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In order to improve the efficiency of fertilizers, we investigate the feasibility of the use of abundant and inexpensive materials that contribute to the slow release of a fertilizer. The aim of this study was to evaluate the effect of a fertilizer, prepared by nitrate (NO3 -) adsorption, a modified bentonite (OBN) and surfactant hexadecyltrimethylammonium (HDTMA), on the growth of lettuce plants (Lactuca sativa L.) and the concentration of NO3 - under a hydroponic system. The hypothesis was that the application of a fertilizer based on OBN and HDTMA, releases NO3 - in the nutrient solution and has a positive effect on the growth of lettuce. The experimental design was totally random. First experiment: eight treatments with six repetitions (48 experimental units): OBN/fertilizer in a Steiner solution 0/100, 25/75, 50/50, 75/25, 25/0, 50/0, 75/0 and 100/0. Second experiment: 15 treatments with six repetitions (90 experimental units): 15 doses of HDTMA with 0 up to 35 mg L-1. OBN reduced the dry biomass between 50 and 70%. The number of leaves decreased from 25 to 50%, the length and width of leaves fell between 40 and 55%. The foliar area fell between 68 and 78%. Water absorption decreased 60%, the concentration of NO3 - decreased between 35 and 60% and electric conductivity descended between 15 and 25% of the concentration applied. The surfactant HDTMA darkened the root and affected the absorption of water and nutrients. Organobentonite can be used as an adsorbent of mineral elements and as a fertilizer, since it released amounts of the NO3 - in the nutrient solution. The type of surfactant can be changed to reduce phytotoxicity and eliminate the negative effects on growth. [ABSTRACT FROM AUTHOR]