PHOSPHORUS SATURATION IN SOILS AND GROUNDWATERS

On average, crops require about 30 kg ha-1 y-1 of P in humid regions. In some regions fertilizer and manure are applied in amounts that exceed this. The surplus of P in agricultural areas is about 20-45 kg ha-1 y-1. This implies an accumulation of P. The surplus of P estimated from the balance of agricultural soils of industrialized countries in Europe in the last four decades is between 800 and more than 1500 kg ha-1 although this distribution is not even. Phosphorus accumulation is more than one order of magnitude higher than average in areas with intensive livestock farming. Consequently, the application of high levels of manure exceed the capacity of the soil to store P. This implies that P saturation may pose a problem. Indeed, about half of the Dutch sandy soils (approximately 300 000 ha) are currently considered to be saturated with P, meaning that there is a hazard of P leaching to groundwaters in the future. Assuming current phosphate application rates, phosphate leaching is a distinct possibility over a period of decades to a century. Vulnerable areas are soils with high inputs, low sorption capacities for P, high water-tables and a low retention time for water. The possible consequence is a time-delayed increase of eutrophication of surface waters (lakes, rivers and the sea) caused by a diffuse load of P in the few next decades. Modelling the influence of different agricultural strategies shows that accumulation of P would not increase further only if the use of P increases by an amount that compensates for the increased inputs. It is proposed to change the current support system of agriculture by installing a system of financial incentives and taxes which seek to promote nutrient balances for each farm.