Extractable aluminium in New Zealand Andisols and Inceptisols

Abstract

It is critical to have a soil test that allows for the measurement of potential toxicity. A laboratory experiment was undertaken to investigate the effects of changing the concentration of salt in the extract and extraction time of the standard CaCl₂ and KCl soil Al tests on the Al concentrations extracted from 13 soils from four New Zealand soil orders. Al extracted by KCl (AlKCl) was 13 times higher than extracted by CaCl₂ (AlCaCl2) across all soils. The effect of changing extract salt concentration and extraction time on Al extracted differed among the four soil orders tested for the two extraction methods. Increasing the concentration of CaCl₂ in the extract increased (P < .001; P < .05) the amount of AlCaCl2 (by 0.02–0.13 cmolc/kg) for the four soil orders, while increasing extraction time resulted in a difference only in the Allophanic soils, where AlCaCl2 decreased (P < .01). The interaction of the concentration of salt in the extract and extraction time for the CaCl₂ extraction did not affect (P > .05) the AlCaCl2 extracted from all soils. An increase in the concentration of KCl in the extract up to 1 M increased AlKCl (P < .01) (by 0.2–0.8 cmolc/kg) on the Allophanic, Brown and Pumice soils, with no increase (P > .05) with a further increase in concentration. Extraction time affected AlKCl (P < .001) for Pallic soils, while the interaction of concentration of KCl in the extract and extraction time resulted in differences (P < .001) only for the Pallic soils. These findings suggest that the Al concentrations measured by the two extraction methods are affected by specific soil properties in the topsoil related to soil order. This means that when measuring the Al bioavailability in soils, extreme care must be taken when interpreting soil Al test results.