The four top nutrient issues which pistachio growers seek input from Kern County’s Craig Kallsen about include copper and zinc deficiencies, too little and too much boron, and proper nitrogen levels in young trees.
However those are not necessarily the top nutrient issues in pistachio production, says Kallsen, pistachio farm advisor, University of California Cooperative Extension, Bakersfield, Calif.
The knowledge of existing levels of some nutrients in leaf tissue is critical in pistachio production especially in young trees. In some areas of Kern County, copper and zinc deficiency in young trees can result in the loss of an entire season’s vegetative growth.
“I am a big believer in knowing the soil’s fertility level and a great believer in tissue analysis,” Kallsen said. “I don’t believe in the wholesale application of fertilizer especially as costs have risen.”
Some locations in the northern SJV can require up to 200 pounds per acre of added potassium to achieve large yields, Kallsen says.
Leaf tissue analyses can greatly assist growers to determine needed additional potassium. Many soils in Kern County fix broadcast potassium fertilizers into forms unavailable to the trees. In these situations, potassium fertilizers can be banded along the tree row, or fertigated in concentrated form, which usually reduces fixation.
Kallsen says potassium sulfate (sulfate of potash or SOP) and potassium chloride are great potassium sources for pistachio. Finely-ground SOP can be applied through irrigation lines while large amounts are best applied to the soil. Soils in Kern County range from sandy to heavy clay. Pistachio trees grow well in clay loam soil.
Mature pistachio tree roots spread out up to 50 feet on each side of the tree which increases nutrient uptake. The extended root structure also minimizes trees blown over by high winds.
Generally, the most accurate method to assess nutrient levels is through leaf tissue analysis. However tissue analysis for some micronutrients can be misleading.
In response to grower questions, Kallsen says many soils in Kern County and the southern San Joaquin Valley are deficient in copper and zinc. He says the suggested range for copper in August leaf samples is 6 to 10 parts per million (ppm). Classic copper deficiency symptoms have been observed in young trees with laboratory analyses in excess of these levels.
“On occasion with copper and zinc nutrition, I have tended to believe what the plant is showing me over the lab tissue results,” Kallsen said.
Growers with low copper and zinc levels should foliar apply the essential metals to prevent the characteristic leaf burn back effect in trees. Kallsen suggests a 10 to 15 ppm application.
Boron is an essential micronutrient important to metabolism, vegetative growth, flowering, and fruit set. Deficient amounts in leaf tissues are commonly found on the east side of the southern SJV while boron levels can top 1,000 ppm on the west side.
“Boron levels in the 150 ppm range are adequate in pistachio production but tissue readings of about 250 ppm are preferred,” Kallsen said. “Boron levels as high as 700 ppm usually are not a concern. Pistachio trees have a high boron requirement and tolerance.”
Highly-elevated boron levels can reduce yields and defoliate trees. Trees that defoliate due to excess boron when the leaf tissue analysis hits 1,000 ppm usually refoliate and produce adequate yields the next year.
For macronutrients, adding too much nitrogen to young trees can be a problem especially late in the season when growers should slow tree growth prior to winter dormancy. Nitrogen stimulates vegetative growth and late October temperatures even a few degrees below freezing can cause extensive freeze damage.
Kern County is California’s top pistachio-producing county. Kern County growers grew about 44 percent of the state’s crop on 49,000 bearing acres in 2007. The next top-producers included Madera, Tulare, Kings, and Fresno counties, according to 2008-2009 California Agricultural Resource Directory.