PLANT-AVAILABLE ELEMENTS IN SOILS AND THEIR INFLUENCE ON THE VEGETATION OVER ULTRAMAFIC (SERPENTINE) ROCKS IN NEW-ZEALAND

Ultramafic (''serpentine'') soils from the Nelson Region contain low t otal levers of calcium, potassium, phosphate and high total levels of chromium, cobalt, iron, magnesium, manganese and nicker. However, the plant-available fraction as measured by extraction procedures, differs considerably from the total elemental abundances in the soils. The so lubility of individual elements is unrelated to total abundance in the soil. Only extractable manganese and iron could be predicted by their total concentration. Extraction experiments indicated that chromium a nd cobalt have low solubility in the pH range to which serpentine plan ts are exposed and therefore unlikely to be toxic. Soil analyses acros s a sedimentary/ultramafic soil boundary indicated that higher levels of extractable nickel and/or magnesium in ultramafic soils seem likely to account for the observed vegetation change. The pH of soils under beech forest was significantly lower than that under serpentine vegeta tion and was probably a result of humic decay of forest litter. Nicker availability increases with decreasing pH. This effect is not limitin g on nickel-poor sedimentary soils, but the increased nickel availabil ity at lower pH on serpentine soils may prohibit forest colonisation o f this ultramafic environment. This hypothesis is supported by the obs ervation that isolated Nothofagus and Pinus radiata have colonised hum us-deficient ultramafics at Hackett Creek and the Cobb asbestos mine.