On-line version ISSN 2224-7904
Print version ISSN 0253-939X
S. Afr. J. Enol. Vitic. vol.36 n.3 Stellenbosch 2015
A.R. MulidziI, *; C.E. ClarkeII; P.A. MyburghI
ISoil and Water Science Division, ARC Infruitec-Nietvoorbij, Private Bag X5026, Stellenbosch 7599, South Africa
IIDepartment of Soil Science, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa
Environmental legislation requires the South African wine industry to find solutions for winery wastewater treatment or reuse. The feasibility of irrigation with diluted winery wastewater was assessed in a pot experiment under a rain shelter over four simulated irrigation seasons. Four soils varying in clay content were irrigated with winery wastewater diluted to 3 000 mg/L chemical oxygen demand (COD), whereas the control received municipal water. The rate of K+ increase in the soil containing 20% clay was higher than in soils containing 13% clay or less. This suggests that heavy soils will aggravate the risk of high K+ levels. The risk of Na+ accumulation increased linearly with clay content. Low Ca2+ and Mg2+ concentrations in the diluted wastewater had no effect on the soil, irrespective of clay content. Irrigation with diluted winery wastewater increased soil pH substantially in all the soils over the four simulated seasons. The soil pH increase was attributed to the addition of organic/bicarbonate salts to the soil. It must be noted that the results represent a worst case scenario, i.e. in the absence of rainfall or crops.
Key words: Calcium, chemical oxygen demand, extractable percentage, magnesium, potassium, sodium
Increased wine production in South Africa is putting more pressure on natural resources such as vegetation, water and soil. Changes in environmental legislation (Department of Water Affairs, 2013) put pressure on the wine industry to find solutions for the treatment or judicious use of winery wastewater (Van Schoor, 2001a). This initiated the development of guidelines for the management of wastewater and solid waste at wineries (Van Schoor, 2005). In many cases, a shortage of good-quality water leads to an increasing need to irrigate crops with poor-quality water, such as saline groundwater, drainage water and treated wastewater (Jalali et al., 2008). The impact of using untreated industrial and municipal wastewater for irrigation is well documented (Bond, 1998; Papini, 2000; Mulidzi, 2001; Arienzo et al., 2009a; Christen et al., 2010; Laurenson & Houlbrooke, 2011; Mosse et al., 2011; Arienzo et al., 2012; Laurenson et al., 2012; Howell & Myburgh, 2014a; Walker & Lin, 2008).
The disposal of winery wastewater through land application has been practised for many years (Mulidzi, 2001; Laurenson & Houlbrooke, 2011). Effective disposal of wastewater depends on the irrigation technology, as well as on soil properties (Oron et al., 1999). In an earlier study, Mulidzi (2001) confirmed that the impacts of using undiluted winery wastewater for irrigation differ substantially between soil types. Under some circumstances, irrigation with winery wastewater can have a beneficial effect. Papini (2000) observed improvements to the soil in terms of pH increases, water retention and the restoration and maintenance of soil micro-flora after the application of distillery wastewater. It was also suggested that using K+-rich wastewater could enhance soil fertility (Mosse et al., 2011). On the other hand, this practice can also have a negative impact on soils. Irrigation with winery wastewater has been shown to result in the leaching of nitrate into the groundwater, increased soil sodicity and/or long-term detrimental effects on the soil physical and chemical properties of arable land (Bond, 1998). In Australia, continued irrigation of pastures with winery wastewater resulted in an accumulation of K+ to levels that leached into the groundwater and other water resources (Christen et al., 2010). In addition, it was observed that using winery wastewater for the irrigation of poorly drained soils could lead to salinisation and water- logging, reducing the long-term sustainability of the land for agriculture (Christen et al., 2010).
