Abstract
The objective of the study was to design and evaluate a pot experiment to determine the effects of irrigation with diluted winery wastewater (WWW) on different soils. Four pedogenetically different soils were included in the experiment, i.e. (i) alluvial sand containing 3.3% clay from Rawsonville, (ii) aeolic sand containing 0.4% clay from Lutzville, (iii) shale-derived soil containing 20% clay from Stellenbosch, and (iv) granite-derived soil containing 13% clay from Stellenbosch. The pot experiment was carried out under a rain shelter at ARC Infruitec-Nietvoorbij. Soils were packed in 3.54 dm3 PVC pots to a bulk density of 1 400 kg/m3. The four soils were irrigated using WWW that was diluted to 3 000 mg/L chemical oxygen demand (COD). Municipal water was used to irrigate the control treatment of each soil. It was possible to irrigate the soils accurately when approximately 85% of the water had evaporated. Since the pot experiment could be continued under the rain shelter during winter, results pertaining to soil chemical responses can be obtained quicker compared to an open field study.
Introduction
Increased wine production worldwide over the last two decades has compelled wine-producing countries to find sustainable wastewater management practices that are in compliance with environmental legislation.1 The negative effects of irrigation with winery wastewater (WWW) on soils are well documented.2 To comply with intensified environmental legislation,3 the wine industry must find solutions for treatment or re-use of WWW. Since negative impacts on soils might be less if the WWW is diluted before being re-used for irrigation, such a practice could be more sustainable compared to undiluted WWW.2 However, knowledge regarding effects of diluted winery wastewater on different soils in South African grape-growing regions is limited. Determining effects of irrigation with WWW on soils and crops in field experiments requires an elaborate infrastructure, particularly if the wastewater has to be diluted to a predetermined level.4 Field experiments are usually carried out with one specific soil type. Since different soils respond differently to WWW irrigation,5 it is essential to determine the effects of diluted winery wastewater on soils that differ pedogenically.6 However, it would be expensive to erect the required infrastructure for a range of soils. A further disadvantage of field experiments is that wineries produce the bulk of their wastewater during the harvest period, i.e. from February to April. Therefore, field experiments can only be carried out annually during harvest. Based on the foregoing, pot experiments seem to be an alternative, since it could include a range of different soils. A further advantage is that WWW can be stored in tanks which will allow experiments to be continued throughout the year if the pots are sheltered from rain. This will reduce the duration of experiments compared to ones carried out in the open field. If pot experiments are carried out correctly, drainage and subsequent leaching of elements can be avoided. The latter can be problematic and difficult to quantify under field conditions.
Therefore, the objective of the study was to design and evaluate a pot experiment to determine the effects of irrigation with diluted WWW on different soils.
Materials and methods
Four pedogenetically different soils from grape-growing regions in the Western Cape Province were included in the study.2,5 A sandy, alluvial soil was collected in a vineyard near Rawsonville in the Breede River Valley. A sandy, aeolic soil was collected near Lutzville in the Lower Olifants River Valley. A shale and granite-derived soils were collected on the Nietvoorbij Experimental Farm of the Agricultural Research Council (ARC) near Stellenbosch. For the purpose of the article, the soils will be referred to as Rawsonville sand, Lutzville sand, Stellenbosch shale and Stellenbosch granite, respectively. The pot experiment was carried out under a 20 m x 40 m translucent fibreglass rain shelter at ARC Infruitec-Nietvoorbij. The control treatment soils were irrigated with water supplied by the Stellenbosch Municipality. For the wastewater treatments, WWW was diluted to a chemical oxygen demand (COD) level of 3 000 mg/L. The undiluted WWW was collected from the wastewater pit at a winery near Rawsonville.
Treatments were applied over four simulated irrigation seasons. Each season consisted of six irrigations. This was estimated as the number of irrigations a vineyard would require during the harvest period, i.e. when the highest volumes of WWW are produced. Hence, a total of 24 irrigations were applied over the four simulated irrigation seasons. Each soil/water treatment combination was replicated three times in a complete randomised block design (Figure 1). Following each simulated season, i.e. after six, 12, 18 and 24 irrigations, the soil chemical status was determined to compare the effect of irrigation with diluted WWW to that of municipal water. Since soil sampling was destructive, a replication “plot” of each soil/water treatment combination consisted of four pots. At the end of each season, one of the four pots was removed for sampling. The volume of water applied to each soil was recorded using water meters.
FIGURE 1. The (A) arrangement of the pots in the rain shelter and (B) the drip irrigation pipes used to irrigate the four different soils in the pot study.
