The apply of high rates of nitrogen to fertilize potato constitutes an economic constraint for the tunisian farmer because of the elevated price of the nitrogenous manures. It constitutes also an environmental constraint because of leaching of nitrates which pollute the groundwater. In order to adjust the crop nitrogen supply to potato needs, an experiment was conducted in 2008 with the objectives of setting up a diagnostic tool (Nitrogen Nutrition Index, chlorophyll meter) to assess the nutrition status of potato and to help the farmer to a better fertilizer management.

For a yield of 50t.ha-1, five urea treatments were applied: 0, 50, 150, 200 and 300 kg N.ha-1. Phosphoric acid and potassium sulphate were applied with 150 kg.ha-1 of P2O5 and 360 kg.ha-1 of K2O. The variety Spunta was planted in a clay loam soil and was drip irrigated.

The relationships between shoot dry matter and total nitrogen concentration corresponding to the nitrogen treatments showed a deficient nitrogen status of plants and a difficulty to establish the critical dilution curve. The calibration of HNT was difficult due to the lowers correlations between HNT and NNI. The method of nitrogen balance and the calculation of the coefficient of nitrogen use showed a non availability of nitrogen to plants explained by elevated nitrogen losses by leaching or denitrification.