Management of nitrogen by fertigation of potato in Lebanon

Reports on soil and groundwater contamination with nitrates in Lebanon and other developing countries could be related to the mismanagement of water and fertilizer inputs. The objective of this work was to determine the N requirements and N-use efficiency of a main-crop potato in Lebanon, irrigated by a drip system, compared to the farmer's practice of macro-sprinkler. In the drip irrigation, fertilizers input was as soil application at the time of sowing or added continuously with the irrigation water (fertigation). Nitrogen-fertilizer recovery was determined using N-15-labeled ammonium sulfate. Fertigation with continuous N feeding based on actual N demands and available sources allowed for 55% N recovery. For a total N uptake of 197 kg ha per season in the lower N rate, the crop removed 66 kg N ha(-1) from fertilizers. The spring potato crop in this treatment covered 44.8% of its N need from the soil and 21.8% from irrigation water. Higher N input increased not only N derived from fertilizers, but also residual soil N. Buildup of N in the soil with the traditional potato fertilization practice reached 200 kg N ha per season. With increasing indications of deteriorating groundwater quality, we monitored the nitrate leaching in these two watering regimes using soil solution extractors (tensionics). Nitrate leaching increased significantly with the macro-sprinkler technique. But N remained within the root zone with the drip irrigation. The crop response to applied N requires a revision of the current fertilizer recommendation in semi-arid regions, with an improved management of fertilizer and water inputs using fertigation to enhance N recovery.