Soils from Andesitic Tephra and their variability, Tongatapu, Kingdom of Tonga

Abstract

The taxonomic units proposed by Gibbs for the sequence of soils from andesitic tephra on Tongatapu have been redefined. The Lapaha series is defined as formed almost wholly from the older more weathered tephra, and the Vaini series as from moderately thick to thick younger tephra overlying the older tephra. Two new series are proposed-Fahefa series for soils from very thick deposits of the younger tephra, and Fatai series for soils with impeded drainage and evidence of gleying. Using the large number of profile observations that have been made on Tongatapu, percentage frequencies of the taxonomic units have been plotted for the mapping units shown on the original soil map of Gibbs, and on a revised map which is being prepared. These have shown that most of the mapping units are very variable and that some of them contain up to four taxonomic units. However, this present study has shown that predictions can be made for any one area on Tongatapu on the range of soils likely, the percentage frequency of occurrence of individual soils, and the dominant soil. Differences between most of the soils are due to variations in the thickness of the younger of the two contrasting tephras. The broad regional pattern of soils is related to the eastward thinning of the younger tephra. Superimposed on this regional pattern there is considerable variability related to local thinning and thickening of the younger tephra. This is partly related to present-day topography, which may have led to erosional redistribution of the younger tephra on sloping land and partly to some other factor unrelated to the present-day topography. This factor is probably related to irregularities in the underlying coral surface.