Plant‐available elements in soils and their influence on the vegetation over ultramafic ("serpentine") rocks in New Zealand

Abstract

Ultramafic ("serpentine") soils from the Nelson Region contain low total levels of calcium, potassium, phosphate and high total levels of chromium, cobalt, iron, magnesium, manganese and nickel. However, the plant‐available fraction as measured by extraction procedures, differs considerably from the total elemental abundances in the soils.The solubility 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 and cobalt have low solubility in the pH range to which serpentine plants are exposed and therefore unlikely to be toxic. Soil analyses across 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 vegetation and was probably a result of humic decay of forest litter. Nickel availability increases with decreasing pH. This effect is not limiting on nickel‐poor sedimentary soils, but the increased nickel availability at lower pH on serpentine soils may prohibit forest colonisation of this ultramafic environment. This hypothesis is supported by the observation that isolated Nothofagus and Pinus radiata have colonised humus‐deficient ultramafics at Hackett Creek and the Cobb asbestos