Technical Yearbook 2024

soil surface is most likely a result of high evapotranspiration during the irrigation season, which concentrates the salts in the upper parts of the root zone. 18 Similar results were reported for vineyard soils in Australia which were irrigated with TMW for at least five years. 3 The accumulation of salts in the soil is concerning as a progressive increase in soil salinity can result in grapevine nutrient deficiencies. 6,19 However, the current study’s relatively small increase in EC e after 11 years of TMW irrigation suggests that winter rainfall leached some of the applied salts beyond the measured depth. In a laboratory study where rainfall cycles were simulated, the EC of the drainage water was considerably higher than that of the input water, 5 indicating a loss of salts from the soil during rainfall events. Therefore, regular rainfall events could help alleviate high soil EC e where TMW containing high salts is used for irrigation. On average, the Bray II K + content of the 0 - 30 cm soil layer increased by 26 mg/kg, 42 mg/kg and 127 mg/kg for the RF, SLD and DLD treatments, respectively (Figure 3). An accumulation of K + in the topsoil due to TMW irrigation has previously been reported. 20,21,22 The high K + content under DLD is of concern since an over-supply of K + to grapevines may result in excessive K + uptake. This could lead to musts with high pH, malate concentrations, and poor colour in red wines. 23 An accumulation of K + in the soil can also have deleterious effects on soil structure 24 and negatively impact soil hydraulic conductivity (K) and infiltration rate (IR) 25 negatively. The baseline’s mean extractable potassium percentage (EPP´) exceeded the recommended norm by far. It increased substantially in all the treatments throughout the study period (Figure 4). There was little difference between the topsoil EPP´ of the RF and SLD treatments, but the DLD was 2% higher than the

Bray II K (mg/kg)

0

50

100 150 200 250 300

0 20 40 60 80

100 120 140 160 180

Baseline RF

Depth (cm)

SLD DLD

FIGURE 3. The effect of rainfed conditions (RF) and irrigation with treated municipal wastewater via single (SLD) and double dripper line (DLD) on the mean Bray II extractable potassium (K) content across the three different landscape positions after 11 years of wastewater irrigation compared to the baseline before irrigation commenced.

EPP´ (%)

0

2

4

6

8

10

12

0 20 40 60 80

100 120 140 160 180

Baseline RF

Depth (cm)

SLD DLD

Figure. Effect of rainfed conditions (RF) and irrigation with treated municipal wastewater via single line (SLD) and double line drip (DLD) on the mean soil extractable potassium (EPP’) across main plots after 11 years of wastewater irrigation compared to the baseline before irrigation commenced (ARC, unpublished data). FIGURE 4. The effect of rainfed conditions (RF) and irrigation with treated municipal wastewater via single (SLD) and double dripper line (DLD) on the mean soil extractable potassium (EPP´) across the three different landscape positions after 11 years of wastewater irrigation compared to the baseline before irrigation commenced.

sustain optimal grapevine growth. 16 Similar results have been reported previously 15,17 where soils were irrigated with TMW. The mean topsoil electrical conductivity of the saturated extract (EC e ) increased with the amount of TMW applied (Figure 2). However, there were no clear trends in the

deeper soil layers that could be related to the different irrigation treatments compared to baseline values. The increased EC e of the topsoil indicates an accumulation of salts at the soil surface. The EC w of the TMW ranged between 0.7 dS/m and 1.2 dS/m 9,10 , which could explain the increased EC e . The accumulation of salts at the

30

TECHNICAL YEARBOOK 2024