Potassium utilization in California wine grape production is a matter of balance, and it can take several years for a grower to determine that equilibrium.

However, one of the first rules in feeding potassium to grapevines is to apply it in doses as needed, according to Lowell Zelinski, co-owner of Precision Ag, a viticulture consulting company in Paso Robles, Calif.

“If you apply 50 pounds of K per acre in the spring, I doubt you will get 10 pounds taken up by the plant,” he said. Because potassium demand is highest when berries (grapes) are developing, he recommends applying 25 percent of the season’s potassium requirements just before bloom.

The remainder should be metered in four or five applications through the growing season, based on potassium readings in petiole samples.

Zelinski has a doctorate in soil-plant-water relations from the University of California, Davis and nearly 30 years experience in agricultural consulting. He is a former UC Cooperative Extension farm advisor in Fresno County, Calif., where he was involved in groundbreaking work in the benefits of potassium for cotton.

Potassium utilization in grapevines is directly related to yields – the higher the yields, the more potassium is needed, according to Zelinski.

Most of Zelinski’s clients apply K through drip irrigation systems. For dryland coastal vineyards, Zelinski recommends applying the potassium at the base of the vine.

“It is very important to monitor potassium in the plant with petiole samples. Use the first bloom petiole sample as the baseline for the season and make applications as dictated by petiole samples taken on the weeks when the potassium is not applied,” he recommended.

The amount of potassium necessary for maximum yields of high quality wine grapes is related to varietal and rootstock. Zelinski recommends starting with a baseline bloom time petiole reading of 2.5 percent.

“It will take a few years to determine the ideal K levels for a vineyard because varietals and rootstock react differently to potassium levels. If you see 2.5 percent at the beginning is not enough, you could raise the potassium rate to achieve 3.5 percent by applying more K,” he said.
“It is very important to pay attention to the petiole samples to not only maximize yields but to achieve high quality wine,” Zelinski says.

Often times, potassium deficiency will negatively impact wine quality before the vines show any K-deficiency symptoms, said Zelinski.

“Talk to your winemaker about brix, pH, and TA titratable acidity and see if there is a potassium issue with wine quality…either too much or not enough. Too much K can impact quality just as a deficiency can. It is all about balancing K with your particular variety and rootstock,” he said.

Zelinski recommends KTS (potassium thiosulfate) (0-0-25, 17 percent sulfur and 25 percent potash). It is a neutral to basic, chlorine-free, clear liquid solution. Each gallon of KTS contains three pounds of potash.

“It has a reasonably high analysis of potash,” Zelinski added.

Although some may recommend foliar potassium, Zelinski does not believe it works as well as soil applied because you cannot get enough on the plant to make a big difference.

Yields in the Paso Robles area average 2.5 to 6 tons of wine grapes per acre. “In the (San Joaquin) valley where growers get 10 to 12 tons per acre, the potassium rate needs to be increased,” Zelinski says.

Consultants find that as the vines age, they need more potassium.