Technical Yearbook 2024

FIGURE 2. Soil pH (KCl) measured before the in-field fractional use (augmentation) of winery wastewater with raw water for vineyard irrigation commenced, as well as soil pH (KCl) of the experimental plots and controls at the end of the trial in September 2021.

the norm of 5.0 to 7.5 recommended for optimal grapevine growth. 14 In a field study at Rawsonville, winery wastewater was diluted to 3 000 mg/L COD, and soil pH (KCl) increased at bud break after winter rainfall. 11 In contrast, other studies showed that irrigation with winery wastewater did not have a pronounced effect on soil pH (KCl) . 12,15 Since irrigation using winery wastewater generally increases soil K and Na, soil pH will consequently increase via alkaline hydrolyses. This reaction is primarily caused by the hydrolysis of exchangeable cations in soils, e.g. K ex and Na ex , or salts, e.g. CaCO 3 , MgCO 3 and Na 2 CO 3 . 16 Exchangeable Ca and Mg are more tightly adsorbed to the exchange complex than K and Na. 16 Therefore, K and Na are more readily hydrolysed and produce a higher pH than exchangeable Ca or Mg. In the present study, excessive soil K after wastewater application in conjunction with winter rainfall could have induced alkaline hydrolysis, thereby increasing the soil pH (KCl) of the experimental plots in September 2021 compared to the respective controls.

At the end of the trial in September 2021, the electrical conductivity of the saturated extract (EC e ) of the C1 experimental plot was similar to that of the control, whereas, for the C2 experimental plot, soil EC e was slightly higher compared to its control (data not shown). This indicated that under the prevailing conditions, rainfall must have leached some of the salts applied via irrigation with augmented wastewater salts from this region’s soil. However, this does not rule out the possibility that winter rainfall could have leached salts beyond the measured depth. Another study reported that large volumes of irrigation plus rainfall must have leached some of the salts applied via the winery wastewater irrigation beyond 90 cm depth, particularly in the last two winters of that particular study. 15 In September 2021, soil EC e of the BR1 experimental plot was lower than that of its control whereas the soil EC e was substantially higher for the BR2 experimental plot compared to its respective control (data not shown). This trend suggested an accumulation

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TECHNICAL YEARBOOK 2024