Two successive lettuce crops were grown in spring 2005 in a completely closed hydroponic system. The ratio of ammonium to total nitrogen (Nr) 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 Nr treatments, the concentrations of total N, K, Ca, Mg, P, and micronutrients in the FNS were identical, but that of SO42– increased as Nr increased, to compensate electrochemically for the enhanced NH4+ and decreased NO3– supply. The highest fresh and dry weights per plant were attained with the highest ammonium supply (Nr = 0.3) but, even when no NH4+ 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 NH4-N over NO3-N. The root zone pH, as indicated by the values measured in the drainage solution, decreased slightly as Nr 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 NH4+ 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 NO3– was the sole N source for lettuce, whereas the P concentration approached the lowest critical level when Nr was 0 or 0.1. The stimulation of lettuce growth as Nr was increased to 0.3 may be a consequence of enhanced P uptake resulting from better control of pH in the root zone.
