Impact of type of use on the phosphorus and potassium requirements of grassland in the Swiss Jura

Grazing differs from mowing in its generally less homogeneous use of forage, and above all in its return of nutrients in the excreta of herbivores. The impact of the type of use (four grazings vs four cuts per year) on the phosphorus (P) and potassium (K) requirements of a grassland in the Swiss Jura was studied from 1992 to 2006 in a trial with various inputs of P (0 to 26 kg/ha/year) and K (0 to 116 kg/ha/year). For the same PK fertilisation, the P and K content of the soil is always higher for grazing than for mowing. Increasing inputs of P and K enable this content to be increased significantly for K regardless of the type of use, and for P only in the case of mowing. Although type of use has a significant effect on botanical composition, PK fertilisation does not. The absence of PK fertilisation results in a significant decrease in the quantity of forage; this decrease appeared later with grazing (2005) than with mowing (2001). The P and K content of the forage increased significantly with PK fertilisation for both types of use. Bearing in mind all of the observed parameters, an annual input of 9 kg P/ha/year may be recommended for this type of pasture that produces approximately 40 dt DM/ha/year. In the case of grazing, K inputs are not necessary to maintain the availability of K in the soil, preserve the botanical composition, and produce the expected forage. This trial confirms that the P and K requirements of pastures are lower than those of hay meadows managed at the same intensity.