Aquaponics using saline groundwater: Effect of adding microelements to fish wastewater on the growth of Swiss chard (Beta vulgaris L. spp. cicla)

Abstract Saline soil and saline groundwater reduce agricultural productivity on drylands. We are developing a new aquaponics system to improve food productivity on such lands while effectively utilizing saline groundwater. In this study, cultivation of Swiss chard (Beta vulgaris L. spp. cicla cv. Seiyou Shirokuki) was carried out using fish wastewater with a high salt concentration (1150 mg L−1 NaCl). The levels of microelements (e.g., Fe, Mn, Zn, and Cu) in the fish wastewater were very low, so we added microelements at 100% (W100), 50% (W50), 25% (W25), and 0% (W0) of the levels in the standard hydroponics solution to the fish wastewater and investigated the effects on growth of Swiss chard. At the first harvest, yields in all wastewater treatments were as high or higher than in the control. At the second harvest, yields in W100, W50, and W25 were not significantly different from the control, while in W0 the yield was significantly lower and chlorosis was evident. At the third harvest, the yield in all wastewater treatments was less than in the control, and chlorosis symptoms were observed in W25 and W0. Since leaf Mn and Zn concentrations in W25 and W0 had decreased to below the critical values for those microelements, Mn and Zn deficiency might have contributed to the observed chlorosis and yield loss. For the cultivation of Swiss chard with fish wastewater, sufficient yield (i.e., comparable to or better than the control) without chlorosis was obtained when microelements were added at 50% of the level of the control solution. In addition, since sufficient yield was obtained even in W0 at the first harvest, it is suggested that longer-term cultivation and higher yield could be achieved by applying 50% microelements after the first harvest.