Technical Yearbook 2024

to high Na + levels present at the beginning of the study period as the subsoil ESP´ decreased in relation to the baseline, except for the SLD at 60 cm and 150 cm, as well as the DLD at 150 cm soil depth. Deeper than 60 cm, the mean ESP´ of the SLD plots were higher than the DLD. This could be explained by the larger volumes of irrigation water applied in the latter plots, which facilitated the leaching of more Na + from the soil profile. The high ESP´ observed in the subsoil of the SLD and DLD treatments remains a concern as it may reduce the movement of water through the soil profile. Lower macro-porosity due to an accumulation of Na + and K + in the soil may affect the drainage capacity of soils, which in turn limits water percolation and, ultimately, the leaching of salts. 29,30 In soil with a permeable A horizon overlying a moderately draining B horizon, irrigation with TMW caused a reduction in K of the soil due to the increase of exchangeable Na + in the B horizon, which reduced the leaching of salts and led to increased soil salinity in the A horizon. 31 Soil permeability problems will also occur if a solution with very low

electrolyte concentration, such as rainwater, percolates through the soil. 32 Therefore, applying large volumes of higher salinity water, as is the case with DLD treatments, might help mitigate the accumulation of Na + at the soil surface and prevent reductions in K and IR. Monitoring the soil chemical status at bud break every year over 10 years showed that the soil ESP´ varied considerably over time and reached excessively high levels in the subsoil (Figure 6A). This is an alarming trend since the water sodium adsorption ratio (SAR) was on average less than 10, 9,10 i.e. within the quality norms for wastewater irrigation. However, results indicated that the accumulation of Na + in the soil profile depended on the winter rainfall, i.e. between May and September (Figure 6B). Generally, this implies that some of the salts will accumulate if the winter rainfall is low. Some will be leached into deeper layers following high winter rainfall. It must be noted that the nature of the rainfall could affect the amount of salts that will be leached. Although the total rainfall could be high, it could consist of numerous small showers. Therefore, fewer salts will be leached compared

to a few heavy downpours, which will add up to the same total. This probably explains the outlier value indicated in Figure 6B. The mean chloride (Cl - ) content of the topsoil increased with the amount of TMW applied (Figure 7). The high Cl - levels were to be expected as the wastewater is disinfected by a chlorination treatment at the WWTW, resulting in a mean Cl - content in the wastewater of 160 mg/L over the 12 year study period. 9,10 An accumulation of Cl - seems evident at a depth of 90 cm, but cannot necessarily be ascribed to the irrigation water, as high Cl - levels were observed in the subsoil of the RF treatments as well.  Acknowledgements • The project was funded by the Water Research Commission (WRC), Winetech and the Agricultural Research Council (ARC). • ARC for infrastructure and resources. • Staff of the Soil and Water Science division at ARC Infruitec Nietvoorbij for technical support. • Messrs Pierre Blake for permission to work in his vineyard, and Egbert Hanekom for managing the vineyard and technical assistance.

References https://www.wineland.co.za/treated-municipal-wastewater-for-irrigation-part-4/

Conclusion Irrigation using TMW increased soil pH and EC e . There was also an accumulation of Cl - in the topsoil, likely due to the chlorine-disinfection treatment process at the wastewater treatment works. Substantial amounts of Na + and K + accumulated in the topsoil due to TMW irrigation. Such soil K + increases could have a negative impact on wine colour stability should it be taken up by the grapevine in sufficient quantities, particularly if the levels of soil K + are such that grapevines excessively absorb it. In general, soil ESP´ increased as a result of TMW irrigation. The increase was more prominent in the subsoil layers, possibly due to the seasonal leaching of salts by rainfall. Furthermore, the application of more water at DLD treatment plots might also have contributed to more Na + being leached from the profile compared to SLD plots. 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. 

For more information, contact Carolyn Howell at howellc@arc.agric.za.

32

TECHNICAL YEARBOOK 2024