Most K is in primary minerals in most soils except silica sand at 30,000 to 50,000 kg per
acre in the top 6 inches. It leaches and accumulates in biomass so biomass export is the worst for flow off site (silage). Manure provides import (animals concentrate). Next to minerals,
wood provides a reserve, liberated in wood ash. Seaweed is high. Granite dust also high. I suspect that where a vegetation is fully engaging the soil profile, and biomass exports are limited you may be able to mine soil for K at a rate that replaces loss. If you are sending biomass out you may need biomass in. With the large soil reserves, using deep rooted K accumulators makes good sense as a realistic strategy. (Mostly from Brady and Weil). If you have sand you have to be even more careful.