Nitrogen and water use efficiency of fertigated processing potato

Abstract

In this paper, we studied the response of spring processing potato to different N level by fertigation and deficit irrigation in the dry Mediterranean Bekaa plain of Lebanon. In the first season between 5 May and 29 August, the impact of different N level was evaluated. Treatments consisted of 125 kg N ha −1 (N125), 250 kg N ha −1 (N250), 375 kg N ha −1 (N375) and 500 kg N ha −1 (N500) applied continuously by drip fertigation. In the second season between 11 April and 16 August, the best N treatment (N125) was set as a standard to study the constraints of deficit irrigation and the possibility of water saving. After plant establishment, treatments consisted of continuously applying a moderate water stress 80% of ET, a severe deficit 60% of ET, a control 100% of ET and an excess irrigation 120% of ET. Labeled N fertilizers in 2000 and 2001, as well as a neutron probe in 2001, were used to estimate N use efficiency and the actual crop evapotranspiration and water use efficiency. Different N level, at recommended water supply (700 mm), did not lead to a crop response. The lower N treatment (N125) gave a significantly higher N recovery (61% of applied N), despite the clay nature of the soil. In the deficit irrigation, water application varied between 500 and 800 mm with 720 mm for the control. Final yield showed the possibility to save 119 mm of water with no reduction in fresh tuber yield. Imposing severe water deficit considerably decreased tuber yield. Extremes of water input, deficit and excess irrigation, lowered the percentage of marketable yield from 84 to 72%. Regardless of the severity of deficit irrigation, the upper 0.4 m soil layer provided up to 90% of crop water demands (uptake). About 40% of crop ET occurred up to the maximum development stage. Under water stress conditions, the effective ground cover was reduced, which can explain the decrease in N recovery from 40 to 20%. This is probably due to higher evaporation rates and N losses. In both trials, increasing N or water input showed inconsistent impact on dry matter production per unit of applied water. Under deficit irrigation, a yield response factor (relationship between relative yield decrease and relative ET deficit) of 0.8 was obtained, while values below 1 justify the implementation of deficit irrigation. Managing continuous deficit irrigation of processing potato in the dry Mediterranean region with 0.80 ET is possible for water saving. Therefore, it could be safe to deplete the soil water content down to 56.6 mm in the upper 30 cm soil layer, which is equivalent to 40% water depletion.