Poor aeration in NFT and a means for its improvement

SummaryBecause water can restrict rapid diffusive exchange of gases we have examined how far the nutrient solution is adequately aerated in NFT systems used for glasshouse tomato production. Measurements were at various places of: the concentrations of dissolved O2 and dinitrogen oxide (nitrous oxide), the maximum rate of O2 flow to an O2-consuming sink (O2 flux density), and the electrochemical (redox) potential. Whalehide pots containing peat-based or peat-sand based composts used to stand the plants in the NFT solution were found waterlogged and poorly aerated. Dissolved O2 in solution drawn from the pots rarely exceeded 1-2 mg l-1. This small amount of O2 was associated with accumulations of dinitrogen oxide, a gaseous product of anaerobic microbial respiration. Oxygen flux density was also small but redox potentials remained above 300 mV which was too high to favour the formation of Phytotoxins such as Mn(II), Fe(II) or H2S. Replacing the composts with more open structured inorganic media such as Perlite, Rockwool or Capogro almost completely eliminated the symptoms of poor aeration observed in the composts. On one occasion the O2 dissolved in the flowing nutrient solution was found to be depleted by c. 50%. Thus the whole system can sometimes be subject to considerable O2 deficits.