Hydric properties of high charge, halloysitic clay soils from the tropical South Pacific region

In humid tropical areas, the spatial distribution of volcanic ash soils generally follows a weathering sequence in which halloysite-rich soils represent an intermediate weathering stage between young soils (Andosols) and strongly weathered soils (Ferralsols). Strongly weathered soils rich in crystalline iron oxide and kaolinite are known for their fast and free drainage as well as for their large water retention in micropores, caused by micro-aggregation. They exhibit, however, low available water. Here, we report on similar properties in halloysitic soils derived from volcanic ash, in Tongatapu island, Kingdom of Tonga, Pacific. The soils are fine clayey (68-96% clay) and contain three dominant constituents: humic substances, halloysite and iron oxide (6-12%). They exhibit a relatively large reserve in alkaline and alkaline-earth cations confined to the clay fraction, and more specifically to exchangeable and non-exchangeable cations associated with high charge clay minerals detected by XRD as smectite. Whatever the soil layer and the soil organic matter content, the soil materials have exceptionally high saturated hydraulic conductivity, ranging from 7 to 85 cm h(-1), and large water retention (30-40%) at low hydric potential (<- 15,000 hPa). The water properties show that these soils are strongly micro-aggregated. We propose that they are caused by specific interactions between halloysite and high surface area iron oxide. (c) 2006 Published by Elsevier B.V