Technical Yearbook 2024

the critical value of 5 mg/L, which is the recommended maximum concentration of Fe 2+ in irrigation water used for irrigation of grapevines. 12 Levels of Mn 2+ in the TMW varied from being absent to a maximum concentration of 0.08 mg/L (Table 2). According to South African guidelines, 13 levels of Mn 2+ should not exceed 1.5 mg/L since Mn 2+ may cause clogging of irrigation pipelines, i.e. similar to Fe 2+ . No arsenic (As 3+ ) and mercury (Hg 2+ ) were detected in the TMW from 2008 to 2013. Chromium was present every season up to 2012/13, with concentrations ranging between 0.000 mg/L and 0.023 mg/L. Concentrations of cadmium (Cd 2+ ) and lead (Pb 2+ ) in the TMW were less than 0.003 mg/L and 0.0002 mg/L, respectively. Due to the low concentrations of the heavy metals and the cost implications, analyses of heavy metals were terminated in 2013/14. The concentrations of each element applied via the TMW in the vineyard, as well as the amount of irrigation water applied, were used to calculate the amount of each element applied on average SLD plots for the three landscape positions’ plots. Annually, 7 kg/ha, 5 kg/ha and 33 kg/ha of

N, P and K + , respectively, were applied. Annual amounts of 77 kg/ha, 14 kg/ha, 201 kg/ha and 267 kg/ha of Ca 2+ , Mg 2+ , Na + and Cl - , respectively, were applied via the irrigations with TWM. The low N content in TMW was insufficient to supply the annual N requirement of grapevines. The TMW provided adequate amounts of P to meet annual grapevine requirements when double the amount of irrigation water was applied. The K + applied via the TMW irrigation to the DLD treatment exceeded grapevine requirements. The amount of Ca 2+ and Mg 2+ applied via the TMW also exceeded annual grapevine requirements. Acknowledgements • The project was funded by the Water Research Commission (WRC), Winetech and the Agricultural Research Council (ARC). • ARC for infrastructure and resources. • Staff of the Soil and Water Science division at ARC Infruitec-Nietvoorbij for technical support. • Messrs Pierre Blake for permission to work in his vineyard, and Egbert Hanekom for managing the vineyard and technical assistance.

Conclusion The quality of the TMW used for vineyard irrigation met the minimum criteria stipulated by the General Authorisations for pH, EC w , COD, faecal coliforms and SAR for irrigation with wastewater in South Africa. The P concentration in the TMW consistently exceeded the long-term critical value of 0.05 mg/L, which demarcates a risk for algal blooms in water storage facilities, as well as biofouling of irrigation equipment. The mean Na + concentration of 120.9 mg/L in the TMW exceeded the critical value of 100 mg/L for irrigating grapevines in South Africa. Chloride levels in the TMW were well below the threshold value of 700 mg/L, at which toxicity in grapevines might occur. Considering the above-mentioned, regular analyses of TMW are essential when using it as an alternative source of water for vineyard irrigation. This will ensure that the legislated limits given in the General Authorisations are adhered to. Furthermore, the analyses will ensure that the chemical load conforms to recommended thresholds and norms. In doing so, irreversible damage to irrigation equipment, soils and grapevines can be avoided. The low N content in the TMW was insufficient to supply the annual N requirement of grapevines. Where double the normal irrigation volume was applied, TMW supplied adequate amounts of P to meet annual grapevine requirements. The amounts of K + applied via TMW irrigation to the DLD treatment were more than grapevine requirements and could negatively affect wine quality. The amount of Ca 2+ and Mg 2+ applied via the TMW also exceeded annual grapevine requirements. In general, using TMW irrigation can supply grapevine nutrients in a plant-available form, but some nutrient amounts may be insufficient, whereas others may be excessive. Consequently, growers are recommended to use an integrated fertiliser program by adjusting fertiliser amounts according to the amount of nutrients applied via the wastewater. Growers could also consider diluting the wastewater with raw water to reduce the oversupply of certain elements if indicated by analysis. Grapevine and soil responses will be presented in subsequent articles. 

References https://www.wineland.co.za/treated-municipal-wastewater-for-irrigation-part-1/

For more information, contact Carolyn Howell at howellc@arc.agric.za.

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TECHNICAL YEARBOOK 2024