A very thin layer of ground peat under the substrate is beneficial for starting the bacteria that is needed there.
Adding KNO3 or KH2PO4 under the substrate isn't likely to be very effective for very long. Those both dissolve readily into the water, and the NO3 and PO4 ions will easily migrate up into the water column. The K ions may be captured by the high CEC Flourite particles and be more available to the plants. I'm not sure how effective that would be, because I have read that potassium can only be taken up by the leaves, not the roots.