Does anyone know of organic sources of Potassium?

I live in FL and have a 4000sqft. organic maintained St.Augustine (floratam) lawn.

The University of Florida IFAS Extension states: In Florida, Turfgrasses require potassium in nearly the same amount as nitrogen, especially in sandy soils where both can readily leach out.

I am currently using soluable seaweed powder, as a foliar spray (npk of 1-0-4) along with my regular feeding program.