The recent start of kernel fill marks the beginning of increased demand of pistachio trees for several key inputs. One, of course, is water.

Average water use (ETc) by pistachio trees in the first 15 days of June is 4.0 inches. This rises to 4.6 inches for the rest of the month, reports Bob Beede, University of California-Cooperative Extension Farm Advisor, emeritus. However, this rate of use more than doubles to 9.8 inches in July. That’s equivalent to 55 gallons per tree per day in an orchard with 150 trees per acre. In August, ETc drops to 8.2 inches or 50 gallons daily for each tree.

“Keep an eye on the temperatures and adjust your schedule accordingly,” he says. “Be sure to meet full ETc by the beginning of nut filling. Do not consider regulated deficit irrigation if you are on shallow soil and are already struggling to adequately irrigate during kernel filling.”

Beede points out that, because of application inefficiency (basin or furrow systems are about 70 percent to 80 percent efficient and low-volume systems are 85 percent to 90 percent efficient), you need to apply more water than the trees actually require.

Nutritional demands of pistachio trees also increase during kernel fill. For example, UC researchers report that 1,000 pounds of dry, inshell pistachios require 28 pounds of actual nitrogen.

“Kernel filling begins in late June and is the most demanding sink for N,” Beede writes in his June Pistachio Tasklist newsletter. “For that reason, I suggest that 75 percent of your total nitrogen management program be applied from late June to early August, when demand is greatest. Your nitrogen management program should include tissue, soil and water analysis to quantify all sources of N and insure that excessive nitrates are not accumulating in the soil from over-fertilization.”

Pistachio trees also require plenty of potassium during kernel fill. When UC researchers applied up to 200 pounds actual K per acre in equal splits over the months of May through August, they significantly increased yield, split nut percentages and nut weights while reducing blank and stained nuts. What’s more, the reduction in staining was associated with less Alternaria leaf infections at harvest.

The research indicates that August tissue levels for K should be about 1.7 percent for optimum plant performance, Beede notes. The high fixation capacity of some soils requires large K applications to saturate the soil exchange sites and increase K tissue levels. That’s why he recommends banding K when using surface irrigation. “This saturates the exchange complex of the clay and provides more K in soil solution for uptake,” Beede explains.

Three continuous years of potassium chloride application did not elevate chloride in the leaf tissue. However, he advises considering orchard health, soil permeability, salinity, stratification and deficit irrigation before performing large-scale KCL applications.

UC researchers calculate the annual K requirement at 25 pounds per 1000 pounds inshell ACP weight, Beede adds.