Potassium (K) is required by plants in large amounts. It has a major role in many plant processes, such as promoting root growth, increasing fruit size, and providing key features in metabolism that include the formation of starch, translocation of sugars, stomata regulation, and the formation of xylem vessels. The K concentration in grapevines can range from 1 percent to 4 percent on a dry weight basis, depending on the tissues and time of sampling.

Harvest removes about 5 pounds K/ton of grapes, although this varies, based upon the rootstock and cultivar being grown. Varieties with high K demand, such as Cabernet Sauvignon, Merlot, Cinsaut, or Syrah, should not be grafted to rootstocks prone to K deficiency if soil levels are low. UC research has shown that vines on rootstocks with Vitis berlandieri genetic background, such as 420A, 110R, 5BB, 5C, and 1103P are sensitive to K deficiency.

Freedom, 1616C, SO4, and 039-16 are examples of rootstocks that provide high K to the scion vines.

Annual soil analyses are of relatively little value in determining vine K needs since there are so many other factors that affect uptake and utilization, including soil type, texture and depth; amount of soil compaction; root pest damage; variety; rootstock; irrigation practice; and crop size. Petiole analysis has been the main tool for assessing K status and the need for K applications to vines.

Petioles are usually collected at bloom from leaves opposite the cluster position on the shoot. Vines are generally sufficient at 1.5 percent to 2 percent and deficiency may occur at 1.0 percent or less. Though it is not a completely reliable tool for making K management decisions, petiole analysis is the most consistent guideline currently available.