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Dr. Scott Johnson’s stone fruit nutrient deficiency studies have led researchers to look anew at traditional nutrient analysis sampling methods and deficiency threshold levels. Based on his research, the recommended optimum range for some nutrients has been fine-tuned.

Throughout the last 50 years, growers have been advised by horticulturists to take a summer leaf sample to analyze the nutrient status of their stone fruit trees.

"We’ve now learned that midsummer is a lousy time to take samples," the University of California Cooperative Extension pomologist said. Johnson and his colleagues are looking at whether or not dormant shoots are better indicators of the nutrient status of stone fruit trees.

Dormant shoots seem to work well when measuring levels for zinc, boron, and phosphorus, but the shoots haven’t worked well for nitrogen.

"We’re still pursuing sampling nitrogen in dormant fruiting shoots by looking at the amino acid arginine," Johnson said. They are studying how arginine, the main storage amino acid in dormant peach trees, relates to vegetative growth and nitrogen deficiency.

But more work is needed on the dormant shoot sampling before it becomes a recommended practice, he adds.

He points to boron as an example of "fine-tuning" stone fruit nutrient guidelines.

Peach and nectarine trees can suffer from boron deficiency—what Johnson calls hidden hunger—before showing leaf symptoms. "The tree may be deficient enough in boron to affect fruit set, but you wouldn’t see any leaf symptoms. That has caused us to reset the boron threshold level from 18 parts per million to 25 ppm, based on our sand tank experiments."

Johnson notes that the old standards were likely set on visual symptoms.

The UC scientists have also refined the recommendations for optimum ranges of nitrogen and phosphorus, but are still working on a new value for zinc. The current deficiency threshold of 15 to 20 ppm is likely too high, Johnson said, but they aren’t ready to suggest new values yet.

"We have refined the nitrogen optimum range based on studies showing the problems of both too much and too little nitrogen," he said. "It used to be 2.4 to 3.3 percent, but now we recommend 2.6 to 3.0 percent for fresh-market peaches. Phosphorus hasn’t changed much, and we still recommend the optimum range to be 0.1 to 0.3 percent."

Johnson is still working to refine deficiency thresholds for other nutrients like calcium, magnesium, potassium, and other micronutrients that have proved difficult to induce deficiencies in their sand tank research trial.