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Remove Unwanted Ions from Water to Ensure Quality Water is Fed to Greenhouse Plants
Getting the right mix of fertilizer to plants is required for optimal productivity.
It 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. 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.
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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
diluted with rainwater or the ions can be removed with reverse osmosis, an expensive water purification system. This system removes all ions (macro- and micronutrients) from the water, although some boron (B) may slip through the membranes (Deckers, 2012). This method of water purification creates a large percentage of saline wastewater which should be well-managed to prevent pollution of soil and rivers.
Ions associated with alkalinity
The alkalinity level in saline feeding water is usually high, due to high levels of one or more of the following: CO3 2-, HCO3- and OH-. In the presence of Ca2+ and Mg2+ and under high pH conditions, a whitish-grey deposit of Ca- and Mg-carbonate (lime) may form when this water is used in kettles and geysers. Gadgets to desalinise or to soften water may be suggested by some institutions, but their claims should be tested by chemical analyses, before and after a water treatment. An inline cylinder, known as ‘Protection against total hardness’ (PTH, 2019), exposes water to a combination of special metals that may lower Ca-, Mg and bicarbonate levels due to the precipitation of these ions as insoluble Ca and Mg-carbonate crystals that remain in suspension. This may allow the water to be used with less damage to elements of electric kettles and geysers. Apart from the danger that Ca- and Mg carbonate particles may block drippers, these crystals will react with applied acids during the acidifying process when nutrient solutions are mixed. Ca and Mg will then be released as ions. Thus, PTH-treated water is not recommended for hydroponics production systems but may soften water used for gardening or households.
Too little iron in the irrigation mix shows up on the leaves of a plant – proof of Fe deficiency.
Too much iron content in the irrigation water is visible in the red spots on the leaves. (Pic: Rutgers)
The alkalinity anions (CO3 2-, HCO3- and OH-) can be replaced by nitrate, phosphate or sulphate, by adding nitric- or phosphoric-, or sulphuric acid respectively. With a very high total alkalinity, special procedures should be followed and provision should be made for the release of CO2-gas. This will be explained in the following edition of Undercover Farming.
By Dr NJJC Combrink from his book; Nutrient Solution Management. Available at njjc@sun.co.za
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