In-field fractional use of winery wastewater with raw water (Part 3): Soil responses

Abstract

Wineries produce large volumes of poor-quality wastewater, particularly during harvest. Since water resources are limited, wine-grape producers will have to use them judiciously to produce grapes. It is also important that the sustainable use of alternative water sources for vineyard irrigation be investigated. In this regard, experimental plots were selected in three selected production areas in the Western Cape Province, namely the Coastal, Breede River and Olifants River regions. The specific locations were selected due to their vast differences in mean annual rainfall. Within each region, two plots were selected which differed in soil texture. Grapevines were irrigated with the in-field fractional use (augmentation) of winery wastewater with raw water for vineyard irrigation. Under the prevailing conditions, soil pH_(KCl) increased consistently in response to winery wastewater irrigation. Soil potassium (K) was substantially higher for the experimental plots compared to their respective controls regardless of mean annual rainfall. Consequently, soil extractable potassium percentage (EPP´) was higher. Results indicated that the increase in soil K and EPP´ was related to soil texture, amounts of K applied via the irrigation water and annual rainfall. In contrast, soil Na of all the experimental plots was similar or even lower compared to their respective controls. This indicated that there was sufficient leaching of sodium (Na) at all the experimental plots, regardless of soil texture. However, where more Na is applied via the irrigation water, Na could accumulate to levels where it could negatively impact soil physical conditions or grapevine growth and yield.

Introduction

Although there is extensive literature available regarding the effect of irrigation with wastewaters of various origins on soil chemical properties,¹ very little is known about the effects of irrigation using augmented winery wastewater on soil chemical status. In Australia, it is estimated that 3 – 5 m³ of winery wastewater, with high organic load, as well as variable salinity and nutrient levels, is produced when a ton of grapes is crushed.² The effects of high concentrations of potassium (K) application to soils have not been extensively researched and are still unclear.^2,3 On the other hand, limited irrigation water supplies could be restricted further in future allocations of irrigation water.^4,5 If winery wastewater could be used to irrigate vineyards with no detrimental impacts on soil chemical status, it could be a possible viable alternative to using either raw river or recycled municipal water.

Land application of wastewater can increase levels of soluble and exchangeable forms of K more rapidly than with conventional inorganic fertilisers,⁶ and most of the K in wastewater is available immediately. Irrigation with K-rich wastewater could be beneficial to overall soil fertility, although long-term application could affect soil chemical and physical properties.^2,7 A further advantage of using winery wastewater as a source of K over the use of conventional fertiliser is that it could be an efficient recycling practice where the soil has low K. In addition to sodium (Na) and K ions, winery wastewater can also contain calcium (Ca) and magnesium (Mg) ions.² Neither of these ions is harmful to soil structure and can ameliorate the impacts of Na via their role in reducing the sodium adsorption ratio (SAR).

Both soil K and SAR increased throughout the soil profile, where winery wastewater was used for irrigation.⁸ The latter practice also resulted in higher Na and K in vineyard soils compared to a control vineyard which was irrigated with river water.⁹ In a field study where grapevines were irrigated with simulated winery wastewater, soil Na levels in the 0 – 20 cm and 20 – 40 cm layers increased.¹⁰ The addition of wine to the simulated winery wastewater enhanced K movement to the sub-soil. In a field study where grapevines were irrigated with diluted winery wastewater, the element concentrations in an alluvial, sandy soil did not respond to the irrigation with the exception of K and Na.¹¹ This was probably due to the low levels of the other elements applied via the irrigation with diluted winery wastewater in relation to the K and Na. Leaching of cations, particularly K and Na occurred from only four of six different soils in a pot experiment where the soils were irrigated with winery wastewater and winter rainfall was simulated.¹² The simulated rainfall was too low for sandy and clay soil to allow leaching. Furthermore, more cations were leached from the sandy soils compared to the two heavier soils.

Taking the above-mentioned into consideration, the aim of this study was to determine the effect of in-field fractional use (augmentation) of winery wastewater with raw water for vineyard irrigation on the chemical status of different soils to assess the fitness for use of winery wastewater for irrigation of different soil types with varying rainfall.

