Common ion effects in zeoponic substrates: wheat plant growth experiment

The National Aeronautics and Space Administration (NASA) is developing a zeolite-based synthetic substrate, termed zeoponics. The zeoponic substrate, consisting of NH4+- and K+-exchanged clinoptilolite, and a synthetic apatite, provides all of the plant-essential nutrients through mineral dissolution and ion exchange, with only the addition of water. Initial experiments with wheat resulted in good vegetative growth, however, a subsequent experiment grown to maturity resulted in poor seed yield. Plant-tissue analyses suggested that high P and low Ca concentrations were possible reasons for the low yield. To address the nutrient imbalances, the Ca-bearing minerals dolomite, wollastonite, and calcite were examined as amendments to the zeoponic substrate to provide a common ion effect (i.e., Ca increases in solution from the dissolution of the Ca-bearing minerals will reduce the dissolution of the synthetic apatite, and thus reduce the amount of P available for plant uptake). Plant tissue Ca for all wheat plants grown in amended zeoponic substrates increased with increasing mineral solubilities of the added Ca-bearing minerals, while plant tissue P decreased with increasing mineral solubilities of the added Ca-bearing minerals. Wheat plants grown with the calcite amendment had the highest Ca concentrations (i.e., 0.45 wt.%) and the lowest P concentrations (i.e., 0.76 wt.%). Seed yield and harvest index were also highest of the plants grown in zeoponic substrate when calcite was the amendment (i.e., 1.57 g/plant and 45.7, respectively). This data illustrates that Ca-bearing minerals do provide a common ion effect in zeoponic substrates. (C) 2003 Elsevier Inc. All rights reserved.