Progressive California fresh fruit growers have significantly reduced nitrogen use on peach, plum and nectarine trees, and the result has been improved quality fruit.
University of California Extension Pomologist Scott Johnson told growers at a University of California Kearney Agricultural Center field day featuring nutrient management that growers have reduced nitrogen use by half, compared to a decade or more ago.
“I am sure some growers continue to over-fertilize, but quite a few have realized that reducing N improves quality without significantly affecting size,” Johnson told growers and pest control advisers (PCAs) on a field tour of his nutrient management research plots.
Over-fertilizing is not all the fault of growers. A demand for increasingly larger fruit by buyers forced producers to pour on the N, and thin and girdle to achieve larger sizes, often at the expense of quality.
The size demand was particularly detrimental to early fruit quality, said Johnson. “It is a little easier to get size with later varieties.”
Johnson said 3 percent leaf nitrogen is sufficient for a healthy tree to produce good fruit, and growers can expect to continue to get good vegetative growth and acceptable quality production at that nitrogen level.
He said he has never identified a phosphorous deficiency, which leaves him wondering why growers continue to use N-P-K triple fertilizer.
However, that is not true of the other major nutrient, potassium.
A potassium deficiency can result in cracked fruit and limb dieback, particularly in the lower areas of trees. A telltale sign of K-deficiency is end-of-the-season leaf roll that can be mistaken for water stress.
Johnson said K levels of less than 1 percent in a leaf sample can cause problems. “One percent to 2 percent is a little low” and could benefit from a potassium sulfate application.
Zinc was the only minor nutrient addressed at the field day.
“Zinc does not represent a major expense for growers,” he noted. “However, zinc prices have gone up with the significant increase in all fertilizers and that is why we continue to look at it.”
Johnson's research shows that timing and sampling for zinc deficiency are critical to determining whether trees need zinc or not. That is of interest to PCAs and growers because with the rising cost of fuel, unnecessary trips through an orchard are costlier than ever.
“Growers also realize that zinc is a heavy metal that does not leach out of the soil. It is an environmental issue — maybe not a major one, but a potential issue nonetheless.”
Johnson says zinc moves within the trees, particularly in the spring when there is a flush of new growth. Then it tends to move to the growing tip where sampling would not necessarily reflect a deficiency within the whole tree. However, a visual symptom of zinc deficiency is thinning of the top of the trees and a lack of fruit there.
“In a deficient tree, our research is indicating that zinc moves from older leaves to younger leaves.”
Johnson said a debate is ongoing about the current university recommendations for zinc deficiency, but a reading of 12 ppm zinc in older, basal leaves should be considered a zinc deficiency signal.
He has had “good success” in identifying zinc-deficient trees by dormant shoot sampling lower in the canopy. “We can get a nice difference between deficient and sufficient trees the first of September. However, I realize that could be too late for most growers.”
It does not take much zinc to mitigate a deficiency, but it takes heavy applications of zinc to get a small amount into the trees. However, applications of zinc can cause fruit and leaf toxicity.
Unlike the movement of zinc within the tree “zinc tends to be immobile” when applied as a foliar spray.
Johnson has been evaluating a wide array of material to see if any are less phytotoxic than zinc sulfate. Pure basic zinc (52 percent) does not cause damage like zinc sulfate. However, not much zinc gets into fruit trees from this formulation.
Bicco's Chelated Zinc 12 percent with ethylene diamine acetic acid (EDTA) is a particular product Johnson used that did not cause leaf shot holes or spots on the fruit.