Are mineral fertilizers essential for a mountain dairy farm?

Changes in the availability of nitrogen (N), phosphorus (P), potassium (K) and magnesium (Mg) have been observed during 10 years on the Swiss dairy farm La Frêtaz/Bullet (1200 m a.s.l.) after the practice of mineral fertilization on permanent grassland was withdrawn in 1994. The amounts of nutrients applied with livestock manure produced on the farm tended to decrease slightly from 1994 to 2003. The nutrient contents of hay (1st and 2nd cut) and the soil content in readily available P, K and Mg (extraction with CO2-saturated water) remained more or less stable during 10 years. The content of extractable soil nutrients (extraction with ammonium acetate + EDTA) decreased in some cases, particularly in permanent meadows (P and Mg). Overall, the withdrawal of mineral fertilization on permanent grassland during 10 years did not cause a marked decrease in the nutrient availability. However, some indicators reveal a slight impoverishment in N, and to a lesser extent in P, which could decrease in the long term the grassland yield. The observed small changes in the N, P and Mg availability agrees rather well with the estimated nutrient balance based on the overall comparison of plant requirements and nutrient supply. The stability of the K availability can be better explained by the comparison of K inputs and outputs at the whole farm level.