California’s 4,000 walnut farmers produce 99 percent of the U.S. crop and rely heavily on added nutrients to develop excellent canopy size and high-quality nuts.

“Many Sacramento Valley walnut groves can be deficient in potassium so potassium fertilizer is needed to provide the proper nutrient levels,” said Joe Connell, walnut farm advisor for the University of California Cooperative Extension, Butte County, Calif.

Connell says the critical level for adequate potassium is at least 1.2 percent in walnut leaf tissue. Foliar symptoms will show when trees are deficient and leaf potassium levels drop to 0.9 percent or below.

“The 1.5 percent range is a good level to maintain if you can manage it.”

“Most growers try to apply what’s needed to maintain the critical value level,” Connell said. “It became very expensive to fertilize with nitrogen and potassium last year so some growers cut back on the amount applied. However, it’s important to provide enough fertilizer to meet essential nutritional levels.”

John Post, president of Agricultural Advisors, Inc., a tree crop consulting firm in Yuba City, Calif., advocates potassium levels in the 1.5 to 2 percent range. While the Chandler variety is the most popular walnut grown in California, Post in early August witnessed signs of potassium deficiency in the Howard variety.

“The Howard variety either doesn’t uptake potassium very well or requires more,” Post said. “I manage a Chandler block next to a Howard block and both receive the same potassium management. The Howard is demonstrating potassium-deficiency symptoms including yellow bronzing of the leaves.”

Potassium fertilizers offer good solutions to deficient soils.

Post prefers potassium sulfate (sulfate of potash or SOP) over potassium chloride since SOP contains virtually no chloride.

Connell added, “Potassium sulfate offers increased safety compared to potassium chloride. The grower doesn’t need to worry about leaching out the negative chloride ion that comes with potassium chloride.”

Potassium products are normally applied in large doses as soil-applied band applications along the tree row in early winter so the winter rainfall can help provide incorporation, Connell says.

About 500 pounds of potassium sulfate per acre can be applied in band applications placed annually in the same location or a mass 2,000 pound/acre application can be applied in bands once every four years.

Nitrogen applied during the spring and summer is crucial to maintain vigor and growth in walnut trees, Connell says. The summer dose is stored by the tree to help develop new tree growth the next season.

Post says walnut trees hold nitrogen better than other deep-rooted tree crops. Nitrogen levels at 3 percent in the leaf can be maintained for several years without an additional nitrogen application.

Walnuts in the Sacramento Valley are grown on fertile, well-drained soils. Some walnut acreage expansion is moving into more marginal soils, according to Post.