Abstract
Topsoil and subsoil samples of nine Tongan soils (eight from andesitic volcanic ash and one from coral limestone) were analysed for exchangeable K, step K, Kc and total K. K intensity (AR0K or pK - ½ pCa) and potential buffering capacity (PBC) values were derived from K/Ca Q/I relationships . The contribution of the sand, silt and clay fraction of topsoil and subsoil samples from three soil to whole soil step K values was also examined. The interrelationships between the various K values are discussed and they are compared with the results of a subtractive pot trial examining the effect of K on crop growth. The soils examined covered a wide range of exchangeable K, step K, and Kc levels, although their total K content was very low. A high proportion of the total K content of the soils was accounted for in exchangeable K and step K. Most topsoils and some subsoils exceeded suggested response levels for exchangeable K (0·2 mequiv./100 g) and step K (0·3-0·5 mequiv./100 g), but only one topsoil exceeded the suggested response level for Kc (0·35 mequiv./ 100 g). The sand and silt fraction of most of the soils probably accounted for an appreciable proportion of non-exchangeable K. Values of pK - ½ pCa varied between 3·15 and l·49. Total K was significantly correlated with cation exchange capacity (CEC) step K, and Kc. There was a significant correlation between step K and Kc and both, together with PBC, were significantly correlated with CEC. Exchangeable K was significantly correlated with total K and K intensity. The lack of significant correlations between exchangeable K and either step K, Kc, or CEC suggested a rapid removal of exchangeable K from the soil system. Over the course of the pot trial the soils behaved largely according to their exchangeable K status. Dry matter yield and K uptake were most significantly correlated with exchangeable K and K intensity. Significant correlations were also given against step K and Kc. Exchangeable K and step K decreased over the course of the trial and both the decrease in exchangeable K, and the decrease in exchangeable K and step K combined, were linearly correlated with K uptake.
