Handbook for Irrigation of Wine Grapes in South Africa

Chapter 5

subjected to simulated winter rainfall (Mulidzi, 2016). A third pot experiment was carried out to determine the effect of diluted winery wastewater on the hydraulic properties of the same soils used in the first pot experiment. In contrast to the other two pot experiments, these soils were subjected to different levels of diluted wastewater in the vineyard where the field trial was carried out (Howell, 2016). Soil analyses carried out after harvest in May and at budbreak in September showed no clear trends in pH (KCl) . However, EC e was substantially higher after the seasonal wastewater irrigations, compared to budbreak. This was probably due to the higher salt content in the diluted wastewaters. There was a close correlation between P applied via the irrigation water and the P levels in the 0 to 30 cm soil layer in the work row. Under the prevailing conditions, soil K (Bray II) increased with a decrease in the dilution of the wastewater during all four seasons. Taking soil samples to a depth of 3 m (Fig. 5.18) showed that even soils irrigated using wastewater containing 3 000 mg/ ℓ COD maintained baseline K levels after four years (Fig. 5.19A). On the other hand, irrigation with the 3 000 mg/ ℓ COD water tended to increase soil Na, compared to river water (Fig. 5.19B). However, the level of Na was substantially lower than the threshold of 0.4 cmol (+) /kg proposed by (W.J. Conradie, personal communication 2010) throughout the 3 m deep profile. Since there were no substantial differences in the amounts of Ca and Mg applied via the irrigation water, these elements did not show any consistent or significant responses in the soil (data not shown).

FIGURE 5.18. Collecting soil samples over 30 cm increments to a depth of 3 m.

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