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 QualityMonitoring 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 theWaikato 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 ma