It is well known that grapes containing excess potassium can cause problems for wine making. This is especially true in red wines – grape skins are sinks for potassium, and red wines are fermented with their skins.
Basically, excessive potassium (K+) in the grape juice or must negatively impacts wine quality by increasing the pH of the must past suitable levels. This occurs because excess K causes precipitation of tartaric acid and other free acids. As that happens, the pH goes up. Ramifications can include reduced respiration in wine, color instability, microbial instability (spoilage), and premature oxidation.
Several research studies have found that juice pH is highly correlated to K+ levels. However, even if the grapes are harvested at recommended pH levels, excessive K+ content in the fruit can cause the grape must pH to rise dramatically in the first few days of fermentation (Sources).
Grape growers can do a number of things to keep grape potassium levels in check.
Moderate shading with good canopy management: Past studies found that potassium is higher in more shaded clusters. The reason for this is not well understood. Prevent excess shading by balancing the canopy with pruning, shoot thinning, leaf thinning, and avoiding excess N fertilization.
Follow soil and foliar nutrient test results: Take soil tests every 3-5 years, and foliar tissue tests every year during bloom or veraison. Stop applying K if the soil test indicates “high” or “excessive.” Soil K concentration over 101 ppm is considered high based on research in Minnesota.
Harvest based on recommended pH levels: K increases linearly with berry pH. Unless you are saving the grapes for ice wine, harvest within suggested pH ranges. — By Annie Klodd, University of Minnesota Extension Educator