South Africa Wine Technical Yearbook 2025

Na + was also linearly related to the cumulative irrigation plus rainfall (Figure 8). Similar to K + , the low clay content of the soil could probably not retain large amounts of Na + . Therefore, leaching of Na + beyond 90 cm was also not inhibited. Although leaching of Na + from sandy or coarse-textured soils during winter rainfall also reduces the risk of accumulation and dispersion, it poses the same environmental risks as the large amounts of K + that were leached from the soil. High concentrations of Na + in soil due to WWW application can reduce soil aggregate stability. 16 When Na + is the predominant adsorbed cation, the clay disperses. When the soil is wet, puddling reduces permeability, and when it is dry, a hard impermeable crust forms. Acknowledgements • This article is an output of WRC Project K5/1881, entitled “The impact of wastewater irrigation by wineries on soils, crop growth and product quality”. This solicited project was initiated, funded and managed by the WRC. The project was co-funded by Winetech and ARC. • Goudini and Koelenhof wineries for their permission to work at their land and utilisation of their wastewater for 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 6. Temporal P-variation in the topsoil (0-30 cm) and subsoil (30-90 cm), as well as irrigation plus rainfall, where wastewater was applied to a Longlands soil near Rawsonville. Vertical columns indicate irrigation plus rainfall.

FIGURE 7. Effect of cumulative (Σ) irrigation plus rainfall on cumulative K + losses beyond 90 cm depth where a Longlands soil was irrigated with winery wastewater near Rawsonville.

reduce soil aggregate stability. 16 When Na + is the predominant adsorbed cation, the clay disperses. When the soil is wet, puddling reduces permeability, and when it is dry, a hard impermeable crust forms. Since there was little change in

Na + extr levels with depth throughout the profile, it suggested that most of the applied Na + was leached beyond 90 cm. Seasonal soil Na + balances confirmed that substantial amounts of Na + were leached (Table 3). Furthermore, the cumulative leached

TABLE 2. Soil extractable K + balances for selected periods in the 0-90 cm depth of a sandy Longlands soil that was irrigated with winery wastewater near Rawsonville.

Soil K + (kg/ha)

Period

Applied K + (kg/ha)

K + loss (kg/ha)

Leached K + (%) *

Beginning

End

Mar 11 - May 11 May 11 - Nov 11 Nov 11 - May 12 May 12 - Nov 12 Nov 12 - May 13 May 13 - Nov 13

3 978 5 909 4 914 6 786 5 148 5 031

5 909 4 914 6 786 5 148 5 031 6 318

2 768 1 561 6 760 3 894 8 879 4 186

837

30

2 556 4 888 5 532 8 996 2 899

164

72

142 101

69 * 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.

25

TECHNICAL YEARBOOK 2025