Mechanisms of acid sulfate soil oxidation and leaching under sugarcane cropping

Abstract

Analysis of acid sulfate soils (ASS) under sugarcane cropping at a site on the Tweed River, north-eastern New South Wales, showed that the majority of the acidity and higher valence ions generated through pyrite oxidation was retained within an individual caneblock. It appears that the oxidation products generated >1 m away from the field drain edge primarily remain where they were formed, and are not exported to the adjacent field drain. Capillary rise and diffusion control the transfer of oxidation products within this area. Leaching and mass movement dominate the transport of ionic species in the topsoil and close to the field drain edge (~1 m). Soluble ion movement within the unsaturated zone also appears to be influenced by nutrient uptake of the growing sugarcane, adsorption and exchange reactions, and convective/dispersive forces. The almost ubiquitous degree and depth of oxidation of ASS profiles along most of the coast, even where no artificial drainage has occurred, leads us to propose natural hydrological and pedogenic processes as the cause. While artificial drainage systems may not have caused the acidity that is stored in backswamps, they do provide the conduit for acidity export. Therefore, management regimes should focus on maximising the retention of acidity in the backswamp and treating that which is exported. Whilst a reduction in the drain frequency appears a logical solution to a reduction in the acidity export from the site, consideration must be given to the benefits field drainage provides before any subsequent changes can be made. An integrated approach of drain minimisation, laser levelling, and active watertable control would appear to be the most appropriate policy in containing the acidity within the soil profile. This approach, combined with the strategic application of lime, offers a means for minimising acid export from the sampled site.