The replacement of bivalent Ca2+ and Mg2+ by monovalent K+ and Na+ during continuous irrigation potentially can lead to the breakdown of the soil structure. Exchangeable Na+ in soils tends to increase where wastewaters containing high levels of Na+ are used for irrigation (Lieffering & McLay, 1996). Where wineries use Na+-based cleaning agents, e.g. sodium hydroxide (NaOH), the accumulation of monovalent cations, such as Na+, on the exchange sites has the potential to degrade the soil structure through clay dispersion and flocculation (Mosse et al., 2011). Consequently, soil hydraulic conductivity can be reduced where winery wastewater is used for irrigation (Laurenson et al., 2012). Indications of poor aeration and water infiltration observed in various soils where winery wastewater was used for irrigation were attributed to structural degradation caused by high Na+ concentrations added to the soil (Mulidzi et al., 2009). This was confirmed when irrigation with diluted winery wastewater reduced the hydraulic conductivity of differently textured soils (Howell & Myburgh, 2014b). Using winery wastewater with very high K+ levels for irrigation may also result in K+ accumulation in the soil, resulting in the leaching of the flocculating cations Ca2+ and Mg2+, thus increasing the instability of the soil structure in the long run (Mosse et al., 2011). Since K+ has an affinity for clay minerals, high soil K+ can cause clay swelling and dispersion where wastewater is used for irrigation (Arienzo et al., 2012). Similar to Na+, K+ in winery wastewater can reduce soil hydraulic conductivity (Arienzo et al., 2009b). However, knowledge regarding the negative effects of K+ on soil structure stability is limited compared to that of Na+.
Soil pH tends to increase when wastewater with high pH and Na+ concentrations is used for irrigation (Lieffer-ing & McLay, 1996). A study carried out in the Western Cape showed that the disposal of grape-processing effluents changed the soil pH from acidic to alkaline (Papini, 2000). This pH increase was attributed to the initial removal of soluble organic matter through the volatilisation of CO2 during biodegradation.
In contrast, the application of wine vinasse containing high bicarbonate slightly reduced the pH of a Mediterranean soil (Bueno et al., 2009). The pH reduction was attributed to the high electrical conductivity of the soil solution (ECe), viz. 9.2 dS/m, and the transformation of organic sugars by microorganisms. These contrasting results of various studies imply that soil reactions to the application of winery wastewater cannot easily be predicted. The soils of the South African winelands are highly heterogeneous and can show a high degree of spatial variation in a relatively small area. Soils range in parent material, texture, structure, drainage, coarse fragment content and chemistry. Parent material is usually largely responsible for the physical and chemical makeup of a soil (Van Schoor, 2001b). In the Stellenbosch region, two of the dominant parent materials are shale and granite, while in the Breede River and Olifants River wine-growing regions, transported aeolian or fluvial sands are important parent materials (Bargmann, 2003). Due to the heterogeneity of the winelands soils, they are likely to respond differently to the application of winery wastewater; however, there has been little work done to determine these responses.
The objective of this study was to determine the effects of irrigation with diluted winery wastewater on selected chemical properties of four soils varying in parent material and clay content. This paper reports on base cations and pH.
MATERIALS AND METHODS
Four differently textured soils from three grape-growing regions in the Western Cape were included in the study, viz. (i) aeolian sand (Garies 1000) from Lutzville, containing 0.4% clay, (ii) alluvial sand (Longlands 1000) from Rawsonville, containing 3.3% clay, (iii) granite-derived soil from Stellen-bosch (Cartref 1100), containing 13% clay, and (iv) shale-derived soil from Stellenbosch (Oakleaf 2210), containing 20% clay, with taxonomic soil classification at the family level according to the South African soil classification system (Soil Classification Working Group, 1991). A composite soil sample was collected from the topsoil layers of the four soils, i.e. approximately 0 to 30 cm deep. The soil samples were analysed to determine particle size distribution, water-holding capacities and initial soil chemical status.
Soils were packed into 3.54 dm3 PVC pots to a bulk density of 1 400 kg/m3 as described by Mulidzi et al. (2015). Control treatments were irrigated with water supplied by the Stel-lenbosch municipality. The winery wastewater treatments were irrigated with wastewater diluted to a chemical oxygen demand (COD) of 3 000 mg/L. The undiluted wastewater was obtained from the wastewater collection pit at a winery near Rawsonville. Treatments were applied over four simulated irrigation seasons, consisting of six irrigations each. Thus, 24 irrigations were applied over the four simulated seasons. The total irrigation amounted to 1 156 mm, 1 126 mm, 987 mm and 728 mm for the Rawsonville sand, Lutz-ville sand, Stellenbosch shale and Stellenbosch granite soils respectively. The pot experiment was carried out under a 20 m x 40 m translucent fibreglass rain shelter at ARC Infruitec-Nietvoorbij near Stellenbosch. Details of the pot experiment, wastewater dilution and the irrigation system are described in Mulidzi et al. (2015).
