Effects of Ammonium Nitrogen on Lettuce Grown on Pumice in a Closed Hydroponic System

Abstract

Two successive lettuce crops were grown in spring 2005 in a completely closed hydroponic system. The ratio of ammonium to total nitrogen ( N r ) in the fresh nutrient solution (FNS) introduced into the closed system to compensate for plant uptake was 0, 0.1, 0.2 and 0.3 on a molar basis. In all N r treatments, the concentrations of total N, K, Ca, Mg, P, and micronutrients in the FNS were identical, but that of SO 4 2– increased as N r increased, to compensate electrochemically for the enhanced NH 4 + and decreased NO 3 – supply. The highest fresh and dry weights per plant were attained with the highest ammonium supply ( N r = 0.3) but, even when no NH 4 + was included in the FNS as an N source, the plants were healthy without apparent nutritional disorders. The ammonium concentration in the drainage solution dropped to nearly zero in all treatments some days after the initiation of recycling, which implies a preferential uptake of NH 4 -N over NO 3 -N. The root zone pH, as indicated by the values measured in the drainage solution, decreased slightly as N r increased, and ranged from 6.5 to 8.0 in all treatments. The leaf K, Ca, Mg, and Fe concentrations were not influenced, whereas those of P, Mn, Zn, and Cu were enhanced by the increasing NH 4 + supply. The increased ammonium supply did not enhance the utilization of N in plant metabolism, although it reduced the nitrate concentration of the internal leaves in the early spring experiment. The leaf micronutrient concentrations were clearly more than critical levels even when NO 3 – was the sole N source for lettuce, whereas the P concentration approached the lowest critical level when N r was 0 or 0.1. The stimulation of lettuce growth as N r was increased to 0.3 may be a consequence of enhanced P uptake resulting from better control of pH in the root zone.