Dealumination as a mechanism for increased acid recoverable aluminium in Waikato mineral soils

Abstract

This paper assesses a potentially interesting soil process, dealumination, as the mechanism for the increase observed in strong acid recoverable Al and associated elements in farmed soils compared with background soils in the Waikato Region of New Zealand. Waikato Regional Council has been measuring an established set of 7 soil quality chemical and physical parameters and concentrations of 33 elements as part of a Regional Soil Quality Monitoring program since 2003. Statistical comparison of farmed to background soils, relative surface enrichments, and inter-element correlations enable us to infer likely and potential sources of those elements which show some form of enrichment. Acid-recoverable Al is 1.5 times higher (P < 0.0001) in the Waikato region’s farmed soils than its background soils. This increase is not readily explained as an external source of recoverable Al (due to lack of enrichment at the soil surface). However, it could be explained as an increase in the concentration of acid-recoverable Al as a result of accelerated weathering or chemical attack of primary crystalline and short-range order aluminosilicates. In keeping with this interpretation, acid recoverable concentrations of several trace elements that are normally retained inside aluminosilicates (in residual phases) are also significantly higher in farmed than background soils but are not selectively enriched at the soil surface. These include (with enrichment-to-background factors) Li (2.5), La (2.1), Mn (1.5), and Ag, Bi, Mo, Sn, and Tl (1.4). Also, this process may contribute one-quarter of the observed increase in acid-recoverable U. If it is occurring, accelerated Al weathering may be a normal part of an increase in soil productivity, or may be facilitated by an external agent capable of attacking crystalline aluminosilicates. A candidate in the latter category is the F (and/or possibly free HF) in phosphate fertilisers, because this substantially increases Al species in soil porewater. Two specific mechanisms that could favour Al mobilisation from clay surfaces include partial dissolution by local areas of high acidity associated with fertiliser granules, and surface complexation and extraction by the fluoride and residual hydrofluoric acid present in phosphate fertilisers. Based on the high reactivity between F and both Al and Si, potential exists for significant production of SiF4(g) as another side-effect of phosphate fertiliser use.