Topsoil and subsoil samples of 14 soils (13 derived from basalt and one from coral sand) from islands of the southern Cook Group were analysed for exchangeable K, step K, Kc and total K. K intensity (ARoK or pK- ½ pCa) and potential buffering capacity (PBC) values were derived from K/Ca Q/I relationships. Results are discussed and related to soil classification (soil taxonomy) and dry matter (DM) yield and K uptake from a subtractive pot trial examining the effect of K on the growth of green panic. The K status of most of the soils was very low and a high proportion of their total K content, which varied from 0·005 to 0·47 per cent, was accounted for in exchangeable K and step K. These soils were mainly oxidic Orthoxic Palehumults and Tropudalli. Basaltic soils from the youngest island, Rarotonga, exhibited the highest K levels and exceeded suggested response levels for exchangeable K and step K, although only one soil exceeded those for Kc and K intensity. These soils were Typic Hapludolls, with one Typic Tropudalf. Although mathematically significant, correlations between the various K values proved inconclusive because of the uneven distribution of experimental data. Results from the pot trial showed that plant uptake of K was usually less than that available as exchangeable K, although some nonexchangeable K was involved in three cases. DM yield and K uptake were significantly correlated with most K values, but probably because these values themselves were significantly correlated with exchangeable K.