Potentialities of Near Infrared Spectroscopy to Assess Nitrogen, Phosphorus and Potassium Nutrient Status of Grasslands in the Reunion Island

Controlled mineral fertilisation practices are an important component for sustainable management of grasslands. The assessment of available nutrients for plants and the general recommendations on the level of phosphorus and potassium to apply to grasslands are classically based on classical soil analysis and average regional levels. For nitrogen, mid or long term recommendations cannot easily be derived solely from soil composition, because it may be rapidly leached from the soil. Recent approaches tended to show that herbage plant N (Lemaire & Gastal, 1997), P, K (Duru & Huché, 1997) mineral analyses associated with actual biomass measurement could be useful for the calculation of combined nutrient indices (IN, IP, IK). Expressing these indices along references curves with a standard optimum value of 100, indicates the limiting factors or excess in the mineral feeding of the plants. It provides a diagnosis of the main nutrient status at a specific local plot situation. The step has been successfully implemented to provide local advice in the management of grasslands on Reunion Island (Blanfort, 1998). Nitrogen content can be predicted from NIRs, but this technique is less used for the other elements. However, the concern is here more related to the development of a combined index, it appeared interesting to test the potential of NIRs to predict these or to rank grasslands according to nutrition levels