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94 WINES&VINES November 2016 GRAPEGROWING PRACTICAL WINERY & VINEYARD P otassium (K) is the major cation in ripe grapes, accounting for about 75% of the total mineral cations. Grape juice pH is strongly correlated with grape juice K and largely determines the pH of the wine after fermentation, with high wine pH negatively impacting wine color, stability and taste. 18 Adjustment of pH with tartaric acid dur- ing vinification is routinely applied to protect against such impacts. 5 The aim is generally to bring pH to below 3.0 for white wines and below 3.5 for red wines. 8 Rootstocks can lead to differences in K concentrations in grape berries and grape juice, 15 which can carry over into wine. Balanced K accumulation by grapevines is important to avoid deficiency (see Malbec and Chardonnay photos above) or excessive uptake. The interrelationships between K and pH in grape juice and wine are discussed below. Potassium accumulation by grapevines In greenhouse studies using potted grape- vines, total K uptake (mg K per grapevine) by ungrafted rootstocks Freedom, Schwarz- mann, 1103 Paulsen, 110 Richter, 140 Rug- geri and 101-14, 9 and by Shiraz grafted to the same rootstocks, 10 was found to be cor- related with total plant dry matter and total root length and surface area at the end of a two-month period. Among the ungrafted vines, rootstock 1103 Paulsen had the highest total K uptake, and rootstock 110 Richter had the lowest total K uptake. 9 For grafted vines, Shiraz on 110 Rich- ter and 140 Ruggeri had higher total K uptake than Shiraz on Ramsey and 1103 Paulsen, indicating a scion-rootstock interactive effect on total K uptake. 10 Potassium translocation efficiency (ratio of K content in shoot to K content in shoot plus roots) of 101-14 was higher than that of 140 Ruggeri for both ungrafted and grafted vines. 9,10 Unfortunately, there are no field stud- Potassium Accumulation by Grapevines Uncovering the relationship between potassium and pH in grape juice and wine By Rob Walker and Peter Clingeleffer A Chardonnay leaf shows potassium deficiency symptoms during the set phase of fruit development on wet clay soils. LEFT: STAN GRANT/PROGRESSIVE VITICULTURE RIGHT: HECTOR BEDOLLA The faded chlorosis typical to potassium deficiency early in Malbec fruit development.