An investigation into the process of dealumination in soils of the Auckland, Bay of Plenty and Waikato regions of New Zealand

Abstract

Dealumination is a term used to describe an increase in the concentration of acid extractable Al as a result of accelerated weathering or chemical attack of primary crystalline and short-range order aluminosilicates. Two specific mechanisms have been proposed: (1) partial dissolution of clay minerals by local areas of high acidity associated with fertiliser granules; (2) surface complexation and extraction by the fluoride and residual hydrofluoric acid present in phosphate fertilisers. This process has been observed in State of the Environment data from the Waikato region, New Zealand, for farmed soils (which receive substantial inputs of phosphate fertilisers) but not for background soils (which are unfertilised).Additional data, including results from XRF analysis for total Al on the original Waikato region samples, acid extractable Al from two neighbouring regions, Auckland and Bay of Plenty, and trends in the data at sites resampled 5years apart are reported in this work. Retrospective analysis of the Waikato samples by XRF for total Al showed that there was no significant difference between farmed and background soils (whereas the increase in acid extractable Al was significant, p<0.0001), indicating total Al concentrations were not increasing in farmed soils but the form of it was changing. In farmed soils, proportionately more Al is becoming acid-extractable over time. Acid extractable Al results were consistent with dealumination occurring in all 3 regions (p<0.015 to <0.0001) and significant (p<0.0001) increases were seen in farmed soils compared to background ones over 5years, with a mean increase in acid extractable Al of about 1000mg/kg/year.The actual mechanisms of dealumination and associated soil processes need to be verified, and the relative importance of possible causative factors delineated (two possible causes are high localised acidity around fertiliser granules, and high annual loadings of F). Whatever the causes, the effects of this process on soil properties and implications to soil management need to be ascertained.