Handbook for Irrigation of Wine Grapes in South Africa

After three years in the vineyard, the near-saturation hydraulic conductivity of the shale-derived and sandy soils decreased substantially with a decrease in the level of winery wastewater dilution (Howell, 2016). In contrast, there was no effect on the K of the granite-derived soil after three years (Fig. 5.23). Furthermore, its water permeability was low, compared to the other soils. The pots in the vineyard were covered when river water was applied in the early part of the season and to flush the system after wastewater irrigations. Therefore, fresh water could not have affected K responses to the wastewater. Likewise, the granite-derived soil under the rain shelter showed poor infiltration, compared to the other soils, although it was irrigated with clean municipal water (Mulidzi et al. , 2015a). A previous study also showed that K of this particular soil was not in line with its clay content, compared to other vineyard soils (Myburgh, 2015). At this stage, there is no explanation for the hydraulic behaviour of the granite-derived soil. However, this does not rule out the possibility that severe reductions in K will occur in the long run if diluted winery wastewater is used for irrigation on these soils. Similar to the pot experiment under the rain shelter, the potted soils in the field trial were not exposed to rainfall which could have caused leaching. Therefore, the salt accumulation reported by Mulidzi et al. (2015b) probably explains why the diluted wastewater reduced K even on the sandy soils after only three years. In fact, visual observations in the field trial revealed no infiltration problems where the sandy soil was irrigated with wastewater for four years and confirmed that winter rainfall prevented salt accumulation that could have reduced K .

1 000

Lutzville sand - 0.4% clay (R 2 = 0.8459)

100

Rawsonville sand - 3.3% clay (R 2 = 0.7685)

10 K (mm/h)

Stellenbosch shale - 21% clay (R 2 = 0.8706)

Stellenbosch granite - 13% clay (R 2 = 0.0317)

1

0

500

1 000

1 500 COD (mg/ℓ)

2 000 2 500 3 000

FIGURE 5.23. The near-saturation K in four differently textured soils after being irrigated with diluted winery wastewater for three seasons in a pot experiment (Howell, 2016). It must be noted that the y-axis is a logarithmic scale.

136 CHAPTER 5 – IRRIGATION WATER QUALITY

12

Pearl millet

Oats