Changes in Nitrate Contents of Hydroponically Grown Leaf Lettuce As Influenced By the Ratio of Ammonium and Nitrate Forms of Nitrogen in Nutrient Solution

Increased levels of nitrate (NO₃^-) in leafy vegetables may be considered hazardous to human health. An experiment was conducted to determine if changes in the ratio of the two different forms (NH₄⁺, NO₃^-) of nitrogen can influence tissue nitrate content. Rockwool-grown seedlings of leaf lettuce (Lactuca sativa) cv. Rex Rz were anchored and grown on Styrofoam boards inside tubs containing 5 different N solutions: 1) 100% NH₄⁺, 2) 75% NH₄⁺+25% NO₃^-, 3) 50% NH₄⁺+50% NO₃^-, 4) 25% NH₄⁺+75% NO₃^-, 5)100% NO₃^- at total 12 of meq/liter N. All fertilizer solutions contained modified levels of other macronutrients from a Hoagland solution. When lettuce plants were harvested after 4 weeks of culture, tissue nitrate levels were higher when the nutrient solution contained increasing percentage of NO₃^- in relation to NH₄⁺ in general. The growth of the plants was the greatest when it was grown with 1:1 mixture of NH₄⁺ and NO₃^- (solution 3). Tissue nitrate content in this treatment was about 4,000 ppm higher compared to solution 2. Difference in biomass yield between the solution 3 and solution 2 was statistically not significant. As tissue NO₃^- levels increased, plant sap pH values increased and the leaf chlorophyll index became lower. It appears that a hydroponic solution containing 75% NH₄⁺+25% NO₃^- can effectively be used for growing lettuce to lower leaf nitrate levels without sacrificing yield and produce quality.