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GYPSUM SOLVES SODIUM ACCUMULATION DAIRY WASTEWATER CAN BE A useful resource for pasture and crops, however high sodium levels in the wastewater may accumulate in soil and cause drainage issues. Excess sodium leads to low soil water availability and poor soil structure. Soil structure effects may reduce the ability of the soil to receive dairy effluent before surface ponding or runoff occurs. Gypsum is ideal to address any building of sodium in soils receiving dairy effluent, extending the number of years that affected land remains useful for receiving wastewater. Gypsum can also enhance soil structure, water infiltration and drainage in the face of high sodium levels. A gypsum programme should ideally start before critical sodium levels are reached. Sodium in wastewater largely comes from the use of caustic soda and sodium hypochlorite in cleaning and sterilising processes. Sodium is relatively easily leached and gypsum accelerates that leaching to reduce the chance of sodium accumulation. Gypsum is hydrated calcium sulphate. Calcium from gypsum replaces sodium in the soil. The sulphate allows the sodium to be effectively leached out of the soil. The soil then has more ability to flocculate and form stable aggregates to improve drainage and soil quality. The combination of calcium and
sulphate effectively addresses sodium. Calcium release from the partially soluble gypsum is faster than from lime (calcium carbonate). Lime is also unsuitable in many cases as it acts to increase soil pH, Applying Gypsum aids with drainage. pushing out acid hydrogen rather than sodium from the cation exchange. Sometimes subsoil structural issues can Gypsum is typically and most easily reduce drainage and impact on the ability applied as a broadcast application to the of a site to receive wastewater without soil surface. This is also the recommended ponding or surface runoff. method if soil crusting is to be addressed. Gypsum may be helpful if the issue Incorporation of gypsum into the soil is is related to sodium and dispersive clay not generally required as the gypsum can content (clay that loses structure in contact work through the profile of most soils. with water), or if there is a problem with Gypsum can be applied annually or high aluminium availability in an acid every two or three years, at higher rates. subsoil. Many soils will have quite high Gypsum can also reduce surface runoff thresholds for sodium before soil structure of phosphorus and other nutrients by is affected. This depends on the balance improving water infiltration (through of the monovalent cations (sodium and soil structural and chemical changes), by potassium) with calcium and magnesium, binding organic matter and soil particles and the electrical conductivity of the together better, and by increasing the soil. It also appears that soil resilience to ability of soil to drain and thus cope with sodium is increased by the organic content larger water inputs. (lactose etc) in dairy effluent (Cameron et Gypsum also increases the binding al., 2003). of phosphate to soil minerals including Application of gypsum will be most calcium and reduces the susceptibility effective if commenced before soil of all forms of phosphorus to drainage structure is impacted by the sodium and losses. potassium. More? Visit www.gypsum.co.nz
New Zealand’s Dairy Effluent Management Specialists | nevadagroup.co.nz | 0800 464 393 NZ Dairy Exporter | Effluent Management Solutions | November 2021
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