The potassium status of some representative soils from the Kingdom of Tonga
AbstractTopsoil 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.