Modelling long-term phosphorus leaching and changes in phosphorus fertility in excessively fertilized acid sandy soils

The sound management of agricultural soils that are heavily loaded with phosphorus (P) involves minimizing the losses of P responsible for eutrophication of surface waters, while ensuring enough P for crops. This paper describes a simple model to examine the compatibility of these two objectives in acid sandy soils in a temperate humid climate. The model is based on several assumptions regarding reversible and irreversible P sorption by P-reactive soil compounds (mainly poorly crystalline Fe and Al oxides) and release of P to water (water-P test). Model inputs are amount of P leached, P uptake by crops, and contents of poorly crystalline Fe and Al oxides in soil. The model predicts that reducing the amount of leached P to what is environmentally acceptable (e.g. 0.44 kg P ha(-1) year(-1), equivalent to 1 kg P2O5 ha(-1) year(-1)) results in the long run in available soil P test values below target concentrations for optimum crop growth. When the amount of leached P is set to a fixed value the model predicts that soils with large contents of Fe and Al oxides can maintain the initial soil P test values for longer periods than other soils. The content in available P decreases if fertilizer P is applied to the soil at a rate equal to P uptake by crops. These results stress the difficulties involved in trying to make agricultural and environmental needs compatible in acid sandy soils.