The endemic tree Acacia koa is used to reforest abandoned agricultural lands in Hawaii.
Growth may be constrained by soil infertility and toxic concentrations of aluminum
(Al) and manganese (Mn) in acidic Oxisols and Ultisols. The effects of phosphorus (P)
fertilization at time of planting, soil type, and seed source on koa growth were studied
for three years. Phosphorus, applied as triple superphosphate and at rates of at least
300 kg P ha−1, significantly increased height, basal stem diameter, crown volume, and
aboveground wood biomass of trees from the local (Oahu) seed source, but not those
from off island (Hawaii). Manganese toxicity in the Oxisol probably slowed growth of
the off-site trees. The local seed source grew similarly in both soil types, but the survival
rate was lower in the Ultisol, possibly due to its lower water-holding capacity. Increased
growth due to fertilization suggested improved P status, but such improvement was not
detected by foliar analysis.