When planning your nitrogen fertilization program for your pistachio orchards, beware of starting your applications too soon.

“Growth between bud break and leaf out is fueled by reserves stored within the tree, not by what it takes  up from soil, says Bob Beede, University of California Cooperative Extension farm advisor, Emeritus, for Kings County.

Adding high levels of N to the soil early in the season does not force the tree into greater uptake unless the tree is deficient. “Nitrogen uptake is driven by demand, not by over-feeding the tree,” he says.

The efficiency of nitrogen uptake from the soil during early leaf out is essentially zero, because nitrogen uptake occurs simultaneously with water use, he explains. Applying N before the tree has about 50 percent foliage risks loss of the nutrient by leaching beyond the main root zone at three to four feet, depending upon rainfall or irrigation amount and soil type.

“Plant N uptake depends upon root health, water management, soil temperature, crop load and overall plant demand,” he says.

In calculating how much N to feed your trees, be sure to consider nitrogen uptake efficiency. It might reach 80 percent

“With multiple applications at low rates under drip or low volume it might reach 80 percent compared to so-called slug treatments, Beede notes. However, as much as half of the N applied by water run can be lost from volatilization and leaching past the root zone.

Also, don’t forget to adjust for the amount of nitrate N in your irrigation water, since 10 ppm nitrate nitrogen provides 27 pounds of actual N per acre-foot of water applied, he adds.

Since nut fill accounts for 90 percent (about 100 pounds,) of the accumulated seasonal nitrogen, the demand for N and the tree’s capacity to take it up from the soil depends on the crop load, Beede reports. It’s greater in the on-year. University of California researchers have calculated the total N requirement for on-year trees at about 175 pounds.

“It is recommended that nitrogen (N) application through the drip system or by calibrated surface equipment begin in mid- to late-April, during early nut development,” Beede says. “A suggested seasonal application schedule for a 5,000-pound crop (200 pounds of N) is 25 pounds in April, 50 pounds in May, 50 pounds in June and 75 pounds in July.”

This amount assumes a 75-percent application efficiency and no nitrate N in the irrigation water, he adds.

University of California studies show that off-year pistachios have less stored N at the beginning of the year than on-year trees. Uptake occurs primarily between mid-May to late August.

“The research suggests reducing off-year applications by one-third of the on-year rate,” Beede says “In this case, one might consider applying half the season’s N prior to shell hardening and the remainder in July and August.”

Also, the high potassium requirement of pistachios calls for applying 125 pounds of potassium (K+) annually, Beede says. One suggestion is to apply 50 pounds K in May, 50 pounds in June, and 25 pounds in July. These fertilization rates are typically achieved using liquid blends applied through the drip system during May, July, and August, he notes. Nitrogen is applied alone in April in the form of UN32.