The hypothesis that the Earth's ''missing'' K may be chemically fractionated into the iron core is examined, in terms of atomic size and electronic structure between K and Fe metals at high pressures. The very compressible nature and possible 4s→3d electronic transitions of K at high pressures would permit K to be chemically fractionated into the Earth's iron core. Models which exclude K from the core on the basis of size and electronic structure are thus inadequate. However, evidence that the terrestrial depletion of K is not due to segregation of K into the core can be inferred from the similar depletion of K on Venus and Mars. The central pressue in Mars (400 kbar) is too low for K to enter a metallic core. However, the K/U ratios for SNC meteorites (assumed to be derived from Mars) overlap terrestrial and Venusian values, implying that the planetary depletion in K relative to CI abundances, is characteristic of the inner solar system.