Methods

Details of the plot selection, augmentation, climatic conditions, irrigation application, water quality and nutrient load were given previously.¹³ Briefly, there was a loamy sand (C1) and sandy clay loam (C2) experimental plot in the Coastal region, a sandy loam (BR1) and sandy clay loam (BR2) experimental plot in the Breede River region, as well as a deep sand (LOR1) and shallow sand (LOR2) experimental plot in the Lower Olifants River region. Experimental plots were irrigated using the in-field fractional use (augmentation) of winery wastewater with raw water for vineyard irrigation.

Baseline soil samples were collected at the six plots between July and August 2017 before the in-field fractional use (augmentation) of winery wastewater with raw water for vineyard irrigation commenced. Each of the vineyards had an experimental plot that was irrigated with winery wastewater, and this was compared to the rest of the surrounding block, which acted as the control at the end of the project in September 2021. Samples were collected at three positions along the grapevine row. Samples for each depth were pooled together to create a composite sample. They were collected over 30 cm increments to a depth of at least 60 cm in all experimental and control plots and up to 300 cm at the LOR2 deep sand plot using a modified soil auger (Figure 1). A commercial laboratory analysed the samples for chemical parameters according to methods described previously.¹¹ The extractable sodium percentage (ESP´) and extractable potassium percentage (EPP´) of the soils were calculated.

FIGURE 1. A modified soil auger was used to collect soil samples to a 3 m depth at the LOR1 deep sand experimental plot.

Results

Soil pH_(KCl) of the C1 and C2 experimental plots was lower in September 2021 compared to the baseline values (Figure 2) and was still within the norm of 5.0 – 7.5 recommended for optimal grapevine growth.¹⁴ In September 2021, the soil pH_(KCl) of the BR1 experimental plot was lower than the baseline values for the 30 – 60 cm and 60 – 90 cm soil layers (Figure 2). The soil pH_(KCl) for the BR2 experimental plot was generally lower in September 2021 compared to the baseline values. In contrast, the soil pH_(KCl) of the LOR1 experimental plot tended to be higher in September 2021 compared to the baseline values (Figure 2). It was previously reported that after two seasons of being irrigated according to the in-field fractional use (augmentation) of winery wastewater with raw water, grapevines at the LOR2 experimental plot were performing poorly. Therefore, the wastewater irrigation had to be terminated at the beginning of the 2019/20 season to prevent any further damage. The soil pH_(KCl) of the LOR2 experimental plot when the winery wastewater irrigation was terminated was similar to the baseline values.

With the exception of the LOR2 plot, in September 2021, the soil pH_(KCl) of experimental plots was higher compared to their respective controls (Figure 2) and was still within the norm of 5.0 to 7.5 recommended for optimal grapevine growth.¹⁴ In a field study at Rawsonville, winery wastewater was diluted to 3 000 mg/L COD, and soil pH_(KCl) increased at bud break after winter rainfall.¹¹ In contrast, other studies showed that irrigation with winery wastewater did not have a pronounced effect on soil pH_(KCl).^12,15 Since irrigation using winery wastewater generally increases soil K and Na, soil pH will consequently increase via alkaline hydrolyses. This reaction is primarily caused by the hydrolysis of exchangeable cations in soils, e.g. K_ex and Na_ex, or salts, e.g. CaCO₃, MgCO₃ and Na₂CO₃.¹⁶ Exchangeable Ca and Mg are more tightly adsorbed to the exchange complex than K and Na.¹⁶ Therefore, K and Na are more readily hydrolysed and produce a higher pH than exchangeable Ca or Mg. In the present study, excessive soil K after wastewater application in conjunction with winter rainfall could have induced alkaline hydrolysis, thereby increasing the soil pH_(KCl) of the experimental plots in September 2021 compared to the respective controls.

FIGURE 2. Soil pH_(KCl) measured before the in-field fractional use (augmentation) of winery wastewater with raw water for vineyard irrigation commenced, as well as soil pH_(KCl) of the experimental plots and controls at the end of the trial in September 2021.

