SOIL ACIDIFICATION - A PROVISIONAL MODEL FOR NEW-ZEALAND PASTORAL SYSTEMS

Soil acidification is a major problem internationally and occurs in pa storal systems as a result of nitrate leaching, nutrient transfer/remo val, and soil organic matter (SOM) accumulation. Existing mechanistic models on soil acidification calculate acidification rates retrospecti vely, based on quantitative information on these acidifying processes. This paper presents an outline of a predictive soil acidification mod el that includes sub-models for estimating the extent of nitrate leach ing, nutrient transfer/removal, and soil organic matter accumulation. The model was used to predict acidification rates in a fertiliser tria l 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 ne eded to include a SOM accumulation sub-model and to refine the various relationships within the existing sub-models. In addition, accurate i nformation on the pHBC of soils is needed to convert soil pH change in to acid addition rate, which will be crucial for validating the model.