Sustainability of New Zealand high-country pastures under contrasting development inputs. 2. Soil inorganic components

Analyses of the <2 mm soil fractions for S, P, pH, Al, and exchangeable cations were made to 40 cm depth 10 years after commencement of two grazed multiple pasture species mixture trials on a Tekapo/Pukaki high country soil. One trial was 30 combinations of 5 superphosphate rates (0-500 kg ha(-1) yr(-1)) x 3 stocking rates x 2 stocking methods, and the other used 27 combinations of P and S fertiliser (0-100 kg ha(-1) yr (1)). Olsen and inorganic P of topsoil increased with increasing P fertiliser rate, and was independent of S fertiliser rate. High levels of P (800 1200 ppm) in subsoil following sample ignition were interpreted as being release from humic complexes rather than organic P. There was an increase in extractable Al. Elemental S was the main S fertiliser used. Sulphate concentrations increased in all soil layers with increasing S fertiliser, and increased slightly with P fertiliser rates. There were high rates of residual elemental S (0 800 ppm). There were large increases in extractable Al and a decrease in soil pH below 5 cm with increasing S fertiliser rates. The pH decrease was attributed to oxidation of elemental S and legume N fixation. Grazing management treatments had small effects on soil inorganic components. The pasture development options were sustainable considering soil nutrient concentrations, except perhaps for increasing extractable Al and decreasing subsoil Ca and pH.