Potassium (K) levels are critical in wine, table and raisin grapes produced in California and the rest of the world. Adequate K enhances disease resistance in vines and is a factor in fruit size, flavor, texture and berry development.

Potassium applications are dependent on whether K is available or tied up in the soil, which is often the case in clay soils. It is difficult to build available soil potassium levels in high clay soils. Potassium often binds with clay, making it unavailable for plant uptake.

“On the east side of California’s San Joaquin Valley, there’s a lot of sedimentary rock that has a very slow (K) release rate,” says Vern Crawford, PCA or pest control adviser with Wilbur-Ellis, Shafter, Calif. “In areas where you have a montmorilinite soil for example, you’ll find a greater need to supplement potassium.”

“On the Central Coast, it varies,” says Mark Battany, San Luis Obispo County University of California Cooperative Extension Farm Advisor. “There are some soils that are high in available potassium, so we don’t have the need to apply it very much, if at all. And then we have other soils that are quite deficient in potassium.”

The need for potassium also varies depending on production and environmental factors.

“We’ve found that as vines age, and we pull more and more crops from them, the vines need more potassium,” says Corky Roche, PCA, Roche Vineyard Consulting in Salinas, Calif. “Also, if we’re pushing yields, we need to have more potassium so the grapes can ripen properly.”

Sulfate of potash or SOP is one of the preferred formulations of potassium-containing fertilizers simply because it does not add undesirable salts to the soil.

“One thing we try to avoid here on the Central Coast because of salt concerns is the chloride-based fertilizers,” Battany says. “We seldom use potassium chloride because the chloride can accumulate in the soil and lead to toxicity problems over time.”

Water is another critical factor in potassium uptake. “The ability of the plant to take up potassium from the soil is going to be dependent upon the plant actually being able to extract that potassium from the soil water,” Battany says.

“There has to be sufficient water in the soil for the potassium to be available to the root. The potassium has to be dissolved as an ion in the soil water. Very dry soil conditions can lead to induced deficiency of potassium as well as other nutrients. By adding more water, a grower can increase the availability of potassium, but that is very site dependent.”

Sulfate of potash can be easily applied through a drip system or broadcast directly to the soil.

“It’s less expensive for growers and they can apply it through a drip system without clogging up the system,” says Roche says. “It’s very effective. We’re able to get good results with it.”

Monitoring potassium levels is best accomplished by tissue analysis rather than soil testing due to the fact that what shows up in the soil may not necessarily be indicative of what the plant is extracting.

“We pull petiole samples throughout the season,” Roche says. “We want to see levels above 1.5 percent K.”

Roche also prefers to spread out applications through the season.

“For example, we can inject up to a quarter of a pound per vine per season,” he says. “We’ll divide that up and put it in every irrigation. If you had 500 pounds of potassium sulfate per acre going out, I would divide that, say, into 20 irrigations. I prefer to put it out in frequent irrigations so the plant is getting a steady dose of what it needs to remain productive and functioning at an optimal level.”