Water sampling and analyses
Water samples were collected prior to each irrigation. The COD in the water was measured using a portable spectro-photometer (Aqualitic COD-reactor®, Dortmund) and the appropriate test kits (COD, CSB, 0 to 15 000 mg/L). The pH and electrical conductivity (EC) were determined by a commercial laboratory (Bemlab, Strand) according to methods described by Clesceri et al. (1998). The water was analysed for Ca2+, Mg2+, K+ and Na+ by a commercial laboratory (BEMLAB, Strand) by means of atomic emission using an optical emission spectrometer (Varian ICP-OES). Total alkalinity was determined through titration with 0.05 N hydrochloric acid. The sodium adsorption ratio (SAR) of the water was calculated as follows (units in meq.L-1):
Soil sampling and analyses
To make provision for destructive soil sampling, each experimental "plot" consisted of four pots. Following each simulated irrigation season, the soil in one of the pots was collected for sampling, i.e. after six, 12, 18 and 24 irrigations. Soil samples were collected from the 0 to 10 cm and 10 to 20 cm layers in the pots of all replications. Soil samples were air dried and passed through a 2 mm mesh sieve. All analyses were carried out by a commercial laboratory (Bemlab, Strand). The pH(KCl) was determined in a 1 M potassium chloride (KCl) suspension. The Ca2+, Mg2+, K+ and Na+ were extracted with 1 M ammonium acetate at pH 7. The cation concentrations in the extracts were determined by inductively coupled plasma optical emission spectrom-etry (ICP-OES), using a spectrometer (PerkinElmer Optima 7300 DV, Waltham, Massachusetts). For this study, the cations will be referred to as extractable calcium (Ca2+extr), magnesium (Mg2+extr), potassium (K+extr) and sodium (Na+extr). The extractable potassium percentage (EPP') was calculated as follows:
where K+extr is the extractable potassium (cmol(+)/kg) and S is the sum of basic cations (cmol(+)/kg).The extractable sodium percentage (ESP') was calculated in the same way to obtain an indication of the sodicity status.
Each soil/water treatment was replicated three times in a completely randomised design. The four soils were randomly allocated within each block. The treatment design was a split-plot, with soil type as the main plot factor and soil depth as the sub-plot factor. Analyses of variance were performed separately for each season using SAS version 9.2 (SAS, 2008). The Shapiro-Wilk test was performed to test for non-normality (Shapiro & Wilk, 1965). Student's "t" least significant difference (LSD) was calculated at the 5% significance level to facilitate comparison between treatment means (Ott, 1998). Linear regressions were calculated using STATGRAPHICS® version XV (StatPoint Technologies, Warrenton, Virginia, USA).
Soils selected for this study were chosen because they represent dominant soils of the Western Cape wine-producing region. Furthermore, it was expected that the impacts of winery wastewater on soils would differ widely between differently textured soils. The Rawsonville soil was formed from the alluvium of the Breede River. The soils in this region are relatively young and often are stratified. The soils selected for this study showed no clear stratification and contained a mottled subsoil, thereby being classified as a Longlands soil form (orthic A-E horizon - soft plinthic B horizon). The topsoil texture of the soil was fine sand.
