South Africa Wine Technical Yearbook 2025

FIGURE 1. Effect of irrigation with diluted winery wastewater on P (Bray 2) in the 0-10 cm layer (solid circles) and 10-20 cm layer (open circles) in (A) Rawsonville sand, (B) Lutzville sand, (C) Stellenbosch shale, and (D) Stellenbosch granite soils over four simulated seasons. Values in brackets indicate the soil pH (KCl) . Dashed lines indicate the P (Bray 2) thresholds for grapevines based on clay content. 10

product quality”. This solicited project was initiated, funded and managed by the WRC. The project was co-funded by Winetech and ARC. • Goudini Winery for providing wastewater for the research. • ARC for infrastructure and resources. • Staff of the Soil and Water Science division at ARC Infruitec-Nietvoorbij for their assistance, and in particular Mr. F. Baron for his dedicated technical support.

FIGURE 2. Variation in relative available P (Bray 2) content with soil pH (KCl) in four different soils that were irrigated with diluted winery wastewater over four simulated seasons. The dashed line was fitted by eye.

Conclusions Where diluted WWW was applied, the level of soluble P in the shale and granite soils increased as the pH (KCl) increased towards the optimum range for P availability over the four seasons. Although the initial pH (KCl) in the aeolic sand was higher than the optimum range, the presence of relatively high levels of Na + extr caused available P to increase as the pH (KCl) increased. In the case of the alluvial sand containing unusually high initial levels of P, the pH (KCl) increased out of the optimum range, thereby causing a substantial reduction in the level of available P. These results indicated that irrigation with diluted WWW will only promote P absorption by grapevines if the pH (KCl) shift is towards the optimum. Since the level of P applied via diluted WWW appears to be generally low, application of P fertilisers will still be necessary to ensure adequate uptake by grapevines. It is worth noting that the results represent a worst-case scenario, i.e. in the absence of rainfall or crops. Effects of irrigation with diluted WWW on the enzyme status of the four soils in the pot study will be presented in the next article. 

Reference https://www.wineland.co.za/winery-wastewater-irrigation-part-6/ For more information, contact Reckson Mulidzi at mulidzir@arc.agric.za.

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TECHNICAL YEARBOOK 2025