Technical Yearbook 2024

JUNE

Treated municipal wastewater for irrigation (Part 4): Soil chemical responses By Carolyn Howell, Karla Hoogendijk, Philip Myburgh, Vink Lategan & Eduard Hoffman

Abstract A long-term trial was conducted in commercial vineyards in the Coastal region of South Africa to assess the impact of irrigation with treated municipal wastewater (TMW) on Cabernet Sauvignon and Sauvignon blanc vineyards. Grapevines were irrigated using TMW for 11 years. They were either rainfed (RF), irrigated with TMW via a single dripper line (SLD) or received twice the volume of wastewater via a double dripper line (DLD). Irrigation using TMW for 11 years increased soil pH (KCl) , and the electrical conductivity of the saturated extract (EC e ) and chloride (Cl - ) compared to baseline values. Substantial amounts of sodium (Na + ) and potassium (K + ) also accumulated in the topsoil due to TMW irrigation. Soil K + increases could have a negative impact on wine colour stability should the levels of soil K + be such that grapevines excessively absorb it. In general, soil extractable sodium percentage (ESP´) increased as a result of TMW irrigation. The increase was more prominent in the subsoil layers. Results also showed that the accumulation of Na + in the soil profile depended on the winter rainfall, i.e. between May and September. It should be noted that the results of this study represent specific in-field situations in three commercial vineyards under one set of climatic conditions. Future research should focus on the use of TMW for irrigation of vineyards or other crops on different soil types in different climatic regions.

Introduction Frequent water shortages and below-average rainfall in the Western Cape of South Africa recently led to the worst drought the province experienced. This drought was especially detrimental to the wine industry due to the carry-over effects of water constraints on grapevine growth and yield. Therefore, water scarcity is an increasingly important challenge to the viticultural sector in the region and has emphasised the need for alternative irrigation water sources. One such alternative could be treated municipal wastewater

(TMW), which has been used as a source of irrigation water in arid and semi-arid countries. 1 In South Africa, approximately 2 000 ha of vineyards in the Swartland region are being irrigated with TMW. 2 However, aside from the current study, no other studies have assessed the feasibility of using TMW for vineyard irrigation under South African conditions. Although there is extensive literature available regarding the effect of irrigation with TMW on soil chemical properties, 3,4 there is very little information regarding the re-use of TMW for vineyard irrigation. In this regard, soil

samples from vineyards in South Australia that were irrigated by means of drip irrigation with either potable metropolitan water (mains water) or TMW for four to 11 years indicated that irrigation with TMW increased soil sodium (Na + ) and magnesium (Mg 2+ ), but reduced soil calcium (Ca 2+ ) of a deep sand, clay loam and a hard setting sandy loam soil. 5 Increased soil salinity as a response to irrigation with TMW was also reported for a vineyard in South Australia. 6 In Spanish vineyards irrigated by means of drip irrigation, the use of TMW increased soil electrical conductivity of the

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TECHNICAL YEARBOOK 2024