Sampling nutrients in stone fruit trees during the dormant period, instead of using only the customary mid-summer leaf sample analysis, would give growers an opportunity to correct any deficiencies before the growing season.

With that idea in mind, Scott Johnson, University of California Cooperative Extension pomologist at the Kearney Agricultural Center, Parlier, continues this year to seek a better way to measure levels of phosphorous, boron, zinc, and other elements in trees.

Johnson recently outlined his progress during the Fertilizer Research and Education (FREP) Conference in Salinas. He said he has been able in a study funded by FREP to correlate productivity and quality of fruit with deficiencies in major and minor nutrients.

“It's been standard practice to make foliar sprays in the early fall, whether the trees really need it or not. We want to figure out how to improve the efficiency of the sprays,” he said.

His observations, begun in 2001, were collected through nutrient analyses of dormant shoots of peach, nectarine, and plum trees grown in 60 large tanks of sand at KAC.

Deficiency thresholds

By 2004, he arrived at proposed deficiency thresholds for nitrogen (1.2 percent), phosphorus (0.12 percent), boron (14 ppm), and zinc (20 ppm).

His next step was to apply those thresholds to commercial orchards early in 2005, but he found those results were not conclusive, since many of the nutrients did not appear to be deficient in any of the orchards tested.

During July of 2004, Johnson did leaf sampling in 60 commercial orchards on sandy soils or in areas where boron deficiency had been diagnosed in nearby vineyards. He did nutrient tests on dormant shoots the following January and treated those orchards found to have deficiencies below or near the thresholds found in his test trees in the sand tanks.

“The use of dormant shoots to test for deficiencies in phosphorus, boron, and zinc,” he said, “still seems reliable even through we have not been able to test the procedure fully in commercial orchards.”

However, Johnson added, the commercial orchard trials prompted some new directions to his research for 2006. In the case of nitrogen, for example, he said studies with arginine and other amino acids have shown they indicate nitrogen status in trees. Arginine is the main storage amino acid in dormant peach trees, so he will evaluate how it relates to vegetative growth and nitrogen deficiency symptoms.

Shortages in potassium, magnesium, and calcium were not observed in the sand tank trees, except for a single plum tree that showed potassium deficiency late in 2005. “One of the problems has been the fairly hard water that is our main source of irrigation,” he said.

To create deficiencies in these elements in the test trees, he will install a water softening system this year to remove magnesium and calcium from the water and a few of the tanks will be irrigated with deionized water.

Role of zinc

His research also raised questions about what role zinc plays. Johnson said he found a range of zinc deficiency in his sand tank trees in the spring of 2003, 2004, and 2005 that correlated well with dormant shoot levels.

“However,” he said, “there have always been some trees with very low zinc levels that showed no symptoms and grew vigorously. Likewise, in the orchard survey there were several sites that tested low in zinc but showed no symptoms.”

That led him to try to unravel more mysteries with zinc with a separate, FREP-funded project on how the micronutrient behaves in peach and pistachio trees.

Zinc, he explained, is universally deficient, causing economic losses throughout the world, particularly in peach, pecan, citrus, and avocado crops. It is the most widely limiting micronutrient in California tree fruit and is most severe in sandy or alkaline soils.

Many growers have responded with foliar applications, but Johnson noted that recent research literature suggests that foliar-applied zinc remains in the leaves and is not moved to other parts of the plant. When the leaves fall, much of the zinc finds its way into the soil.

This year he will continue studies begun in 2005 with a group of practices to promote the trees efficient recovery of zinc from both soil and foliar applications.

One is a method used in Germany to add iron and zinc to ammonium fertilizer applied to acidified portions of the root zone to lower the pH and increase uptake of the micronutrients.

Another is seeding with barley as a companion crop. Johnson wants to see if barley roots close to the tree roots will interact to capture more zinc for the trees. He has been able to obtain for his trials a scarce barley variety having much greater zinc efficiency than normal varieties.

He will test the rootstock Hiawatha, found to have considerably more zinc in its foliage than the standard Nemagard, even without being treated with any zinc fertilizer.

He plans to again evaluate the ZN68 isotope to trace zinc mobility in the tree. “In one experiment, we have been able to show about 7 percent of the zinc applied to peach leaves in late fall is taken into the perennial parts of the trees before leaf fall.”

Greater mobility

Noting that this suggests zinc mobility is much greater than originally thought, at least in peach, he said of the amount taken up, almost half is transported to the roots but is then remobilized in the spring and mostly ends up in new growth.

Johnson has planned a series of experiments with rates, timing, and additives to further study the movement of zinc from foliar applications into other parts of the tree.