At the end of the trial in September 2021, the electrical conductivity of the saturated extract (EC_e) of the C1 experimental plot was similar to that of the control, whereas, for the C2 experimental plot, soil EC_e was slightly higher compared to its control (data not shown). This indicated that under the prevailing conditions, rainfall must have leached some of the salts applied via irrigation with augmented wastewater salts from this region’s soil. However, this does not rule out the possibility that winter rainfall could have leached salts beyond the measured depth. Another study reported that large volumes of irrigation plus rainfall must have leached some of the salts applied via the winery wastewater irrigation beyond 90 cm depth, particularly in the last two winters of that particular study.¹⁵ In September 2021, soil EC_e of the BR1 experimental plot was lower than that of its control whereas the soil EC_e was substantially higher for the BR2 experimental plot compared to its respective control (data not shown). This trend suggested an accumulation of salts during the grapevine growing season partly due to irrigation with augmented winery wastewater containing salts.³ Furthermore, less effective leaching in heavier soils is more likely to result in salt accumulation.

Soil K of the C1 experimental plot in September 2021 was higher compared to the baseline levels for all the soil layers (Figure 3). This indicated that under the prevailing conditions, irrigation with the in-field fractional use (augmentation) of winery wastewater with raw water led to an accumulation of soil K even in the loamy sandy soil in the higher rainfall region. Soil K of the C2 experimental plot was substantially higher than baseline levels up to a depth of 90 cm (Figure 3). The accumulation of the K was substantially higher in the sandy loam soil compared to the loamy sand one. In heavier soils, less effective leaching is more likely to result in salt accumulation. In September 2021, soil K in all soil layers of the BR1 and BR2 experimental plots was higher than the baseline values (Figure 3). Furthermore, K accumulation was substantially higher in the sandy clay loam soil compared to the sandy loam one. Soil K levels of the LOR1 experimental plot in September 2021 were substantially higher than the baseline values up to a depth of 150 cm (Figure 3). The greater accumulation of soil K in the LOR1 experimental plot was a result of higher amounts of K applied via the irrigation water in conjunction with lower winter rainfall.¹³ The soil K of the LOR2 experimental plot when the winery wastewater irrigation was terminated was substantially higher compared to the baseline values (Figure 3).

At the end of the trial in September 2021, soil K was substantially higher for all of the experimental plots, except for LOR2, compared to their controls (Figure 3). This trend indicated an accumulation of K during the grapevine growing season partly due to irrigation with augmented winery wastewater which contains salts.³ In heavier soils, less effective leaching is more likely to result in salt accumulation. In another study, winter rainfall could not leach basic cations, particularly K and Na, from two of six soils in a pot study as the amount of the simulated rainfall was too low.¹⁷ Furthermore, more cations are leached from sandy soil than clayey soils. These trends indicated that the leaching would be a function of soil texture, as could be expected, as well as rainfall. The simulation with low rainfall events showed that the basic cations are more likely to accumulate in soils if climate change results in lower winter rainfall in these regions. It was also previously reported in a study representing the worst-case scenario, i.e. large amounts of wastewater disposed of on a small surface, particularly during harvest and in winter, that land application of winery wastewater resulted in the accumulation of high levels of soil K.¹⁸ In a field study where the re-use of winery wastewater for irrigation was investigated with micro-sprinkler irrigated Cabernet Sauvignon/99 Richter in the Breede River Valley region of South Africa, soil K also increased.^11,19 Where winery wastewater was used for irrigation for over 30 years, an accumulation of K was reported.²⁰ Likewise, soil surface K increased where winery wastewater was irrigated on two soils typical of the South Eastern Australia Riverine plains for three years.²¹ However, there were no changes in sub-soil K due to the slow mobility of K in the soils, which contained 50 – 60% clay. Soil K levels were also higher in vineyards which were irrigated with winery wastewater compared to control vineyard soils.⁹ The addition of wine to simulated wastewater used for vineyard irrigation enhanced K movement to the sub-soil.²⁰ Although the fate of K in soils and grapevines irrigated with winery wastewater has received limited attention,³ it is almost certain that high soil K could lead to an increase in K uptake by grapevines. This could have negative consequences on grapevine responses, such as musts with high pH, malate concentrations and poor colour.^22,23,24 However, the effect of soil K on K concentrations in must is often negligible unless excessive amounts are applied.²²

FIGURE 3. Soil K measured before the in-field fractional use (augmentation) of winery wastewater with raw water for vineyard irrigation commenced, as well as soil K of the experimental plots and controls at the end of the trial in September 2021.

