The studies where carried out since 1999 on a permanent grassland placed on an alluvial soil from France, in the Central Pyrenees (Ercé). The experiment consisted from 2 annual doses of fertilization with N and P arranged in 4 randomised blocks. The analysis of IN and IP index using the cycle of growth method will provide the opportunity to analyze the effect of N and P levels on the nutrition of the grassland, and their impact on plants growth. The method for determining the nutritional index are based on plant analysis and show the concentration of mineral elements N and P depending on their accumulation in the plant biomass. The literature data have shown that considering the biomass of the legumes fraction in the calculation of IN and IP index, there are resulting errors in the estimation of nitrogen and phosphorus nutrition level, the index being overestimated in comparison with its real value. The data presented were collected only on the N0P0 and N0P1 fertilisation doses (where N0= 0 kg N ha-1 yr-1; P0= 0 kg P ha-1 yr-1; P1= 50 kg P ha-1yr-1), for cycles in that a legumes percentage over 10% was registered in 2000 and 2002. IN and IP index of mixture fractions were calculated from N% and P% concentration of mixture fraction; IN and IP index of fractions without legumes (nonleg) were calculated from the levels of N% and P% of nonleg fractions. The difference between the index calculated from the mixture fraction and calculated for the nonleg fraction is even more important than the contribution of legumes harvested biomass. The results for the IN as well as for the IP confirm the need of doing tests on mineral-free forage legumes, if is necessary to eliminate the inaccuracies of the obtained results when the percentage of legumes in mixture is higher then 10%.

nitrogen nutrition index; phosphorus nutrition index; legumes

Presentation: oral