South Africa Wine Technical Yearbook 2025

Results Chemical composition of the water and amount of elements applied The COD in the municipal water was substantially lower compared to the diluted WWW (data not shown). The COD in the diluted WWW was comparable between the four simulated seasons and was reasonably close to the target level of 3 000 mg/L. Most of the other WWW quality variables were considerably higher compared to the municipal water. On most irrigation days, the WWW pH was lower compared to the municipal water. The average sodium adsorption ratio (SAR) of the WWW was close to five, which is the limit for irrigation with wastewater according to the South African water quality legislation. 27 Due to the differences in the chemical composition of the municipal and diluted WWW, considerably more cations were applied to the soil via the WWW compared to the municipal water. The average amount of K applied per season via the irrigation water was 15 kg/ha and 2 785 kg/ha for the municipal water and diluted WWW, respectively. The average amount of Na applied per season was 110 kg/ha and 1 162 kg/ ha for the municipal water and diluted WWW, respectively. Soil potassium and EPP' Municipal water irrigation had no effect on soil K + extr , irrespective of clay content (data not shown). In contrast, irrigation with the diluted WWW increased K + extr substantially over the four seasons. The K + extr in the 0-10 cm soil layer was slightly higher compared to the 10-20 cm layer, irrespective of clay content (Figure 1). It was previously reported that a higher amount of exchangeable K + is retained by soils higher in clay content than soils low in clay content following WWW irrigation. 19 This may have resulted in a similar trend being observed in the four soils. Furthermore, K + extr in the four soils increased linearly with the cumulative amount of K + applied via the irrigation waters (Figure 1). In the 0-10 cm

were given previously. 23 In brief, a pot experiment was carried out under a 20 m x 40 m translucent fibreglass rain shelter at ARC Infruitec-Nietvoorbij. Four different soils, namely a Rawsonville sand, Lutzville sand, Stellenbosch shale and Stellenbosch granite, were included in the pot experiment. The control treatment soils were irrigated with water supplied by the Stellenbosch municipality. For the wastewater treatments, WWW was diluted to a chemical oxygen demand (COD) level of 3 000 mg/L. The undiluted WWW was collected from the wastewater pit at a winery near Rawsonville. Treatments were applied over four To make provision for destructive soil sampling, each experimental “plot” consisted of four pots. Following each simulated irrigation season, the soil in one of the pots was collected for sampling, i.e. after six, 12, 18 and 24 irrigations. Soil samples were collected from the 0-10 cm and 10-20 cm layers in the pots of all replications. A commercial laboratory analysed the samples for chemical parameters according to methods described previously. 23 The extractable sodium percentage (ESP') and extractable potassium percentage (EPP') of the soils were calculated. Statistical procedures Each soil/water treatment was replicated three times in a complete randomised design. The four soils were randomly allocated within each block. The treatment design was a split-plot with soil type as the main plot factor and soil depth as the sub-plot factor. Analyses of variance were performed separately for each season using SAS. 24 The Shapiro-Wilk test was performed to test for non-normality. 25 Student’s “t” least significant difference (LSD) was calculated at the 5% significance level to facilitate comparison between treatment means. 26 Linear regressions were calculated using Statsgraphics. simulated irrigation seasons. Soil sampling and analyses

Soil pH tends to increase when wastewater with high pH and Na + concentrations is used for irrigation. 16 A study carried out in the Western Cape showed that disposal of grape processing effluents changed the soil pH from acidic to alkaline. 6 This pH increase was attributed to initial soluble organic matter removal through volatilisation of CO 2 during biodegradation. In contrast, application of wine vinasse containing high bicarbonate concentrations slightly reduced the pH of a Mediterranean soil. 20 In this case, the pH reduction was attributed to the high electrical conductivity of the soil solution (EC e ), i.e. 9.2 dS/m, and transformation of organic sugars by micro-organisms. These contrasting results of various studies imply that soil responses to the application of WWW cannot be readily predicted. The soils of the South African winelands are highly heterogeneous and can show a high degree of spatial variation in a relatively small area. 21 Soils range in parent material, texture, structure, drainage, coarse fragment content and chemistry. Parent material is usually largely responsible for the physical and chemical composition of a soil. In the Stellenbosch region, two of the dominant parent materials are shale and granite, whereas in the Breede River and Olifants River wine growing regions, transported aeolian or fluvial sands are an important parent material. 22 Due to the heterogeneity of the soils in the winelands, they are likely to respond differently to the application of WWW. However, there has been previous research to determine these responses. Therefore, the objective of this study was to determine the effects of irrigation with diluted WWW on selected chemical properties of four soils varying in parent material and clay content. Materials and methods Experimental layout Details of the pot experiment

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TECHNICAL YEARBOOK 2025