Managing wine pH in a changing climate

Abstract

The role of potassium in berry acidity was investigated in six cultivars in the Riverina and Orange wine regions of NSW. It was found that a high magnesium to potassium ration ([Mg]/[K]) in the soil and in the grapevine was associated with lower berry pH and higher titratable acidity. The monitoring of the [Mg]/[K] ration should become part of an integrated soil management strategy that is focused not only on productivity, but also on berry composition. A questionnaire and interviews highlighted drivers and barriers to the implementation of sustainable grapegrowing and formed the basis of an adoption strategy specific to address berry composition.

Summary

Warmer climates are associated with higher evaporative demand, accelerating plant transpiration and the associated uptake of nutrients through the vascular streams. Potassium (K+) is the dominant cation of the grape berry. This nutrient regulates berry sugar accumulation and has a strong influence on wine microbiological stability and fermentation processes. Past research has demonstrated that high berry K+ concentration [K], is associated with high total soluble solids (TSS), however links with juice pH and titratable acidity (TA) requires substantiation for Australian warm grape growing regions.

Soil, leaves and berries were sampled at several phenological stages in a survey of vineyards in the Riverina and Orange wine regions of NSW to better understand the role of nutrition on berry composition. Juice pH was positively correlated with juice [K], while juice TA was negatively correlated with juice [K] in several cultivars. Grapevine [Mg] and [Ca] status, assessed using petiole or laminae samples, had an influence on vine and juice [K] and on juice pH and TA. Higher concentrations of the potassium cation antagonists, Mg2+ and Ca2+, in the juice were associated with lower pH and higher TA. The vine [Mg]/[K] ratio was particularly important to grape berry composition suggesting that targeted vine nutrition management offers an avenue to ameliorate climate driven losses in wine acidity.

Subsequently, potted-vine and field-vine trials were implemented to test the effectiveness of the potassium cation antagonists at manipulating berry composition. In the second season of the potted-vine study, high cation concentrations in the petioles were associated with high levels of the corresponding cation in the juice. Notably, juice [K] was inversely correlated with petiole [Mg]. Furthermore, the petiole and blade [Mg]/[K] ratios were inversely correlated to juice pH. Juice [K] was positively correlated with pH, but there was no association with TA. Despite this, the fertiliser treatments of the potted and field vines did not alter berry pH or TA. Only minor changes in vine nutrient levels, unseasonably wet conditions and the characteristics of the soils may all have contributed to the absence of treatment effects. Overall, these findings indicate that the application of cation antagonist fertilisers could be utilised to alter juice characteristics, but this would require further refinement and investigation. It is expected that several seasons of nutrient application would be required to alter vine nutrient levels to the extent that they will have an impact on berry composition.

Based on the extensive regional survey results and bearing in mind the need for further nutrition trials, we have proposed several sustainable vineyard practices to modify berry composition. These are grounded on reducing excessive K application and increasing Mg application if deficient or marginal. Excessive vine [K] can be addressed by ensuring there is no over-application of K, adoption of K excluding rootstocks, crop manipulation to slow berry ripening and ripening-related K accumulation, reduction in bunch exposure, and curtailing excess irrigation and nutrient uptake through soil and plant water status monitoring. To address marginal Mg status or deficiency, Mg fertilisers should be applied, and non-acidifying fertilisers used to avoid soil acidification. The pH should be adjusted in acidic soils with soil alkalinity-enhancing fertilisers to increase Mg availability, although this treatment would be most effective prior to planting.

The drivers and barriers to adoption of sustainable vineyard practices were investigated through an online questionnaire and interviews. The main drivers for implementing these practices were long- term viability and ‘doing the right thing’. The foremost impediments were cost, confidence in region- specific solutions, and complexity. The CSIRO ADOPT tool was used to assess how quickly the adoption of practices that lead to improved berry acidity will spread and what percentage of adoption we could expect. The targeted populations are those growers that are rewarded for berry quality or those that process their own fruit into wine. The modelling predicts that the peak adoption rate will be 46% and that this will occur in seven years. The monitoring of the [Mg]/[K] ratio can become part of an integrated soil management strategy that is focused not only on growth or yield, but also on berry composition.

This work was the result of collaboration between NSW Department of Primary Industries, the Gulbali Institute of Charles Sturt University, and CSIRO with strong support from the NSW Wine Industry.