Here's something about synthetic soil from the WIPO (World Intellectual Property Organization) website.
" . . . Mineral zeolites have been found to be a class of very useful ion exchange media. Many natural species are prevalent and numerous synthetic species have been made in the laboratory. Zeolites are hydrated aluminosilicates of alkali and alkaline-earth cations that possess infinite, three-dimensional crystal structures (i.e.,
tektosilicates). The primary building units of the zeolite crystal structure are (AI,Si)è4 tetrahedra. When Al3+ and sometimes Fe3+ substitute for Si4+ in the central cation position of the tetrahedron, a net-negative charge is generated. This negative charge is counterbalanced primarily by monovalent and divalent "exchange cations." Zeolites have shown the ability to exchange most of their constituent exchange cations as well as hydrate/dehydrate without major changes in the structural framework. Most zeolites have large channels and/or cages that allow exchange cations easy access to charge sites and provide unique cation selectivity.
The use of zeolites as a major soil component has a relatively recent past. U. S. Patent 4,337,078 to Petrov et al. describes the use of a natural zeolite clinoptilolite with vermiculite and peat in a synthetic soil. The term zeoponics has been coined to describe synthetic soils containing zeolites in horticulture. "
Ok, I get it now. Go ahead Dick, explain it to everybody else.