Closing in on upper limits for root zone aqueous ozone application in mineral wool hydroponic tomato culture

Abstract Aqueous ozone is increasingly used to treat irrigation source water in greenhouse and nursery operations; however, the ozone is typically removed prior to crop application. Until recently, this was appropriate as there was a paucity of data to suggest any other course of action. Recent research examining the application of aqueous ozone, through drip irrigation, to a mineral wool grown hydroponic tomato crop suggests that ozone can remain in solution during distribution to the crop without negative effect. What is not known from this recent work is the upper limit or phytotoxic threshold for aqueous ozone application, beyond which crop damage would occur. The present study examined the effects of high aqueous ozone concentrations applied daily in this same production system. Suppression of some growth metrics was observed at 6.0 mg L −1 . The ancillary effect of enhanced oxygenation of the irrigation solution, as a result of the ozonation process, was also examined to disentangle the oxygen/aeration effects from the ozone effects. No significant productivity effects were observed at elevated dissolved oxygen levels relative to the fully aerated control, therefore any productivity influence was attributed to aqueous ozone. The degree to which aqueous ozone penetrated the mineral wool substrate was also examined to aid in the development of horticultural management protocols. Even at volumetric ratios well in excess of typical commercial irrigation events, aqueous ozone still did not penetrate the mineral wool medium beyond 4 cm, with most of the ozone being lost in the upper 2 cm. Only after excessive volumes of aqueous ozone solution had been applied was a residual observed past 4 cm. Based on the results of this study and previous work, an upper limit of 3.0 mg L −1 for this type of production system is recommended until further research is completed.