South Africa Wine Technical Yearbook 2025

JULY

Winery wastewater irrigation (Part 6): Effect on soil phosphorus By Reckson Mulidzi & Carolyn Howell Abstract dominant cation that will react with PO 4 3- to form a general sequence of

Due to environmental legislation, the wine industry requires solutions for wastewater treatment or its reuse. The feasibility of reusing diluted winery wastewater (WWW) was assessed in a pot experiment under a rain shelter over four simulated irrigation seasons. Four soils varying in parent material and clay content were irrigated with WWW diluted to 3 000 mg/L chemical oxygen demand (COD), whereas the control received municipal water. Irrigation with diluted WWW raised the level of phosphorus (P) in shale and granite-derived soils into the optimum pH (KCl) range for P availability. Although pH (KCl) in the aeolic sand was initially above the optimum range, relatively high sodium (Na + ) levels also caused available P to increase as the pH (KCl) increased. The pH (KCl) in the alluvial sand increased out of the optimum range, thereby causing a reduction in the available P. This indicated that irrigation with diluted WWW may only enhance P absorption if the pH (KCl) shift is towards the optimum. It is worth noting that the results represent a worst-case scenario, i.e. in the absence of rainfall or crops.

calcium phosphates. 11 The formation of these compounds decreases the solubility of phosphate. On the other hand, PO 4 3- solubility 9 can also increase in alkaline soils when exchangeable sodium (Na + ) releases inorganic PO 4 3- . When Na + replaces exchangeable Ca 2+ , magnesium (Mg 2+ ) and Al 3+ , the negative potential of the surface increases, which results in desorption 12 of PO 4 3- . It was also reported that water-soluble PO 4 3- increases as Na + saturation increases in alkaline soils. 13 Similar to the effect of exchangeable Na + , soluble P increased above pH 7 when a silty clay soil was alkalinised with potassium hydroxide. A number of field and laboratory studies have shown that irrigation with WWW increases soil pH, particularly if the water contains high levels of potassium (K + ) and Na + . 8, 14, 15, 16, 17, 18 However, it was also reported that the opposite effect on soil pH is possible. 19 Therefore, the extent to which the P applied via WWW can be absorbed by plants will indirectly depend on the impact of the WWW irrigation on the soil pH. Taking the above-mentioned into consideration, the objective of this study was to determine the effects of irrigation with diluted WWW on

Introduction Most previous studies and reports have focused on the detrimental effects of irrigation with winery wastewater (WWW) on the soil’s physical and chemical status. 1, 2, 3, 4, 5, 6, 7 However, reusing stillage for irrigation and as a fertiliser had positive effects on soil, such as increases in pH, improved water and mineral salt retention, as well as restoration and maintenance of soil micro-flora. 8 It was also proposed that reusing potassium-rich wastewater could enhance soil fertility. 5 In this regard, P (phosphorus) applied via

WWW irrigation could contribute to the nutrient requirements of agricultural crops. Solubility of phosphate (PO 4 3- ) compounds, or P availability to plants, strongly depends on the soil pH. 9, 10, 11 In acidic soils, particularly where pH < 5.5, aluminium (Al 3+ ) and iron (Fe 3+ ) will react with PO 4 3- to form amorphous phosphates. 11 The amorphous Al 3+ and Fe 3+ phosphates gradually change to insoluble PO 4 3- compounds that are unavailable to plants. Phosphate becomes more insoluble if the soil pH (KCl) exceeds 7. 10, 11 In alkaline soils, i.e. pH > 7, calcium (Ca 2+ ) is the

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TECHNICAL YEARBOOK 2025