The relationship between soil solution pH and Al3+ concentrations in a range of South Island (New Zealand) soils

Concentrations of Al3+ were calculated in soil solutions from concentration s of the monomeric 'reactive Al' species ([Al3+] + [Al(OH)(2+)] + [Al(OH)(2 +)] + [AlF2+]) obtained using a recently reported flow injection analysis ( FIA) chelating resin technique. Soil solution samples came from 7 sites enc ompassing a range of New Zealand soils (Brown, Gley, Pallic, Podzol, and Re cent Soils) and vegetation types (pasture, shrub lands, and indigenous and exotic forest). Previously published data from a further 7 sites, obtained using a rapid (7 s) FIA technique, were transformed to give compatible resu lts. The resultant data (n = 85) covered the pH range 2.7-7.6, and showed a single curvilinear relationship for log [Al3+] v. soil solution pH, regard less of vegetation or soil type. At pH >5.6, the data had a slope of -2.98 and fell between the amorphous Al(OH)(3) and gibbsite solubility lines. At pH <5.0, the data had a slope of -0.46; further, the soil solutions were un der-saturated with respect to both minerals. These results are interpreted as indicating control of Al solubility by Al(OH)(3) (s) (at pH >5.6) and so il organic matter (at pH <5.0), respectively. This interpretation is suppor ted by data from a pH-dependent Al-fulvic acid binding curve, for which cal culated values of [Al3+] follow the same curvilinear relationship determine d from the soil solution samples.