Influence of ammonium to total nitrogen supply ratio on growth, yield and fruit quality of tomato grown in a closed hydroponic system

Abstract

Summary The influence of NH 4 -N/total-N supply ratio ( N r ) via the nutrient solution on plant growth, yield and fruit quality was investigated in a tomato crop grown in a closed hydroponic system using perlite as a substrate. N r was set at 0.05, 0.10, 0.15, 0.20 and 0.25 on a molar basis. Increasing N r to values above 0.10 restricted progressively the vegetative growth, the fruit yield and quality, and enhanced markedly the incidence of blossom-end rot. Furthermore, the increase ofNrabove 0.15 resulted in long-term decrease of pH in the rhizosphere to levels below 5, as indicated by the pH values measured in the drainage water. The high ammonium supply ratios had no effect on the leaf K, P, and micronutrient concentrations, as well as on the aluminium content in leaves and roots, but restricted significantly the Ca and Mg concentrations in roots, leaves and fruits to values below critical levels. In contrast, the concentrations of both chlorophyll and organically bound nitrogen in the leaf were enhanced by the increasing N r . It is postulated that tomato is susceptible to increasing N r in hydroponics due to suppression of the rhizosphere pH below critical levels and competitive restriction of Ca and Mg uptake by ammonium, which acts synergistically with the low pH.