In wine production, it is important to know the best climate and soil type combination for each cultivar in order to get the best out of each terroir.
While the role of climate as one of the important terroir attributes on wine style and quality is generally understood, that of soil type carries much uncertainty.
The study investigated the effects of soil types on the performance, wine style and quality of Merlot through:
- characterisation of selected climate and soil parameters,
- assessing grapevine responses and
- conducting sensorial evaluation of wines.
Study layout and measurements
A Merlot/Ruggeri 140 vineyard in a commercial wine estate at Elgin was studied for five years (2005 – 2010). The vineyard was established in 2000, at a 487 m altitude, on a 20% slope, with SSW aspect, 1.3 x 3.0 m planting density and under drip irrigation (supplemental). The vineyard had two experimental plots representing two different soil forms (Bainsvlei and Hutton). An automatic weather station was installed at the site and some of the climatic data were obtained from three weather stations positioned in Elgin. Soil and root profile studies were performed at the commencement of the study. Soil water contents and grapevine water stress levels were monitored, while selected grapevine parameters were quantified. Grapevine nutritional status was assessed in leaf petioles and blades, as well as juice. The target sugar levels for harvesting were between 23.0°B to 24°B. The must were analysed chemically, thereafter experimental wines were prepared in duplicate by using a standard production technique. An experienced panel of judges performed sensorial evaluations on the wines. Data from the five seasons were used as replicates and analysed statistically in order to determine differences due to soil types (p ≤ 0.05).
Findings and discussion
Seasonal weather conditions
2005/06: Annual rainfall was 97 mm above the long term mean (LTM), while spring and summer rainfall were 33 mm and 25 mm, respectively below their respective long term means; summer maximum temperature was 1°C higher than the LTM (data not shown) (2005/06 – dry spring, dry and warm summer). Researchers discussed climatic conditions for 2006/07 to 2009/10,1 therefore only a summary is supplied:
- 2006/07: Wet and slightly warm summer.
- 2007/08: Slightly wet and warm summer.
- 2008/09: Wet and cool spring, slightly dry and warm summer.
- 2009/10: Wet and cool spring, dry and warm summer.
Site climatic conditions
The site climatic data was the same as that discussed by researchers for Sauvignon blanc as both vineyards were in the same estate.1 In general, cool and windy conditions prevailed at the site, and with the Winkler index of 1 308 degree days, it could be placed in Region I, and regarded to have a potential to produce high quality red wines.
Besides the difference in soil forms (Table 1), the Hutton generally had lower soil water content values and was well-drained compared to the Bainsvlei (data not shown). The pH values were low for both soils, and the Bainsvlei was characterised as acidic, but the Hutton as very acidic (Table 1). Furthermore, the Bainsvlei had higher K levels that were due to more fertilisation than the Hutton.
The root system in the Hutton was considered better developed and possibly more efficient (superior) compared to that in the Bainsvlei, due to a higher fine root density (304 vs. 136 fine root density/m2) and a fine:medium root density ratio that was above 3.5. Therefore, despite being very acidic, good drainage conditions for the Hutton might have created an environment that was sufficient for adequate root development.
Grapevine water stress, vigour, yield, bunch and berry parameters were not affected significantly by soil type, but must pH levels of vines on the Bainsvlei were significantly higher (pH = 3.68) than those of vines on the Hutton (pH = 3.52). This was attributed to the generally higher juice K levels, because of the soil K levels. Furthermore, some noteworthy differences that might have affected grape composition and subsequently wine quality were observed during some seasons. Grapevines on the Hutton generally experienced higher water constraints than those on the Bainsvlei during 2007/08 (data not shown). This was attributed to the Hutton having generally lower soil water content levels than the Bainsvlei, especially during the warm summer period of 2008. In contrast, vines on the Bainsvlei experienced higher water constraints during 2009/10 (data not shown). As already discussed, the Bainsvlei had a less developed root system than the Hutton, and this may have influenced grapevine water absorption during the dry and warm summer period of 2010.
Additionally, the must level content from vines on the Hutton that were harvested in 2009 were above the desired values for winemaking purposes, and 1.2°B higher than that of must from vines on the Bainsvlei. This was expected to affect wine quality negatively.
Wine style and overall quality
There were no significant effects of soil type on wine attributes when data from all vintages were combined (Table 2). However, it is noteworthy that soil type was found to have significant effects on certain wine attributes in some vintages. The Hutton produced better wines than the Bainsvlei, pertaining to berry aroma (2006 vintage), colour (2008 vintage) and vegetative aroma (2010 vintage) (Table 2). In contrast, the Bainsvlei produced better wines than the Hutton for the 2009 vintage. The stronger berry and vegetative aromas were attributed to grapevines on the Hutton experiencing higher and lower water constraints during 2005/06 and 2009/10, respectively compared to those on the Bainsvlei. The weaker colour for the Bainsvlei wines (2008 vintage) compared to the Hutton wines was attributed to a high K uptake by grapevines as reflected on juice K and must pH levels during a season characterised by a slightly wet and warm summer period. Also, grapevines might have received supplemental irrigation, resulting in increased diffusion of K into the roots in the Bainsvlei soil already containing higher K levels. Researchers found that K fertilisation and supplemental irrigation increased grapevine K status.2 The superiority of the 2009 Bainsvlei vintage over the Hutton one, might had been largely due to the lower and desired sugar level of its must, which had already been discussed.
The effects of soil types on grapevine performance, wine style and quality were assessed in a Merlot vineyard at a high altitude (487 m), with two soil types (Bainsvlei and Hutton) over five seasons (2005 – 2010). The study site was in Elgin (Overberg district, Western Cape) and cooler than most of South Africa’s traditional wine growing areas. The Hutton soil was more acidic and better drained, with lower K levels and water contents than the Bainsvlei soil. The root system in the Hutton was regarded as better developed than that in the Bainsvlei. The must pH from vines on the Bainsvlei was significantly higher than that from those on the Hutton, which was attributed to juice and soil K levels. Wine attributes, namely berry character, colour and vegetative aroma were affected significantly by soil type, but only during the 2006, 2008 and 2010 vintages, respectively. These differences were mainly attributed to soil nutrition, moisture and drainage conditions, possibly influencing grapevine water status and subsequently grape composition under specific weather conditions. Therefore, the indication is that soil type affects wine style and quality via grape composition, but this is largely dependent on prevailing seasonal weather conditions.
Funding for this project was provided by the Agricultural Research Council (ARC) and Winetech. The authors wish to thank Dr Kobus Conradie for starting the study and guidance, the staff of Soil and Water Science Division of the ARC Infruitec-Nietvoorbij for technical assistance, D. Rowswell and I.O. Joubert from the ARC Institute of Soil, Climate and Water for climate data.
Augustine, P., Conradie, K. & Olivier, P., 2017. Soil type effects on Sauvignon blanc wine quality at a high altitude in Elgin. WineLand, April 2018.
Sipiora, M.J., Anderson, M.M. & Matthews, M.A., 2005. A role of irrigation in managing vine potassium status on a clay soil. Soil environment and vine mineral nutrition. American Society for Enology and Viticulture. pp. 1 – 9.
– For more information, contact Reckson Mulidzi at email@example.com.