The Effects of Nitrogen Fertilization on Yield and Quality of Spinach Grown in High Tunnels

A study was conducted under a high tunnel during the spring season of 2014 at the University of Wyoming Research and Extension Center located in Powell, WY, USA, to investigate the effect of nitrogen (N) fertilization on growth, yield, and quality of spinach. Four N fertilization rates (56, 112, 168 and 224 kg N ha^-1 as urea) were used in a completely randomized design with three replications. Eight weeks after planting, the spinach were harvested. The results indicated that increasing Nfertilization rates up to 224 kg N ha^-1 increased plant growth (fresh and dry plant weight, leaf area and leaf area index) and yield (fresh and dry weight as ton ha^-1). Increasing N fertilization rates significantly decreased spinach quality parameters (moisture and dry matter contents and nitrate content). The nitrogen utilization and use efficiency were decreased significantly with increasing N fertilization rates. Increasing N fertilization rates generally increased the spinach content of N, P, K, Fe, and Cu and decreased the contents of Mn, and did not affect the contents of S, Ca, Mg, and Zn. The results suggest that increasing N fertilization rates up to 224 kg N ha-1 under a high tunnel conditions has positive effects on spinach yield and adverse effect on spinach quality.