The analysis of a representative sample of herbage from a pasture gives evidence of insufficient or excessive absorptions of P and K. Herbage analysis, as an a posteriori evidence of the availability of these nutrients (in the soil and through mineral or organic dressings) and of the pasture’s aptitude to take them up, is a very useful diagnostic tool for the steering of the P and K fertilization of the pastures. The P and K nutrition indices of a pasture were calculated from herbage analysis results, on the basis of the physiological considerations described in this paper. The agricultural interpretation of these indices and the method of herbage sampling are given. Two levels of diagnosis may be considered, according to one’s objectives, the one bearing on the farmer’s fertilizing practices, and the other giving an assessment of the short-term availability of the P and K reserves in the soil. Two examples are given to illustrate how this method can be applied as a regional tool of development : one in Puy-de-Dôme, for a group of farmers, and the other in Pas-de-Calais, on the scale of a small region.