Pistachio growers are stepping up to the plate to be the best stewards of nitrogen usage as possible – hence, avoiding heavy nitrogen applications before fruit set. Remember that early shoot growth and fruit development is all from stored nitroge. A good nitrogen management program includes soil, plant and irrigation water N assessment.
Take samples of each so you can make a better assessment as to how much synthetic N needs to be applied to the soil to meet the plant/crop requirement. Assess your potential crop shortly after fruit set, check past soil analyses and then begin N fertilization at 30 to 50 pounds, depending on your irrigation method and estimated crop load. Apply the higher rate under flood irrigation.
Some growers and consultants favor N application shortly after bud break. That support this early timing with the accurate statement that it takes about 14 days for ammonia-based fertilizers to convert to nitrate for uptake. So, they want the nitrate conversion completed by the time there is sufficient leaf area for its uptake by transpiration.
In my opinion, this practice assumes there is inadequate nitrogen stored in the tree and the soil to meet the growth potential of the tree. Based on UC nitrogen research, the pistachio tree’s entire seasonal N requirement for growth and development is only about 25 pounds.
Where Most Of The Nitrogen Goes
The bulk of the N requirement is associated with crop development. Thus, the nitrogen status of the tree, soil and irrigation water would have to be quite low to require N application at bud break.
Research by Dr. Patrick Brown, UC Davis, and Dr. Ismail Siddiqui indicates that pistachios remove 28 pounds actual N per 1,000 pounds of ACP weight from the orchard system.
This value does not account for application inefficiency. Fertigation applications may be 80% efficient. Furrow/broadcast applications may be as low as 50% efficient. For that reason, pistachio growers have largely gone to drip irrigation for improved water and nutrient management.
When To Back Off
Back off on the N applied this season if you find soil nitrate nitrogen levels above 35 ppm per foot in the root zone. Nitrate nitrogen levels in the irrigation water must also be considered in your budget. A 10 ppm nitrate N level supplies 27 pounds of actual N per acre foot of water applied.
Adding high levels of N to the soil early in the season will not result in greater plant uptake unless the tree is deficient. Available data indicates pistachio growth and yield is not improved with July tissue levels above 2.5 percent.
A validated model for predicting July nitrogen and potassium levels from tissue samples taken earlier in the season can be found at:
Created by Dr. M.I. Siddiqui during his doctorate studies with Dr. Patrick Brown, UCD Pomology Professor, this model allows growers to assess the future nitrogen status of their orchards from late April and May tissue samples. For example, a 3% nitrogen tissue analysis 40 days after full bloom would predict that you would have 2.68% in July, a level sufficient to begin kernel filling.
I recommend you begin using this model, since laboratory testing last season showed it was an accurate prediction tool.
Micronutrient Sprays: You’ll Find A Hog Here
Research shows good zinc uptake at 50% leaf expansion (late April). Use only 2 pounds of zinc sulfate 36%. Research by Patrick Brown and Qinglong Zhang indicates it is safe to add onehalf pound of Copper EDTA or one pound of Solubor to the foliar zinc sulfate rate.
Buffering this mixture to a pH of about 5 also improves zinc uptake by increasing the amount in solution. Acidification should be done with citric acid (powder) rather than phosphoric buffer to prevent zinc phosphate precipitation.
Many growers prefer to avoid the “Betty Crocker” mix and apply liquid materials formulated for pistachios. Check with your supplier for suggestions, but be sure they have adequate amounts of copper, boron, and zinc. This is especially true of two and three year-old trees. Deficiencies severely limit canopy development and reduce early bearing potential.
They are also very common, suggesting growers are not taking young pistachio plant nutrition sufficiently seriously, and suffer loss in orchard development because of it. My experiences suggest second leaf orchards are especially susceptible to micronutrient deficiency, partly because of all the tipping performed to create branching.
Three sprays are often needed to prevent loss of canopy development during this critical training year.
AgFax Weed Solutions
Shriveling flower clusters do not necessarily indicate fungal infection. Clusters that remain green and shatter off the tree could be associated with low boron levels. Boron deficient leaves have crinkled edges but remain uniformly green and normal in size.
Tissue levels less than 60 ppm in May suggests the need for boron fertilization. Boron is taken up by the leaves throughout the season, so application can be made at any time during the spring. Correcting severe deficiencies may require up to 2 ounces of Solubor PER TREE (16 lb/ac) soil applied over time. Yes, you read it right. It’s not a typo. Pistachios are boron pigs.