South Africa Wine Technical Yearbook 2025

TABLE 2. Effect of winery wastewater application on soil P (Bray II) in mg/kg at a newly irrigated grazing paddock at a winery near Stellenbosch. Depth (cm) Season Mar 11 May 11 Nov 11 May 12 Nov 12 May 13 Nov 13 10 50 64 77 95 63 70 76 20 54 44 64 75 70 64 71 30 55 44 70 67 73 63 80 60 42 34 50 53 53 46 59 90 31 19 22 27 31 25 22 Mean 46c * 41c 57ab 63a 58a 54b 62a * Means in the same row followed by the same letter, do not differ at p = 0.05. TABLE 3. Seasonal soil K + balances in the 0-90 cm depth of a sandy Kroonstad soil that was irrigated with winery wastewater near Stellenbosch. Period Soil K + (kg/ha) Applied K + (kg/ha) K + loss (kg/ha) Leached K + (%) Beginning End Mar 11 - May 11 2 457 2 633 5 236 5 060 97 * May 11 - Nov 11 2 633 2 984 2 883 2 532 88 Nov 11 - May 12 2 984 4 154 17 030 15 860 93 May 12 - Nov 12 4 154 3 452 9 105 9 807 108 Nov 12 - May 13 3 452 3 686 15 751 15 517 99 May 13 - Nov 13 3 686 3 744 5 934 5 876 99 * Amount lost through leaching expressed as percentage of the amount applied, a figure of >100 indicating that more was lost through leaching than what was applied during that period.

years of irrigating with WWW, the soil P levels were still in the acceptable range for plant growth, i.e. the P levels were below 100 mg/kg. The magnitude of the increases in the top 60 cm within only three years indicates that irrigating with the WWW could lead to P reaching unacceptably high levels in a few more years. Soil nutrient balances Since there was little change in K + levels with depth throughout the profile, it suggested that most of the applied K + was leached beyond 90 cm. In fact, seasonal soil K + balances showed that substantial amounts of K + remained in solution, and were leached (Table 3). Furthermore, the cumulative leached K + was linearly related to the cumulative irrigation plus rainfall (Figure 5). Due to the low clay content of the soil, the exchange complex could not retain large amounts of K + . Therefore, leaching of K + beyond 90 cm was not inhibited. Although leaching of K + in sandy or coarse textured soils during winter rainfall reduces the risk of accumulation and dispersion, it increases environmental risks such as groundwater recharge and/or lateral flow into other freshwater resources. A previous study showed that the K + accumulation in soil upon WWW irrigation could be high if it is not absorbed by plants, but adsorbed to soil particles thereby reducing the possibility of leaching. 9 Visual observations revealed that the grassroots did not extend beyond 30 cm depth. This suggested that the large amounts of the K + that was applied via the wastewater could not be utilised by the grass, since it had died back. Since there was little change in Na + levels with depth throughout the profile, it suggested that most of the applied Na + was leached beyond 90 cm depth. Seasonal soil Na + balances confirmed that substantial amounts of Na + were leached (Table 4). Furthermore, the

FIGURE 5. Effect of cumulative (Σ) irrigation plus rain on cumulative K + losses beyond 90 cm depth where a Kroonstad soil was irrigated with winery wastewater for two and a half years near Stellenbosch.

May 2012 and November 2013 (Figure 4B). The WWW contained too low amounts of Mg 2+ to have any significant impact on the soil to which it was applied. Soil phosphorus Application of WWW over three years increased available soil P (Table 2).

It increased from 50 mg/kg to 76 mg/kg in the topsoil layer after three years of WWW applications, while for the 90 cm soil depth, it decreased from 31 mg/kg to 22 mg/kg. Although there was P build-up over time due to WWW application, the P had accumulated in the top 60 cm. At this stage, after three

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TECHNICAL YEARBOOK 2025