Thank you both for the prompt replies. Would adding a layer of laterite on the bottom gain me much in terms of happier plant roots in general?
No, a rich sediment can impart a backup if you forget to dose the water column.
People who are not consistent, or under dose the ferts will see, obviously.........an increase in growth for plants if they add more ferts(go figure), the location does not matter.
I've had massive swords, Crypts for 20+ years with simple sand and water column ferts. That said, I see no good reason to avoid using a rich sediment, or ferts in the sediment.
They will leach out over time and go into the water column, likewise, water column ferts also diffuse into the sediment in plain sands etc. There's no avoiding either case in our tanks, so all methods are in both locations really.
One of the few research papers of root vs shoot preference showed no differences between root vs shoot source of ferts, they even cut the roots off and the shoot had the same relative growth rate.
Large root systems do not imply root uptake preference also, it could be they are from streams where they need strong roots for anchorage, or the roots act as storage organs(much more likely the case with swords and Crypts) or for when water levels drop and they need a source of water etc. None of these possible reasons imply anything about uptake of ferts from the sediment.