Exactly, but my doubts were related to adding the KNO3 under the soil, which under probable anaerobic conditions could turn into ammonia.
This will never occur in a planted tank.
It can occur in some wetland sediments that have extremely high organic matter loading and are extremely anaerobic.
No one in aquariums has such conditions.
This is way beyond Sulfur reduction, and it's a very slow process.........
Who ever told you this is full of beans and has no idea under what conditions are required for this to occur.
This takes over 8 electrons of energy per NO3 converted, this is not a fast process or one that easily achieved. It's actually the hardest of all the transformations.
The bacteria required must be under pure anaerobic conditions with no O2 even remotely close, if you have plant roots, this is never going to occur as they give off lots of O2, as well as the pore/grain sizing in most sediments is too large to for this as well.
The redox levels must be below 0mV, which is extremely low.
In order to do that, you need huge amounts of organic carbon coming in, this is not possible without also depleteing the O2 in the water column and the NH4 leaching as well.
Not good conditions for any fish.
But after reading your suggestions i think it will be a lot safer to dose it to the water column, allowing me to dose as required.
Much safer............let the plants reduce the NO3 to NH4, not bacteria........plants do the same thing, but without the requirement of very low redox, no O2 and NH4..........
If you want non toxic forms of N, add fish and KNO3.......