2018 ASHS Annual Conference

Understanding the pattern of growth, nutrient uptake and utilization is a prerequisite to effectively managing fertilization programs in any cropping system including leafy vegetables in hydroponic production systems. An experiment was carried out in an unheated and naturally-lit hoop-house in Uvalde, TX to determine the effect of different concentrations of nitrogen (N) nutrient solution on growth dynamics, leaf N concentration and accumulation patterns, N nutrition index (NNI), yield, and N use efficiency (NUE) of lettuce grown over three consecutive seasons (fall, winter, and spring) in a recirculating hydroponic system. During each season, three lettuce varieties including Buttercrunch, Dragoon, and Sparx were grown in six N concentrations, i.e. 100, 150, 200, 250, 300, and 400 mg L⁻¹ using a nutrient film technique (NFT). Leaf number, accumulated dry weight and N, and leaf area index (LAI) all followed a somewhat logistic trend over time characterized by a slow increase during the early growth period (14 to 21 days after transplanting, DAT) which was followed by a linear increase to a maximum level by the harvest. In contrast, total N concentrations in plants were highest at early stages and slightly decreased over time possibly due to a dilution effect. There was a more pronounced effect of season and variety on these growth traits than that of the N concentrations of the nutrient solution. Averaged over variety and N concentrations, accumulated biomass at harvest during the spring season was 73 and 34% greater than those of fall and winter, respectively, mostly due to the highest relative growth rates. At each sampling date, there were linear, quadratic, and cubic effects of N concentrations on each of these variables. Furthermore, the results revealed that Sparx is the most productive variety which exhibited 63 and 32 % higher fresh marketable yield than those of Buttercrunch and Dragoon, respectively during the fall season and 145 and 114 % in spring. Overall, increasing N concentrations from 100 to 400 mg L^-1 increased the marketable fresh yield linearly or curvilinearly (quadratic or cubic) from 5.9 to 6.7 kg m^-2 in fall, 8.1 to 10.7 kg m^-2 in winter and 10.3 to 12.6 kg m^-2 in spring. Additionally, NUE values were highest at the lowest N concentration (100 mg L^-1) and significantly decreased in response to increasing N concentrations in the nutrient solution. Similarly, NNI values during mid to late growth stages were near or above 1 even for the lowest N nutrient solution (100 mg L^-1). These results demonstrated that N concentrations of 100 to 150 mg L^-1 can maximize growth and yield of hydroponic grown lettuce.