FREE ALUMINUM AND RELATED PARAMETERS IN SOIL SOLUTION FROM A SOUTH ISLAND (NEW-ZEALAND) HIGH COUNTRY SITE UNDER CONTRASTING S AND P FERTILIZATION

Soil solution samples were taken from 7 plots of varying Hieracium inf estation at the AgResearch (NZ) Mt John trial site, Lake Tekapo. The s oils, which had been fertilised, oversewn, and graced, were analysed f or free (reactive) Al, total Al, pH, NOM (natural organic matter), and phosphate. The plots had received annual fertiliser combinations of 0 -100 kg P and 0-20 kg S/ha . year. Soil solution pH was in the range 4 .33-5.76 and total Al concentrations were in the range 13-68 mu M. Fre e Al concentrations were in the range 0.48-4.8 mu M. The pH dependence of log(free[Al]) (slope: -0.69+/-0.37) indicated control of soluble A l by NOM rather than Al(OH)(3) solubility. Soil solution phosphate con centrations depended strongly on P application rate, reaching over 100 mu M. At high P concentrations, the product [Al3+][PO43-] indicated a pproximate saturation with respect to variscite. One comparison indica ted that effects on pH and NOM in the upper 5 cm were due to the prese nce or absence of gracing.