The soil was slightly acidic, with a pH(KCl) of 5.7. The geology of the Lutzville region is dominated by metamorphic rocks of the Nama Group in the north and sedimentary rocks of the Cape Super Group in the southern and south-western parts (Department of Water Affairs, 2011). However, the soils in this area are mainly derived from Aeolian-deposited sand (Saayman & Conradie, 1982). The soil was classified as the Garies form (orthic A - Red apedal B horizon - with dor-bank as the underlying material). The topsoil texture was fine sand and the soil was neutral, with a pH(KCI) of 6.7. The Stellenbosch shale soil was located on the foothills of Simons-berg Mountain. The lower subsoil was derived in situ from shale, but the upper subsoil and A horizon were derived from colluvial material of shale origin. The soil was classified as a red Oakleaf soil form (orthic A - red neocutanic B horizon - unspecified material). The topsoil texture was a fine sandy clay loam and the soil was acidic, with a pH(KCl) of 4.2. The Stellenbosch granite soil was also located on the foothills of Simonsberg Mountain. The subsoil was derived in situ from granite, although the A and E horizons were derived from granitic colluvium. The soil was classified as a Cartref form (orthic A and E horizon - lithocutanic B horizon). Both the A and E horizons were highly leached and hard setting. The topsoil texture was coarse sandy loam. The soil was acidic, with a pH(KCl) of 4.4.
RESULTS AND DISCUSSION
Chemical composition of the water and amount of elements applied
The mean COD levels in the municipal water and diluted winery wastewater were 28 ±4 and 3 210 ± 43 mg/L respectively during the four simulated seasons. The COD in the diluted winery wastewater was reasonably close to the target level of 3 000 mg/L. As expected, most of the other winery wastewater quality variables were considerably higher compared to the municipal water (Table 1). On most irrigation days, the winery wastewater pH was lower compared to the municipal water. The average SAR of the winery wastewater was close to 5 (Table 1), which is the upper permissible limit for irrigation with wastewater according to South African water quality legislation (Department of Water Affairs, 2013). Due to the differences in the chemical composition of the municipal and diluted winery wastewater, considerably more cations were applied to the soil via the wastewater compared to the municipal water (Table 2). Total irrigation amounts applied to the Rawsonville sand (1 156 mm), Lutzville sand (1 126 mm) and Stellenbosch shale (987 mm) over four simulated seasons were comparable, but the Stellenbosch granite (728 mm) received substantially less water. According to Mulidzi et al. (2015), this particular soil had a lower water-holding capacity and high coarse sand content compared to the other three soils.
Extractable potassium and EPP'
Where municipal water was applied, K+extr amounted to 0.21 cmol(+)/kg, 0.42 cmol(+)/kg, 0.35 cmol(+)/kg and 0.31 cmol(+)/kg for the Rawsonville sand, Lutzville sand, Stellenbosch shale and Stellenbosch granite respectively after the four seasons (data not shown). Since these values were comparable to the baseline values (Table 3), this indicated that irrigation with municipal water had no effect on the K+extr, irrespective of clay content. In contrast, irrigation with the diluted winery wastewater increased K+extr substantially over the four seasons. The K+extr in the 0 to 10 cm soil layer was slightly higher that that in the 10 to 20 cm layer, irrespective of clay content (Fig. 1). According to Arienzo et al. (2009b), a higher amount of exchangeable K+ is retained by soils higher in clay content than soils low in clay content following winery wastewater irrigation. This may have resulted in the similar trend being observed in the four soils. Furthermore, K+extr in the four soils increased linearly with the cumulative amount of K+ applied via the irrigation water (Fig. 1).
In the 0 to 10 cm layers, the degree of K+ extraction was similar for the four soils, with an increase of 0.0002 cmol(+)/ kg per kg K+ applied. After the four seasons, EPP' amounted to 4.6%, 11.5%, 13% and 9.5% for the Rawsonville sand, Lutzville sand, Stellenbosch shale and Stellenbosch granite soils respectively where municipal water was applied (data not shown). Similar to K+extr, the EPP' values were comparable to the baseline values (Table 3), indicating that the municipal water irrigation did not affect EPP'. In contrast, irrigation with the diluted winery wastewater increased EPP' over the four seasons (Fig. 2). The EPP' in the 0 to 10 cm soil layer was slightly higher compared to that in the 10 to 20 cm layer, with the exception of Stellenbosch granite soil. In the case of the sandy soils and Stellenbosch shale soil, the EPP' in the 0 to 10 cm layer showed a slower increase following the second season (Figs 2A, 2B & 2C). The EPP' in the 10 to 20 cm layer showed an almost linear increase with applied K+. These trends did not occur in the case of Stellenbosch granite, as EPP' was comparable in both soil layers (Fig. 2D). After the fourth season, EPP' was similar in both layers, which suggests that the granite soil was no longer retaining high amounts of K+ in the 0 to 10 cm layer.
