By Kathryn Carter (OMAFRA), Dr. Tejendra Chapagain, (OMAFRA) and Christoph Kessel (formerly OMAFRA)
Potassium (K) plays an important role in vineyards, with K deficiency resulting in negative impacts on vigour, yields and fruit ripening. High levels of K in fruit at harvest has a negative impact in wine quality resulting in increased pH and a shorter shelf life of wines (Gardner, 2016). Hence, monitoring K levels in the vines is very important to ensuring healthy vines that produce good quality fruit.
Traditionally in North America grape petiole tissue is most commonly used to evaluate nutrient levels in the vine due to ease of sampling and large amounts of historical data. In Europe, vineyards rely on the use of leaf blades which are considered to provide a more accurate understanding of nutrient (N, P, K) concentrations in the vine. Other grape growing regions (e.g. Pennsylvania and Washington) also have developed nutrient recommendations for sampling grapevines at bloom. The benefit of sampling grapevines at bloom allows growers to amend their nutrient applications in the current season, resulting in more timely information for making fertilizer decisions.
Currently Ontario guidelines recommend soil sampling in vineyards (0 to 15 cm depth) every three years to monitor soil K. In addition, petiole analysis at veraison (onset of ripening) is used to evaluate K levels in the vine. However, Heck et al. (2014, Unpublished) suggested that there may be opportunities to improve the accuracy of nutrient sampling in vineyards as there wasn’t a strong relationship between soil K levels (0 to 15 cm) and petiole analysis at veraison. They found that petiole K had a greater correlation with soil K at greater soil depths (15 to 30 cm)indicating the opportunity to improve sampling guidelines for K in vineyards.
To test and validate these relationships, OMAFRA conducted a three-year project (2019-2021) looking at refining the use of soil testing and petiole analysis as tools for making K management decisions. The objectives of this project were to answer the following questions:
- Can soil sampling at a greater depth (15 to 30 cm or 0 to 30 cm) provide a better understanding of K levels in the vine than the current soil testing guidelines (0 to 15 cm)?
- Can sampling leaf blades provide a better understanding of K levels in the vine as opposed to sampling petioles?
- What is the impact of K levels in the vine (Petiole and Leaf Blade) on bud hardiness?
- Can petiole sampling at bloom be a viable alternative to monitoring K levels in vineyards?
- Does K level in the vine (petioles and leaf blades) impact yields?
- Do K levels in the soil or vine impact grape juice quality?
Unlike Heck et al. (2014, Unpublished), this study found that there was a significant relationship between soil test (0 to 15 cm) with petiole K and leaf blade K at veraison in medium soils. With petiole K at veraison more significantly correlated with soil K from 0 to15 cm depth compared to leaf blade K, these results support OMAFRA’s current guideline for soil sampling at 0 to15 cm along with petiole analysis at veraison to determine nutrient levels for vineyards.
This study further demonstrated that leaf blade K at veraison were more correlated with soil K in deeper soils (15 to 30 cm depth). For the 0 to 30 cm soil samples, only the leaf blade K were correlated to soil K indicating that leaf blade analysis may be an effective tool for monitoring K levels in mature vineyards with deeper roots.
The data we collected suggests that the relationship between soil K (at all depths) and petiole K at bloom were poor indicating that bloom petiole sampling is not a good indicator in predicting K levels in the vine. In comparison, tissue sampling at veraison (petiole and leaf blade) was significantly correlated to soil K levels, indicating that this is the best timing for monitoring K levels in the vine.
This study found a positive association between petiole K at bloom and yields, indicating that petiole K at bloom may give an idea about yields, however petiole K was not associated with soil K. There was no relationship between yields and soil K or tissue K (petioles and leaf blades) at veraison.
Juice K level was significantly and positively associated with petiole K at veraison indicating that petiole samples at veraison may be used to get a better understanding of the K levels in the fruit at harvest. We did not observe any association between juice K with petiole K at bloom.
Also, we found that juice K and pH was significantly (and positively) associated (i.e., as juice K levels increased, there was a corresponding increase in pH of the juice).
Although bud hardiness was monitored at all blocks throughout the trial, we had insufficient data to determine if K levels in petioles at bloom or veraison impacted bud hardiness, as only one site had low K levels throughout the entire project.
Take home message
The data we collected provides a better understanding of optimal timing for tissue sampling in vineyards, suggesting that growers should continue to sample petioles for K at veraison instead of bloom.
The results of this trial do not support Heck et al. (2014, Unpublished) that increasing soil sampling depth to 15 to 30 cm results in a better relationship with K levels in petioles. Instead, soil sampling at 0 to 15 cm should provide adequate results in medium soils. However, the results suggest that leaf blade sampling at veraison provides a better understanding of how K levels in the vine relate to soil K in deeper soils (15 to 30 and 0 to 30 cm).
In sites or years where wineries are concerned about K levels in grape juice, there may be some benefits to monitoring K in petioles at veraison as they may indicate a relationship with juice K.
Acknowledgements: We are grateful to OGWRI for funding this project. Special thanks to the grower co-operators including Malivoire, 30 Bench, Glen Elgin Vineyards and Falk vineyards. Thanks to Christoph Kessel, SGS labs, Brewster Consulting, CCOVI labs for their assistance.
Gardner, 2016. Making [red] wine from fruit high in potassium | Penn State Extension Wine & Grapes U. (wordpress.com)
Heck, R., C. Kessel, K. Carter. (2014, Unpublished) Potassium fixation and availability in the dominant vineyard soils of the Niagara Region to improve soil potassium fertilizer application guidelines.
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