The extent, significance and amelioration of subsurface acidity in southern New South Wales, Australia

Soil pH is seldom uniform with depth, rather it is stratified in layers. The soil surface (0-0.02 m) commonly exhibits relatively high pH and overlies a layer of acidic soil between 0.05-0.15 m deep, termed an acidic subsurface layer. Commercial and research sampling methods that rely on depth increments of 0.1 m either fail to detect or under report the presence and/or magnitude of pH stratification. The occurrence of pH stratification and the presence of acidic subsurface layers may cause the extent of acidity in NSW agricultural land to be underestimated. Though the cause of pH stratification in agricultural systems is well understood, the effect on agricultural production is poorly quantified due in part to inadequate sampling depth intervals resulting in poor identification of acidic subsurface layers. Whilst liming remains the best method to manage acidic soil, current practices of low pH targets (pHCa 5), inadequate application rates, and no or ineffective incorporation have resulted in the continued formation of acidic subsurface layers. Regular monitoring in smaller depth increments (0.05 m), higher pH targets (pHCa >5.5) and calculation of lime rate requirements that account for application method are required to slow or halt soil degradation by subsurface acidification. If higher pH is not maintained in the topsoil, the acidification of subsurface soils will extend further into the profile and require more expensive operations that mechanically place amendments deep in the soil. Whilst the use of organic amendments has shown promise to enhance soil acidity amelioration with depth, the longevity of their effect is questionable. As such proactive, preventative management of topsoil pH with lime addition remains the most cost-effective solution for growers.