For healthy grapevine growth in soils with a pH below 6, it is recommended that a K+ saturation of 4% is required at the exchange sites (Conradie, 1994). Prior to irrigation, the EPP' was greater than 4% in all the soils, except for the Rawsonville sand, which had an EPP' of 3.7%, which was close to the threshold (Table 3).
Thus, for the soils investigated, K+ added via the wastewater did not represent a benefit in terms of nutrient balance and supply. In fact, high K+extr levels may cause excessive absorption by grapevines, which could result in high wine pH and eventually reduce the colour stability of red wines where winery wastewater was applied (Mpelasoka et al., 2003; Kodur, 2011).
Under normal cropping conditions there is a possibility that K+ applied via wastewater can be beneficial if it can maintain optimum levels when K+ is absorbed by the grapevines and/or inter-row crops, or if K+ is leached by rainfall in winter. It should be noted that the observed K+ accumulation occurred in the absence of rainfall or crops. Determining the effect of leaching by winter rainfall where diluted winery wastewater is used for irrigation is part of an ongoing study.
Extractable sodium and ESP'
Where municipal water was applied, Na+extr amounted to 0.15 cmol(+)/kg, 0.17 cmol(+)/kg, 0.16 cmol(+)/kg and 0.25 cmol(+)/kg respectively for the Rawsonville sand, Lutzville sand, Stellenbosch shale and Stellenbosch granite soils after the four seasons (data not shown). Being comparable to the baseline values (Table 3), this indicates that municipal water irrigation had almost no effect on the Na+extr, irrespective of clay content. On the other hand, irrigation with the diluted winery wastewater increased Na+ substantially over the four seasons. In all the soils, the degree of Na+extr accumulation in the 0 to 10 cm layer was higher than in the 10 to 20 cm layer (Fig. 3). The difference between the layers was most prominent in the shale, followed by the granite and sandy soils (Figs 3C & 3D). These trends indicate that more Na+ was extracted in the 0 to 10 cm layer of the heavier soils than the sandy soils. The increased extraction of Na+ from the top layer may be as a result of less sorption of Na+ to the soil and the evaporative concentration of Na+ in the evaporating soil solution. In fact, previous studies have shown that the adsorption of Na+ was reduced by the presence of high K+ levels where winery wastewater was applied (Laurenson et al., 2012 and references therein).
In all soils the Na+extr increased linearly with the cumulative amount of Na+ applied via the irrigation water (Fig. 3). However, the rate of increase in Na+extr with increase in applied Na+ (Na+extr/Na+appl) differed between the soils. The Na+extr/Na+applincreased with clay content in the 0 to 10 cm layer, but no correlation was observed in the 10 to 20 cm layer (Fig. 4). Where municipal water was applied, the ESP' amounted to 3.2%, 4.4%, 2.9% and 4.3% in the Rawsonville sand, Lutzville sand, Stellenbosch shale and Stellenbosch granite soils respectively after four seasons. The ESP' values were comparable with the baseline values, with the exception of the Stellenbosch granite soil, which had a higher baseline ESP' (Table 3). Where winery wastewater was applied over four seasons, the ESP' did not show a definite linear increase with the amount of Na+ applied in any of the layers (Fig. 5).
