South Africa Wine Technical Yearbook 2025

TABLE 1. Particle size distribution and textural class of the six soils used in the study. Particle size and textural class Rawsonville sand Lutzville sand Stellenbosch shale

Stellenbosch granite

Stellenbosch sand

Robertson clay

3.3

0.4

20 13 50

13 17 33

7 6

35 20 35

Clay < 0.002 mm Silt 0.002-0.02 mm

1

1

Fine sand 0.02-0.2 mm Medium sand 0.2-0.5 mm Coarse sand 0.5-2 mm

60 29

69 26

39 26 22

5

3

7 3

8

2

12

35

Textural class

Sand

Sand

Sandy clay loam Sandy loam

Sand

Clay loam

TABLE 2. Chemical properties of the six soils before the application of the diluted winery wastewater irrigation treatments. Variable Rawsonville sand Lutzville sand Stellenbosch shale Stellenbosch granite Stellenbosch sand

Robertson clay

pH

5.8

6.5

4.2

4.3

4.3

6.6

EC e (mS/m)

30

20 21

30

40 11 99

20 28

70

Bray2 P (mg/kg) Bray2 K (mg/kg)

227

8

102 702

66

240

95

206

Org C (%)

0.8

0.2

1.5

1.3

0.9

0.6

extr (cmol c .kg

-1 )

0.17 0.01 2.79 0.95

0.61 0.05 3.22

0.24 0.06 1.64 0.61

0.25 0.05 1.21 0.51

0.53 0.02

1.79 0.37 9.17 3.02

K +

extr (cmol c .kg extr (cmol c .kg extr (cmol c .kg

-1 )

Na + Ca 2+ Mg 2+

-1 )

0.7

-1 )

0.8 3.4

0.22

CEC (cmol c .kg

-1 )

3.9

4.3

3.6

2.9

8.3

wastewater (WWW) with high organic load and variable salinity and nutrient levels is produced per tonne of grapes crushed. 4 Most WWW is currently irrigated onto pastures, but because of the drought and climate change, the idea of using WWW to irrigate vineyards has been encouraged. 2 Winter rainfall after WWW irrigation may lead to the leaching of nutrients to the groundwater. Changes in soil structure due to wastewater irrigation depend on the quality of wastewater, i.e. salinity levels, organic matter content, and the amount of total suspended solids. 5 The rate and amount of pollutants from WWW reaching groundwater resources depend on several factors such as: sorption, degradation, chemical properties of the wastewater, soil characteristics, environmental conditions, rainfall and water management practices. 6 Saline-sodic irrigation water in low rainfall and high evaporation areas will increase soil sodicity. Furthermore, a major side effect associated with wastewater irrigation is the potentially irreversible deterioration of the groundwater quality. 7 The electric conductivity (EC e ) of Fluvisol soils decreased as a result of leaching of salts by the Autumn Spring rainfall. 6 However, due to high cation exchange capacity (CEC) and high water retention capacity, the soils

still retained high levels of sodium (Na + ). Winter rainfall on soils irrigated with WWW will lead to the reduction of soil electrolyte concentrations regardless of soil type. Low rainfall areas are likely to experience less soil structural hazard, which is linked to high exchangeable monovalent cation concentrations, while high rainfall areas will experience more soil structural hazard. 8 Taking above-mentioned into consideration, the objective of the study was to investigate the effect of simulated winter rainfall on leaching of basic cations in soils irrigated with diluted WWW. Materials and methods Soils used Six pedogenetically different soils, commonly found in the Western Cape Province of South Africa, were included in the study. Soils will be referred to as Rawsonville sand, Lutzville sand, Stellenbosch shale, Stellenbosch granite, Stellenbosch sand and Robertson clay. The Robertson soil had the highest clay content, i.e. 35%, whereas the Lutzville sand contained only 0.4% clay (Table 1). The soil was packed into PVC pots to a specific predetermined bulk density and volume of the pots as described previously. 7,9 The results of the chemical properties of the six soils are presented in Table 2. The pH

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TECHNICAL YEARBOOK 2025