GREENHOUSES I SHADE NET I HYDROPONICS I AQUAPONICS
Getting the right mix of fertilizer to plants is required for optimal productivity.
REMOVE UNWANTED IONS FROM WATER to ensure quality water is fed to Greenhouse plants
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t is found that high levels of soluble (ferrous) iron are present in wells or boreholes from the mountainous areas of the Cape, along the Drakensberg and a few other areas. This iron is in a reduced state (Fe2+). When the Fe-rich water is used for sprinkler irrigation, ferrous iron is oxidized to form an insoluble ferric iron Fe3+ and it may be visible as a brownred substance on leaves and garden walls. Manganese (Mn) is also soluble in its reduced state and precipitates as insoluble MnO2 when oxidized. When using water with high Fe or Mn concentrations for drip irrigation, the ions are oxidized, and these insoluble salts block the drippers. Apart from oxidation due to aeration, ferric and manganese bacteria are chemotropic and contribute to oxidize Fe and Mn. These ferric and manganese bacteria cause the oxidized residues to
accumulate among the bacterial waste, creating a slimy residue, also blocking drippers. Fe and Mn concentrations in water are important feeding water quality parameters in areas with high levels of these ions in the water. Good quality is regarded as safe to use, where Fe and Mn levels are <0.1 and <0.02 mg L-1, respectively. Medium quality water may contain Fe at 0.1 to 0.5 and Mn at 0.02 to 0.3 mg L-1. Poor quality water contains Fe at >0.5 or Mn at >0.3 mg L-1, needing pre-treatment to lower their concentrations where drippers are used. By aerating feeding water, Fe and Mn can be oxidized. Small oxidized particles can then be removed with filters. The oxidizing process is extremely slow in acidic water. By increasing the pH of the water before aeration, the oxidation time can be reduced substantially.
18 Undercover farming I January/February 2022 I Volume 19 No 1
The oxidation of ferrous iron may also be accelerated with UV tubes, the addition of chlorine gas (Cl2), ozone (O3) or peroxide (H2O2). Growers who make use of drip irrigation should remove as much Fe as possible from the feeding water. According to Deckers (2002), the natural Fe content of feeding water cannot be absorbed by plant roots and should be considered as not available. Other nutrients in feeding water can be topped up, but the total Fe-need should be applied, using Fe-chelate. With production systems, where drippers are not used, removal of Fe is not critically important. Sodium, chloride and other unwanted ions Should Na+ and Cl- levels exceed the maximum limits, it should not be used as feeding water. However, it may be
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