Koa (Acacia koa) is an important endemic tree species in Hawaii for ecological and economic reasons. Previous research has indicated that koa growth could be limited by low plant available soil phosphorus (P), but koa's nutrient dynamics are poorly understood. A nutrient cycling study was conducted in a 25 year-old koa forest on the island of Hawaii growing on 3,000-5,000 year-old Andisols. Preliminary results suggest that koa growth responds favourably to P fertilisation. The objective of this study was to determine how thinning, grass competition control (CC), and CC + P amendments (300 kg P ha ^-1 every 6 months) alter soil P cycling. P availability was measured using PRS™ probes at 5 and 15 cm depths andincubated for 2 months before they were replaced. Aboveground litterfall were collected monthly, dried and weighed, and analyzed for nutrient content. Preliminary data for January to March 2004 showed that CC + P amendments increased overall available P from 9.2 to 256.2 μg P m^-2 day^-1. Depth of soil had little effect on available P for the control and CC treatments; however in the CC + P amendments treatment, available P was higher at 5 than at 15 cm. For March to July 2004, CC + P amendments increased available P at 5cm from 11.3 to 681.6 μg P m^-2 day^-1, and at 15 cm from 11.2 to 132.1 μg P m^-2 day^-1. Competition control had no effect on P availability. Despite the very high P sorption capacity of these Andisols, P amendments combined with competition control were able to increase and sustain a higher level of available P long after the fertiliser had dissolved. This may increase koa P uptake and ultimately net primary productivity. The dynamics of available P over 12 months and its relation to litterfall will also be presented.