In the case of the Rawsonville sand, the ESP' exceeded the critical threshold of 15% for sustainable agricultural use from the second season onwards in the 0 to 10 cm layer (Fig. 5A). Wastewater irrigation increased the ESP' above 15% from the first season in the Lutzville sand, but also only in the 0 to 10 cm layer (Fig. 5B). From the first season, the ESP' exceeded 15% only in the 0 to 10 cm layer of the Stellenbosch shale soil (Fig. 5C). Although no infiltration problems occurred after four seasons, this does not rule out the possibility that sodicity could have negative effects on soil structure in the long run. In the case of the Stellenbosch granite soil, the ESP' exceeded 15% after the third season, but also only in the 0 to 10 cm layer (Fig. 5D). Although the ESP' in the two sandy soils seemed to have reached a plateau at c. 20%, it might induce negative effects on grapevine growth and yield if the ESP' remains near the threshold over time. Given the higher ESP' in the heavier soils, sodicity will have negative effects on plant growth and soil physical conditions if these soils are irrigated with winery wastewater, even when diluted. The Stellenbosch shale soil showed no visual signs of infiltration problems, but water infiltration into the Stellenbosch granite soil was considerably slower where wastewater was applied rather than municipal water. It should be noted that the infiltration problems occurred right from the first season, i.e. when the ESP' in the top layer was around 15% (Mulidzi et al., 2015). It is well documented that Ca2+ and Mg2+ can counter the negative effects of Na+ on water infiltration, but the Ca2+extr and Mg2+extr in the Stellenbosch shale and granite soils were comparable (Table 3). It was previously reported that the saturated conductivity of a topsoil of a similar granitic soil at Nietvoorbij was 112 mm/h (Myburgh, 2015). Since the drip application rate was 115 mm/h (Mulidzi et al., 2015), it could be that the infiltration rate of the granitic soil was exceeded, thereby causing the slow water infiltration. Another possible reason for the slow infiltration rate in the granitic soil is the dispersive nature of the bleached topsoil. Bleached topsoils are pale in colour due to the loss of Fe2+ from the horizon. Iron oxides play an important role in stabilising clays against dispersion (Tombacz et al., 2004). The lack of Fe2+ in the granitic topsoil might make this soil more susceptible to clay dispersion and surface sealing when irrigated with wastewater containing high levels of Na+ and K+. The red Oakleaf soils in the Stellenbosch region have a high Fe2+ content (Le Roux, 2015). This may explain why infiltration in these soils was unhindered, despite the poor quality of the irrigation water.
Calcium and magnesium
After the four simulated irrigation seasons, Ca2+extr and Mg2+extr did not show any trends that could be related to the amounts of these elements applied via the municipal water and diluted winery wastewater respectively (Table 4). The lack of response could be expected in view of the small amounts of Ca2+ and Mg2+ applied through the irrigation water (Table 2). In fact, irrigation with the wastewater reduced the Ca2+extr in the Rawsonville sand after the four seasons. The Mg2+extr in the Lutzville sand showed a similar trend (Table 4). Where wastewater was applied to the Stellenbosch granite soil, Mg2+extr also was lower compared to Mg2+extr in the 0 to 10 cm layer of the municipal water irrigation. The foregoing implies that irrigation with winery wastewater is unlikely to have any benefits in terms of Ca2+ and Mg2+ supply to plants. Furthermore, if applied in such small amounts, these elements will not be able to counter possible structural problems caused by high levels of Na+ applied via winery wastewater.
The pH(KCl) of the soils prior to any treatment is given in Table 3. The Stellenbosch soils had a low pH(KCl) (4.2 to 4.4), while the Rawsonville and Lutzville sands had substantially higher values (5.7 and 7.6 respectively). Where municipal water was applied, soil pH(KCl) was 5.9, 7.6, 4.5 and 4.6 for the Rawsonville sand, Lutzville sand, Stellenbosch shale and Stellenbosch granite soils respectively after the four seasons (data not shown). In contrast, irrigation with diluted winery wastewater increased pH(KCl) substantially in all the soils over the four seasons (Fig. 6). In all the soils, pH(KCl) in the 0 to 10 cm soil layers tended to be very slightly higher than that in the 10 to 20 cm layer.
