Optimum Strength and NH4+:NO3- Ratio of Nutrient Solution for Romaine Lettuce Cultivated in a Home Hydroponic System

Abstract

Concentration of nitrogen, one of the major elements, and ratio of two nitrogen forms (NH4+ and NO3–) in the nutrient solution affect the quality and food safety of fresh vegetable produce. This study was conducted to find an appropriate strength and NH4+:NO3– ratio of a nutrient solution for growth and development of a Romaine lettuce (Lactuca sativa L. var. longiflora) ‘Caesar Green’, a representative leafy vegetable, grown in a home hydroponic system. In the first experiment, plants were grown using three types of nutrient solution: A commercial nutrient solution (Peters) and two strengths (GNU1 and GNU2) of a multipurpose nutrient solution (GNU solution) developed in a Gyeongsang National University lab. Plants grown with the GNU1 and GNU2 had greater shoot length, leaf length and width, and biomass yield than Peters. On the other hand, the root hairs of plants grown with Peters were short and dark in color. Tissue NH4+ content in the Peters was higher than that of the GNU1 and GNU2. The higher contents of NH4+ in this solution may have caused ammonium toxicity. In the second experiment, eight treatment solutions, combining GNU1 and GNU2 solutions with four ratios of NO3–:NH4+ named as 1, 2, 3 and 4 were used. Both experiments showed more growth in the GNU2 group, which had a relatively low ionic strength of the nutrient solution. The growth of Romaine lettuce showed the greatest fresh weight along with low tissue NO3– content in the GNU2-2. This was more advantageous in terms of food safety in that it suppressed the accumulation of surplus NO3– in tissues due to the low ionic strength of the GNU2 subgroup. In addition, this is preferable in that it can reduce the absolute amount of the input of inorganic nutrients to the nutrient solution.