Soil acidification: a provisional model for New Zealand pastoral systems

Soil acidification is a major problem internationally and occurs in pastoral systems as a result of nitrate leaching, nutrient transfer/removal, and soil organic matter (SOM) accumulation. Existing mechanistic models on soil acidification calculate acidification rates retrospectively, based on quantitative information on these acidifying processes. This paper presents an outline of a predictive soil acidification model that includes sub-models for estimating the extent of nitrate leaching, nutrient transfer/removal, and soil organic matter accumulation. The model was used to predict acidification rates in a fertiliser trial on sheep-grazed pastures, receiving superphosphate fertiliser for 35 years. The predicted rates ranged from 1.46 to 3.83 kmol(c)/ha.year, which was in moderate agreement with acid additions rates of 3.09 to 3.43 kmol(c)/ha.year as calculated from measured changes in soil pH and the pH buffer capacity (pHBC). Further development of the model is needed to include a SOM accumulation sub-model and to refine the various relationships within the existing sub-models. In addition, accurate information on the pHBC of soils is needed to convert soil pH change into acid addition rate, which will be crucial for validating the model.