Nutrient Management Research Database

Potassium (K) fertilization of pistachio trees in the Central Valley of California substantially increased both nut yield and quality during a three-year study. Results indicated that the presently suggested leaf K critical level of 1.0 percent should be increased to 1.7 percent or higher.

This study investigated the effects of potassium (K) fertilization on leaf K concentration, nut yield, and pistachio quality.

There were three study locations: Madera, Yolo, and Orland, CA. All orchards were of the 'Kernan' variety.

K was applied at rates of 0, 120, 240, and 360 lbs K₂O per acre applied as potassium sulfate (K₂SO₄)

Initial available soil K in the top 0-6 inches of soil was 82, 97, and 125 in the Madera, Yolo, and Orland sites respectively.

Leaf K Concentrations

K fertilization reliably increased leaf K concentrations. In all but a single case, no differences in leaf K resulted from applying more than 120 lbs K₂O per acre.

Leaf K uptake is low at the start of the season, and increases as fruit development proceeds from July-September. Maximal K demand occurs at the end of this period, and declines rapidly following harvest.

K Fertilization Increases Yield

K fertilization reliably increased yields when additions ranged from 0-240 lbs K₂Oper acre. However, further increasing K application to 360 lbs K₂O per acre decreased yields.

K Fertilization Increased Nut Quality

Higher percentages of split nuts, higher 100-nut weight, and lower percentages of blank and stained nuts resulted from K fertilization. However, no differences were seen between the different levels of K added.

Diagnostic Criteria for K Nutrition

A critical value of 1.67 to 2.02 leaf K% was identified as a level to attain 95% of maximum yield.

Recommendations

Pistachio orchards should be fertilized at a rate of 120-240 K₂O/acre in California.