Deficiency symptoms include slow growth. Leaves can become pale, and leaf size and shoot growth are reduced. Leaf tips sometimes curl upward. He said that by the time you see deficiency symptoms, trees are already deficient and yield is lost.

Duncan said that while “the word on the street says K (potassium) values should be 2 percent or higher,” research has shown “increasing potassium levels above 1.4 percent may not lead to higher yields.”

He said data suggests that 1.4 percent levels of potassium in leaves sampled in July is “pretty close to the correct economic threshold.”

“I don’t argue with any grower who wants 2 to 3 percent K,” he added. “It’s their money.”

Duncan said it is important that any drip hoses should be over the applied band of potassium fertilizer, not distant from it. He adds that “banding,” applying the fertilizer in a line down the row of trees within the rooting zone, is not efficient in drip orchards.

“It’s better to inject,” he said.

Doll said potassium can be applied in granular form or though the irrigation system. Ground applications are often made during dormancy or in early spring. Depending on the soil type, he said the amount could vary from 50 percent to 100 percent of the potassium budget for the year.

Fertigated materials can be applied in season or during dormancy. Those products are thought to have a faster rate of tree uptake due to being suspended in soil water.

But Doll said those products are often more costly than ground applied fertilizers. Examples include potassium thiosulfate, potassium nitrate, potassium chloride, potassium carbonate, or high grade potassium sulfate.

For sandy soils, Doll said it’s best to apply a reduced later application during dormancy. The remainder should be applied from April to August.

With this program, he said 50 percent of the potassium budget could be applied as potassium sulfate in late January or February with the remainder applied in-season using potassium thiosulfate.

With heavier soils, Doll said it’s best to apply a larger percentage of the potassium budget during dormancy. Those soils might also require the application of gypsum or other calcium products to help saturate the soil and move potassium into the soil water.

Doll said potassium is pulled into the tree’s root zone at a constant rate throughout the season.

Other researchers have pointed to potassium fertilization as also being important for productivity of pistachio trees as well.