A NITROGEN AND PHOSPHORUS HERBAGE NUTRIENT INDEX AS A TOOL FOR ASSESSING THE EFFECT OF N AND P SUPPLY ON THE DRY-MATTER YIELD OF PERMANENT PASTURES

In order to propose consistent decision rules for fertilizer supply, a study was made on the effect of additions of N and P fertilizers and of their interaction on the above-ground dry matter yield of pastures during spring. The interaction between N and P could occur through nut rient acquisition or nutrient efficiency for growth. We therefore char acterised the herbage N and P status (N and P index) from previously e stablished critical curves of herbage mineral content according to abo ve-ground dry matter. First we studied the effect of N and P addition on herbage nutrient status. Secondly, we expressed the above-ground dr y matter as a function of the herbage nutrient status. This study cons isted of four treatments applied to four permanent pastures which had a low phosphorus availability. The results showed a positive effect of P supply on the herbage nitrogen status, which may be due to an incre ase of organic matter mineralization or root growth. The P herbage sta tus decreased only if N was supplied without P. The dry matter yield w as positively related to the herbage nitrogen status, but a low P herb age status reduced the slope of the relationship. For the pastures stu died, the indirect effect of P supply on above-ground dry matter, reve aled by an increase in N index, was greater than its direct effect. Th is methodology allows us to distinguish the direct and indirect effect s of N and P addition on herbage growth at field level. It could be us ed to propose consistent rules to manage jointly both N and P supplies .