The effect of kauri (Agathis australis) on grain size distribution and clay mineralogy of andesitic soils in the Waitakere Ranges, New Zealand

Kauri (Agathis australis) is generally associated with intense podzolisation, but little research has been carried out to substantiate this. We studied soil profiles, grain size distribution patterns and clay mineralogy under kauri and broadleaf/tree fern vegetation in the Waitakere Ranges, North Island, New Zealand. Kauri appears to cause increased breakdown of fines (0–10 μm) in the top of the profiles due to lower soil pH, relative to broadleaf/tree fern forest. The low soil pH under kauri apparently causes the loss of Al-hydroxy-interlayers from interlayered vermiculite and possibly the conversion of vermiculite to smectite. The effect of microtopographic differences in the landscape on the grain size distribution within the topmost horizons can be observed using laser grain size diffraction. Kauri does not cause podzolisation on andesitic material, and although similar weathering of clay minerals was observed, there was no evidence for transport of clay particles.