Results
Since only topsoil was used in the study, characteristics of the deeper horizons were considered to be irrelevant. With the exception of the Stellenbosch granite soil, which had a high coarse sand fraction, fine sand dominated the sand fraction (Table 1). It must be noted that the Stellenbosch granite soil contained approximately 47% gravel. All soils compacted with relative ease to a bulk density of 1 400 kg/m3. When the soils were packed into the pots, the mean soil water content (SWC) was 14.9%, 11.7%, 12.1% and 14.5%, respectively, for the (i) Rawsonville sand, (ii) Lutzville sand, (iii) Stellenbosch shale soil and (iv) Stellenbosch granite soil. Irrigation amounts applied to the Rawsonville sand, Lutzville sand and Stellenbosch shale soil over the four simulated seasons were comparable, but the Stellenbosch granitic soil received substantially less water (Table 2). As expected, the COD in the municipal water was substantially lower compared to the diluted WWW (Table 3). The COD in the diluted WWW was comparable between the four simulated seasons and was reasonably close to the target level of 3 000 mg/L.
The SWC at field capacity of the soils were comparable, except for the Stellenbosch granite soil (Figure 2). This indicated that this particular soil had a lower water holding capacity compared to the other soils and was probably due to the high gravel content and coarse sand contents (Table 1). Initially, the SWC was restored to field capacity following irrigation in all soils. However, in the case of the Stellenbosch granite, field capacity was only restored following the first two irrigations (Figure 2D). From the third irrigation onwards, visual observation revealed that the irrigation water ponded on the soil surface due to poor water infiltration. Consequently, the target soil water depletion level was reached following irrigation, but field capacity could not be restored (Figure 2D). Although actual SWC was not measured in the pots, it can be assumed that only the upper section of the profile in the Stellenbosch granite soil was wetted.
FIGURE 2. Temporal variation in soil water content (SWC) in (A) Rawsonville sand, (B) Lutzville sand, (C) Stellenbosch shale, and (D) Stellenbosch granite soils measured in a pot experiment. Arrows indicate when soil chemical status was determined after each of the simulated seasons. “FC” and “RP” indicate field capacity and refill point, respectively.
Although the level of COD differed substantially between the municipal water and WWW (Table 3), water infiltration problems occurred where municipal, as well as WWW, were applied. The sodium adsorption ratios in the municipal and WWW were 0.8±0.1 and 4.6±0.6, respectively (unpublished data). This confirmed that poor water quality did not cause the problem. Since the soil was not saline, irrigation with low salinity water could not have caused the problem in the case of clean water treatments. When irrigated with clean river water and a range of diluted WWW, the near-saturation hydraulic conductivity of this particular soil was considerably lower compared to the other soils, irrespective of the level of water quality. Since the poor infiltration could not be related to water quality, there was no other obvious explanation for the poor water infiltration.
With the exception of the Stellenbosch granite soil, the SWC was managed between field capacity and the refill point (Figure 2). This indicated that the soils were well-aerated between irrigations. Since the lower part of the Stellenbosch granite must have remained dry, it implied that this soil was also well-aerated between irrigations. Visual observation revealed that no drainage occurred after irrigations had been applied. Therefore, it can be assumed that no leaching occurred of elements applied via the municipal water or diluted WWW. The foregoing indicated that the lysimetric approach provided an accurate measure of the irrigation volumes required. It is important to note that the pot experiment was completed in approximately two and a half years, whereas it would have taken four years to do the wastewater irrigation in a field experiment.
Conclusions
It was possible to subject more than one soil to irrigation with diluted WWW by using a single mix and irrigation infrastructure. Since the pot experiment could be continued under the rain shelter during winter, results were obtained quicker compared to an open field study. Results from the study can form a solid basis for future research on the effect of WWW application on soil.
Effects of irrigation with diluted WWW on the chemical status of the four soils in the pot study will be presented in subsequent articles.
Acknowledgements
- This article is an output of WRC Project K5/1881, entitled “The impact of wastewater irrigation by wineries on soils, crop growth and product quality”. This solicited project was initiated, funded and managed by the WRC. The project was co-funded by Winetech and ARC.
- Goudini Winery for providing wastewater for the research.
- 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.
References
- Mulidzi, A.R., Clarke, C.E. & Myburgh, P.A., 2018. Annual dynamics of winery wastewater volumes and quality and the impact of disposal on poorly drained duplex soils. S. Afr. J. Enol. Vitic.39, 305-314.
- Mulidzi, A.R., Clarke, C.E. & Myburgh, P.A., 2016. Design of a pot experiment to study the effect of irrigation with diluted winery wastewater on four differently textured soil. Water SA 2, 20-25.
- Department of Water Affairs (South Africa), 2013. Revision of general authorizations in terms of Section 38 of the National Water Act, 1998 (Act No. 36 of 1998), No. 665.Government Gazette No. 36820, 6 September 2013. Department of Water Affairs, Pretoria.
- Myburgh, P.A., Lategan E.L. & Howell, C.L., 2015. Infrastructure for irrigation of grapevines with diluted winery wastewater in a field experiment. Water SA 41, 643-647.
- Mulidzi, A. R., 2016. The effect of winery wastewater irrigation on the properties of selected soils from the South African wine region. PhD dissertation, Stellenbosch University.
- Mulidzi, A. R., 2001. Environmental Impact of winery effluent in the Western and Northern Cape Provinces. Masters dissertation, University of Pretoria.
For more information, contact Reckson Mulidzi at mulidzir@arc.agric.za.
Click here to get your copy of WineLand Magazine.
