Three sequential surveys of exchangeable potassium (Exch. K) in the surface 10 cm of major Tasmanian soils are presented, together with data from surface samples of profiles collected earlier. Soils with Exch. K less than 0.4 m.equiv 100 g-1 are considered to be low or marginal in potassium readily available for pasture growth. Lateritic podzolic soils were lowest in Exch. K, with four-fifths of the sample sites below 0.4 m.equiv 100 g-1 (median value 0.21 m.equiv 100 g-1). Podzols and humus podzols were similarly low. Available potassium was low to marginal in yellow podzolic soils on sandstone and solodized solonetz and solodic soils. Moderate contents were found in yellow podzolic soils on mudstone, the only soils with appreciable illite, and in organic alluvial soils. On basalt, krasnozems had moderate available potassium but black earths and prairie soils were marginal. On dolerite, non-calcic brown soils had ample potassium (median Exch. K 0.93 m.equiv. 100 g-1) but grey-brown podzolic soils had much less. Most soils showed high variability between farms and one krasnozem soil series had high variability between paddocks within farms. Coefficients of variation ranged from 42 to 58%, in agreement with corresponding values reported by other workers. The contents of Exch. K found are interpreted in terms of factors of soil formation and of land use.