Rubidium-potassium ratios were determined on foliage, litter, and surface soils of plots in two plantations of Pinus resinosa 41 to 46 years old previously fertilized once with potassium. Calculations based on indigenous soil rubidium as the "tagging" ion demonstrate that after 9 years some 60 percent of the foliage potassium is still derived from the fertilizer, and after 23 years about 40 percent of the foliage potassium is derived from the fertilizer. Additional fertilizer potassium is present in soil and litter, indicating the high retention of this mobile element in the pine ecosystem.