Soil EPP´ at the C1 experimental plot in September 2021 was higher than baseline levels for all the soil layers (Figure 4). It should be noted that in the Western Cape fruit industry, the recommended ratio of exchangeable K is 3 – 4% of the cation exchange capacity.²⁵ Soil EPP´ at the C2 experimental plot in September 2021 was higher than the baseline levels up to a depth of 90 cm (Figure 4). The greater accumulation of K in the soils of the Coastal region was a result of higher amounts of K applied via the irrigation water in the 2020/21 season.¹³ Soil EPP´ in all the soil layers of the BR1 and BR2 experimental plots was higher than the baseline values (Figure 4). This trend suggested an accumulation of salts during the grapevine growing season due to irrigation with winery wastewater, which contains salts.³ In heavier soils, less effective leaching is more likely to result in salt accumulation. Soil EPP´ of the LOR1 experimental plot in September 2021 was substantially higher up to a depth of 150 cm compared to baseline values (Figure 4). The soil EPP´ of the LOR2 experimental plot when the winery wastewater irrigation was terminated was substantially higher compared to the baseline values (Figure 4).

At the end of the trial in September 2021, soil EPP´ was substantially higher for all of the experimental plots compared to their controls (Figure 4). This was expected given that the in-field fractional use (augmentation) of winery wastewater with raw water for vineyard irrigation consistently increased soil K of the experimental plots compared to their respective controls (Figure 3).

In September 2021, soil Ca and Mg were higher for the C2 and BR2 experimental plots compared to their respective controls (data not shown). In contrast, soil Na of all the experimental plots was similar or lower compared to their respective controls (data not shown), indicating sufficient leaching of Na at all the experimental plots, regardless of soil texture. Consequently, the ESP´ of the soils from the experimental plots and their respective controls were similar (data not shown).

FIGURE 4. Soil EPP´ measured before the in-field fractional use (augmentation) of winery wastewater with raw water for vineyard irrigation commenced, as well as soil EPP´ of the experimental plots and controls at the end of the trial in September 2021.

Conclusions

Under the prevailing conditions, soil pH_(KCl) increased consistently in response to the in-field fractional use (augmentation) of winery wastewater with raw water for vineyard irrigation. As expected, soil K was substantially higher for the experimental plots compared to their respective controls regardless of mean annual rainfall. Consequently, soil EPP´ was higher where the in-field fractional use (augmentation) of winery wastewater with raw water was used for vineyard irrigation. Results indicated that the soil K accumulation and consequent increase in soil EPP´ was related to soil texture, amounts of K applied via the irrigation water and the mean annual rainfall. The K increases could have a negative impact on wine colour stability should K be taken up by the grapevine in sufficient quantities. In contrast, soil Na of all the experimental plots was similar or even lower compared to their respective controls. This indicated sufficient Na leaching at all the experimental plots, regardless of soil texture. However, where more Na is applied via the irrigation water, Na could accumulate to levels where it could negatively impact soil physical conditions or grapevine growth and yield.

Grapevine and wine responses will be presented in the next article.

Acknowledgements

• This report is an output of WRC Project K5/2561, entitled “Use of winery wastewater as a resource for irrigation of vineyards in different environments”. This solicited project was initiated, funded and managed by the WRC. The project was co-funded by Winetech and ARC.
• ARC for infrastructure and resources.
• Staff of the Soil and Water Science division at ARC Infruitec-Nietvoorbij for their assistance, and in particular Mr. F. Baron for his dedicated technical support.
• Backsberg, Madeba, Lutzville Winery and Spruitdrift Winery for permitting the project team to work at their wineries and in their vineyards. Colleagues at the wineries for their assistance and support.
• Mr W. Smit from Netafim for advice and designing the irrigation systems.

References

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

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