How about including expanded clay pellets of the kind used in hydroponics in the substrate?
On browsing this excellent forum it has just dawned on me that these pellets may well have been responsible for the only plant-happy aquarium we ever had...
...many years ago I mixed some of these pellets into the substrate in desperation to bulk it out as I hadn't bought enough "proper" gravel. ( I seem to recall that the new plants and fish were waiting patiently in their bags for their new home
This small 20g tank was so good for the plants that I took out an armfull of cabomba and friends every fortnight.
About 12 months later we moved house and I took the chance to sieve out the offending pellets and top up with "proper" gravel. We have never had the same success with plants since. Until just now I put this down to differences in the local water supplies.
After reading here about "binding sites", and "ion exchange", though without fully understaing the chemistry, I am suddenly gripped with the conviction that these pellets were responsible for some sort of plant magic!
Could this be?
( And as it happens I still have the original sack full of pellets in my shed!