By Chuck Ingels, Paul Verdegaal, Stuart Pettygrove, and Ria DeBiase: UC Cooperative Extension

Potassium in vines

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 to 2.0 percent, and deficiency may occur at 1 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.

Potassium deficiency

Grapevines tend to show K deficiency when they are heavily cropped and maintenance applications of K have not been made in the vineyard. Deficiency can be more likely to occur under these conditions:

• Soil cut areas

• Areas where the K-rich surface soil was removed during land leveling

• On sandy soils that have low native K fertility

• On clay soils of certain geologic origin K deficiency

• Shallow soil areas

• Poorly drained soils

• Where soil pests have caused root problems

• Water stress can also increase this deficiency by reducing vine uptake of K; keep this in mind when using deficit irrigation on red grapes.

Deficiency symptoms can appear in early spring in cool, wet years, into June, but mild deficiencies will not be seen until just before harvest. The first symptom is a fading of green color at the leaf edges and between the main veins, while leaf margins tend to curl upward. The leaves may turn chlorotic and begin to turn brown on the margins, and some leaves may die as the deficiency becomes more severe.

Severe K deficiency also reduces vine vigor and crop yield, and can result in defoliation. Oftentimes petioles can remain attached as blades defoliate. Vines also tend to have fewer and smaller clusters that are tight, with unevenly colored, small berries.