider:  
 
Clay and Soil. Potter's Clay. Clay is one of the non-organic elements in loam. The slippery quality of wet clay and of potter's "slip" — or of bentonite drilling mud — is caused by the microscopic sheets called colloids. The extremely fine silty quality of potter's clay, and calcium and magnesium it may contain, limit its usefulness in aquaria. I haven't experimented with potter's clay myself. The form of clay most aquarists are using instead is laterite.
 
"Greensand." Greensand is glauconite (hydrated silicon-iron-potassium), a soil additive that counters chlorosis in high alkalinity soils, where nutrients may be bound and unavailable. Some fishkeepers use a little greensand as a substrate additive. In general, potassium is the scarcest major nutrient in the substrate. I have no experience with greensand, however.
 
Potting soil or compost. From time to time experimentive aquarists have tried pasteurized compost mix, and their informed consensus is, to avoid composts or soil mixes that have been amended with fertilizers. Enriched "potting soil" is too rich for aquarium use. Earthworm castings, even used very sparingly are also too rich. In general you want to avoid enrichments that will rot. Besides the sand and clay, which you're getting from other sources, potting soil contains some organic components. If you're like me, you might prefer to control the proportions of these three ingredients yourself. There's no reason to dig subsoil out of your backyard terrain, either.
 
You might consider adding separately, in very limited amounts, some of the following:
 
"Compost" is vegetal matter that has blackened and broken down part way, but it still contains plenty of nitrogenous matter. I feel it is too active for substrate enrichment. (Christel Kasselmann, in Aquarium Plants, 2003, p.52, says that in an ideally light and porous, acidic and lime-free substrate, there should be few or no substances that could rot, though at the same time "a nutrient-rich substrate should be used." Laterites fill the bill, by the way.)
 
"Humus" has degraded much more than compost and from here on it will continue to break down only very slowly. Unenriched humus may be available in bags at a garden center. 
 
Leaf mulch. Dried fallen leaves have already given up their fast-acting nitrogenous substances. The remaining cellulose and lignin break down slowly, like Sphagnum peat.
 
Sphagnum peat (see the following page) is a good substitute for humus.
 
Planting in pots. If you need pockets of soil as rich as potting soil, it's safer to try planting individual plants in pots. For pot planting, you mix an unenriched soil in small terracotta "thumb pots" and plant in those, topping with a scant inch of your upper substrate mix, to keep the soil in place and to disguise the pot's rim.