Nutrition index are used on a wide scale for the diagnsosis of the mineral nutrition of the grasslands and to facilitate the decisions regarding their fertilisation mode.In literature are available few data regarding the inter-annual evolution of the nutrition indexes in relationship with the cumulated mineral balances. The problems that are coming from are to find out the measure in that those indexes are able to provide piece of information on the evolution of the N and P supplies level in the case that the mineral balance is positive or in the case in that the mineral balance is negative. The experimental fields were set by the Orphée team (INRA Toulouse) on two grassland types different from the pedological and climatic point of view, placed in Ercé in Central Pyrenees in 1999 (0° east, 43° north) and in Gramond in Massif Central in 1998 (2° east, 44° north). There were applied four treatments: N 0 P 0 , N 0 P 1 , N 1 P 0 et N 1 P 1 ; where 0 is correaponding to a treatment wihout N or P input; 1 is correasponding to a variable input depending by year and P correasponds to a unique annual dose of 50 kg ha –1 . On the ensemble of the plots was provided an input of 400 kg ha –1 year –1 of K as KCl applied in every year to provide a non-limited potassium level. The results are showing that IN are able to provide pieces of information regarding the finish the nitrogen supply level under the influence of the balance cumulated in time. When the available P level is low, the IP nutrition indexes measured for P 0 treatments show that the level of the available P supplies are finishing when the cumulated balance I–O becomes negative. In some situations the cumulative effects haven’t diminished significantly the available P level from soil, the IP nutrition indexes being sensitive to the evolutions of P balance during time.

nitrogen and phosphorus balance, nutrition index, grasslands fertilization

Presentation: oral