Technical Yearbook 2024

Methods The field trial was carried out in full-bearing, commercial vineyards on a farm near Philadelphia in the Coastal region of the Western Cape from the 2006/07 until 2017/18 seasons. Three experiment sites were selected in different landscape positions. The first site was in a Sauvignon blanc vineyard located on the shoulder of a hill. The second and third sites were in two Cabernet Sauvignon vineyards situated on a back- and a footslope, respectively. Details of the characteristics of the vineyards, irrigation treatments and application, as well as an assessment of the water quality and nutrient load, were reported previously. 8,9,10 Grapevine water status and vegetative and yield responses have also been reported. 8,11,12,13 Baseline soil samples were taken in 2006 before wastewater irrigation commenced. Following 11 years of irrigation with TMW, soil samples were taken at budbreak (September) of the 2017/18 season. Soils were sampled in 30 cm increments to a depth of 90 cm in all plots and up to 180 cm in all treatment plots where it was possible to sample deeper. Soil chemical analyses were carried out by On average, the topsoil pH (KCl) of the SLD and DLD plots increased by 1.3 units after 11 years of irrigation with TMW (Figure 1). The increase in pH was most likely due to the pH of the irrigation water, which varied between 6.7 and 8.0 throughout the study period. 9,10 The decarboxylation and hydrolysis of organic acids and bicarbonate anions present in the TMW could also have contributed to the increased pH. The increased pH 14 did not cause concern as it remained near neutral and would therefore have little effect on biological functioning. 15 In addition, the pH at all of the treatment plots was within the recommended range of 5.0 to 7.5 to a commercial laboratory. Results and discussion

pH (KCl)

4.4 4.8 5.2 5.6 6.0 6.4 6.8 7.2 7.6

0 20 40 60 80

100 120 140 160 180

Baseline RF

Depth (cm)

SLD DLD

FIGURE 1. The effect of rainfed conditions (RF) and irrigation with treated municipal wastewater via single (SLD) and double dripper lines (DLD) on the mean soil pH across the three different landscape positions after 11 years of wastewater irrigation compared to the baseline before irrigation commenced.

EC e (dS/m)

0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60 80

100 120 140 160 180

Baseline RF

Depth (cm)

SLD DLD

FIGURE 2. The effect of rainfed conditions (RF) and irrigation with treated municipal wastewater via single (SLD) and double dripper lines (DLD) on the mean soil electrical conductivity (EC e ) across the three different landscape positions after 11 years of wastewater irrigation compared to the baseline before irrigation commenced.

saturated extract (EC e ), nitrogen (N), potassium (K + ), Na + , Mg 2+ and manganese (Mn 2+ ) compared to well water. 7 Topsoil pH and EC e were also higher where TMW was used for vineyard irrigation rather than freshwater or for unirrigated pastures adjacent to vineyards. 3 The soil exchangeable sodium percentage

(ESP) also increased, particularly in the soil layers up to 60 cm below the soil surface. Considering the above, the study’s objective was to assess the effects of long-term irrigation with TMW on soil chemical properties in commercial vineyards in the Coastal region of the Western Cape.

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TECHNICAL YEARBOOK 2024