Ion exchange PRS™ probes (Western Ag Innovations, Saskatchewan, Canada) have been successfully used in a range of temperate agricultural soils to measure plant available nutrients. The 8.5 cm2 resin membrane within a plastic stake makes it significantly more user friendly than traditional ion exchange resin techniques. The manufacturers claim that the probe acts like a passive root surface and the resin exchange surface as an infinite sink. However, some researchers have found that the membranes in forest soils act instead as a dynamic exchanger, i.e. ions can be exchanged between the soil and the resin surface. The ion exchange membrane could also act differently in acidic tropical soils. If these concerns are true, then estimates for annual plant available nutrients, especially phosphorus (P), could be underestimated. The objective of this study is to test if PRS™ probes act as an infinite sink or as a dynamic exchanger. This study was located in a 27 year-old montane tropical forest on the island of Hawaii dominated by the N-fixing Acacia koa. The soil is an Andisol developed from 3,000-5,000 year-old a'a lava. Two sets of PRS™ probes were installed in March 2005 in control plots (C) and in plots with application of both herbicide to control grass competition and fertiliser (300 kg P ha ^-1 every 6 months) (HF). One set of probes was collected every two weeks for 8 weeks; the other set was left in the soil for the entire 8-week duration. The results from this study are forthcoming. These results will be compared with standard chemical sequential extraction techniques. This study will provide a critical test of the nature of ion exchange on the surface of PRS™ probes in tropical forests on volcanic-derived soils.