This means that, despite the wastewater having a fairly low pH (4.9 to 6.0), it actually increased the soil pH. The Lutzville, Rawsonville and Stellenbosch shale soils showed a pH increase of approximately 2 pH units, while the granite soil, which received less irrigation water, only showed a pH increase of 1 unit. Although this may seem counterintuitive, it is not an unusual phenomenon and has been recorded in numerous studies where organic substrates are added to a soil (Yan et al., 1996; Li et al., 2008; Rukshana et al., 2011; 2012). When salts of organic acids are added to a soil, decarboxylation and hydrolysis of the organic/bicarbonate anions increases the pH (Li et al., 2008). The winery wastewater used in this study had an extremely high total alkalinity (Table 1). It is likely that this alkalinity comprised a number of deprotonated organic acids as well as bicarbonate ions. The charge on these anions is largely countered by K+ and Na+ cations, thus when applied to soils this results in a pH increase due to decarboxylation and anion hydrolysis reactions, as described by Li et al. (2008). These authors found that Na+ and K+ organic salts are more effective at increasing soil pH than Ca2+ and Mg2+ organic salts. This would explain why the soil pH(KCl) increased linearly with the cumulative amount of K+ plus Na+ applied via the diluted winery wastewater (Fig. 6). Similar increases in pH were reported by Laurenson et al. (2012) when high alkalinity winery wastewater was applied to vineyard soils.
Initially, pH(KCl) in the Rawsonville and Lutzville sands (Table 3) was higher than the lower threshold of 5.5 for vineyard soils (Conradie, 1994). However, where these soils were irrigated with diluted winery wastewater, the high pH(KCl) levels (Fig. 6) could have detrimental effects on the availability of plant nutrients (Busman et al., 2002). Where the pH(KCl) initially was lower than 5.5 in the Stellenbosch shale and granite soils, irrigation with the diluted winery wastewater had a beneficial effect by raising the pH(KCl) to the optimum range after the first season (Fig. 6C and D). In sandy soils, where the pH is not well buffered, vineyard soils may become acidic under intensive irrigation, particularly drip irrigation (Myburgh, 2012). Such soils, e.g. the sandy vineyard soils in the Olifants River region, require frequent liming. Therefore, irrigation with diluted winery wastewater containing high levels of K+ may reduce the rate of acidification in these poorly buffered sandy soils.
Irrigation with winery wastewater containing relatively high levels of K+ and Na+ affected the soil more than irrigation with municipal water, which served as the control. Since the K+extr increase with increasing amounts of K+ applied was comparable for the four soils, it is suggested that clay content did not play a significant role. The EPP' was above the critical level of 4% in all the soils before the experiment commenced. This means that, under the prevailing conditions, there is a high risk of K+ accumulating to levels that could have negative effects on wine colour if the excess K+ is not leached out in the winter or absorbed by inter-row crops in the summer. In the heavier soils the increase of Na+extr with increasing amounts of Na+ applied was almost double than in the sandy soils. This indicates that the risk of Na+ reaching excessive levels will be less when vineyards in sandy soils, compared to heavier soils, are irrigated with diluted winery wastewater. Although the ESP' exceeded the threshold of 15% only in the 0 to 10 cm layer, Na+ accumulation in the deeper layers could increase ESP' to excessive levels in the long run. Due to low Ca2+ and Mg2+ concentrations in the diluted winery wastewater, their extractable concentrations in the soil were comparable to the initial levels after four seasons.
This indicates that these elements are not contained in the cleaning detergents used in wineries to the extent that they would accumulate in the soil, irrespective of clay content. The increase in soil pH(KCl), irrespective of clay content, could be attributed to organic anions added to the soil via irrigation with diluted winery wastewater. In the sandy soils, where the pH(KCl) approached 8 or even higher values, nutrient solubility and absorption could be reduced if winery wastewater was used for vineyard irrigation. It must be noted that the foregoing results represent a worst-case scenario, i.e. in the absence of rainfall or crops. Determining the effect of seasonal leaching by winter rainfall on the chemical status in soils irrigated with diluted winery wastewater is part of an ongoing study.
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Submitted for publication: August 2015
Accepted for publication: October 2015
Acknowledgements: We are grateful to the Water Research Commission, for initiating and funding the project, and to Winetech, THRIP (TP1208066038) and ARC Infruitec-Nietvoorbij, for co-funding. We thank the staff of Soil and Water Science at ARC Infruitec-Nietvoorbij, for their technical support, and in particular Mr Francois Baron, for his dedicated effort
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