The 2012 ASHS Annual Conference

The low cost and simple design floating hydroponic systems makes it an attractive and productive system for small farmers who lack greenhouses or other protective structures. However, nutrient management strategies for outdoor floating systems affected by rain events is uncertain. The objectives of this study were to determine optimum nutrient solution concentration and impact of dilution on lettuce production. Nutrient solution treatments were installed in 15 L plastic tanks under greenhouse conditions at two locations in north Florida for production of lettuce (Lactuca sativa L. var. ‘Green Wave’) from transplants. For the first experiment, increasing amounts of NO₃-N (0, 22, 56, 91, 125, 159, 193 mg·L^-1 of NO₃-N) were supplied by adjusting the amount of fertilizer solution added to a finale solution volume of 12 L. For the second experiment, nutrients were dissolved in equal amounts of water (8 L) to supply 91 mg·L^-1 of NO₃-N for all treatments. At 14 days after planting, increasing amounts of water (0, 0.96, 1.93, 2.89, 3.86, 4.82, 5.78 L) were added to simulate dilution of hydroponic solution by rainfall (0 to 7.62 cm depth). At 5 weeks after planting, lettuce was harvested, fresh and dry weights determined, and nutrient content measured. Maximum lettuce yield was obtained with NO₃-N concentrations from 91 to 159 mg·L^-1. Nitrate concentration in lettuce tissue increased and uptake efficiency decreased with increasing N rates. The results suggest 91 mg·L^-1 of N (726 mg per plant) was optimum for lettuce production. Dilution of nutrient solution with up to 5.78 L (equivalent to a 7.62-cm depth rainfall) did not impact yield, quality, or N uptake efficiency of lettuce. This indicates the amount of N and nutrients in the hydroponic solution could be more useful for management than concentration. Large rain events that dilute nutrient solution without compromising the amount of nutrients available do not require adjustment of hydroponic solution for lettuce production.