Balanced phosphorus fertilization in a mixed grazing and mowing system on grassland; results after 26 years

In many affluent countries the excessive use of phosphorus (P) in agriculture has led to a high P content in the soil, thus threatening surface water quality by run-off and leaching. In the Netherlands P fertilization is limited to the net offtake from the land, so called balanced P fertilization, for soils with a sufficient soil P status. Balanced P fertilization, however, might affect grassland yield and quality negatively. In the short term a decrease in herbage P content is expected. In the longer term decreases in herbage yield can be foreseen. The objective of a long-term field experiment, initiated in 1997, was to examine the effects of balanced P fertilization compared with two levels of positive P surpluses in a mixed grazing and mowing system. Herbage yield, P content and soil P status were measured. In the last ten years DM yield did not respond to P fertilization. Herbage P content and soil test P responded positively to P fertilization. At balanced P fertilization the P content of herbage and soil test P decreased implying that the system was not in a steady state. Due to this it cannot be ruled out that DM yield will be affected negatively to balanced P fertilization eventually. On relatively P-rich soils, however, the DM yield seems not to be affected by balanced P fertilization in the first 15 to 25 years.