South Africa Wine Technical Yearbook 2025

TABLE 1. The chemical status of the Kroonstad soil near Stellenbosch before the study began. Depth (cm) pH (KCl) P (mg/kg) Basic extractable cations (cmol c .kg -1 )

EPP’ (%) 28.1 31.4 35.6 29.8 30.4

ESP’ (%)

K

Ca

Mg

Na +

+

2+

2+

extr

extr

extr

extr

10 20 30 60 90

4.4 4.6 4.4 4.7 5.0

50 54 55 42 31

0.17 0.13 0.10 0.10 0.12

0.6 0.5 0.4 0.4 0.5

1.1 0.7 0.5 0.6 0.8

0.3 0.2 0.1 0.1 0.1

8.2 8.4 9.2 8.1 7.6

were applied in the course of the study (Figure 2A). Almost immediately, the application of WWW (May 2011) doubled the K extr in the 0-10 cm layer, after which it remained almost constant throughout the study period (Figure 2B). The K extr increased steadily up to May 2012 at 10-20 cm depth, after which it decreased again, to end up at almost the same level as before the start of the study. During 2013, there was an accumulation of K + in the 90 cm soil depth indicating that K was leaching to the deeper horizons. Substantial amount of applied K + via the WWW from November 2011 until May 2012 could be the reason for the relatively high soil K + concentrations recorded at all depths in May 2012. The potential for K + accumulation in soil after WWW is high, because it has a low leachability and K + ions that are not adsorbed by plants are then adsorbed by soil particles thereby reducing the risk of leaching. 9 This happened to some extent in the 0-10 cm layer in this study. The actual value is still low, probably due to the low cation exchange capacity (CEC) of the soil and thus its low capacity to retain cations. In the present study, high levels of K + were recorded in the 90 cm depth (Figure 2B) towards the end of the sampling period, which showed that K + had leached into the subsoil and then possibly to the water table and nearby streams through lateral flow. This also indicates a low capacity for retaining cations in the soil. The effect of K + ions on soil structure relative to Na + is well documented, 15 but limited research

FIGURE 2. Temporal variation in (A) rainfall and winery wastewater irrigation, (B) soil K + , (C) soil Na + , and (D) soil pH (KCl) where winery wastewater was applied to a Kroonstad soil near Stellenbosch.

data on the effect of high levels of K + in soil due to WWW irrigation on soil structure stability is available. 9 Application of WWW immediately after the start of the study in May 2011 more than doubled the soil Na extr in the 0-10 cm layer (Figure 2C). In the 10-20 and 20-30 cm layers, it also increased somewhat. Thereafter, it dropped down to its original level for the duration of the rest of the

study. The Na extr trend is in line with the Na + content in the WWW, which decreased after July 2011 and remained low during 2012 and 2013 where the average Na + concentration in the WWW was 41 mg/L and 46.2 mg/L, respectively. 2 As Na + is not an essential element, this trend is good for environmental sustainability of WWW management. The soil pH increased at all soil

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TECHNICAL YEARBOOK 2025