Free aluminium and related parameters in soil solution from a South Island (New Zealand) high country site under contrasting S and P fertilisation

Soil solution samples were taken from 7 plots of varying Hieracium infestation at the AgResearch (NZ) Mt John trial site, Lake Tekapo. The soils, which had been fertilised, oversewn, and graced, were analysed for 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. Free 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 Al by NOM rather than Al(OH)(3) solubility. Soil solution phosphate concentrations depended strongly on P application rate, reaching over 100 mu M. At high P concentrations, the product [Al3+][PO43-] indicated approximate saturation with respect to variscite. One comparison indicated that effects on pH and NOM in the upper 5 cm were due to the presence or absence of gracing.