PURPOSE:To reduce content of water-soluble oxalic acid in a plant and to improve quality of plant by injecting an anion and an ozone-containing gas to a culture solution for water culture to disinfect and to activate the culture solution. CONSTITUTION:A culture solution 14 for water culture supplied to a re-culture water tank by a pipe 9 for discharging a disinfected and activated culture solution is returned through a return pipe 15 to a water tank 10 for preparing the culture solution. On the other hand, an excessive supplied solution of a return pipe 4 is sent to a device 5 of mixing the culture solution with air, vigorously blended with an anion and an ozone-containing gas which are generated by ozone and ion evolution through high-voltage application plasma discharge and adjusted to proper concentration with active oxygen introduced from an outlet 3 of an ozone.ion activated oxygen feed pipe 2, dispersed as bubbles 6 into the water tank 10 to disinfect and to activate a culture solution 7, which is sent through the pipe 9 to a culture tank.
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