California Almonds: Zinc And Boron Management Before Leaf Drop

Urgent and important tasks are priorities in any business. What orchard nutrition tasks are urgent and important given the limited time and energy before leaf drop?

Zinc and boron are important elements for the beginning of growth in the spring and the successful nut set. Specifically, zinc is needed to ensure timely bud break and shoot extension and boron is needed for pollen germination and pollen tube growth. Because only small amounts of these nutrients are needed compared to nitrogen and potassium, a single foliar spray can deliver needed Zn and B at bloom. The fall is an excellent time to make this spray to orchards with healthy canopies because the nutrients can be stored overwinter in the tree. Zn and B can be applied in pink bud sprays, but the impacts of these nutrients on bee health are largely unknown.

In addition, B sprayed after flowers open at full bloom can reduce nut set, not improve it. Bloom timing variation between varieties makes pink bud a tough target to hit for all varieties without spraying at least one variety at full bloom.

Zinc (Zn) is critical for timely bud break and expanding shoot growth, so adequate Zn must be present in deciduous tree crops at bud break. Since Zn is readily tied up in soils, a foliar treatment is the most cost effective Zn fertilization method if summer leaf levels are at or below 15 ppm Zn. If ziram or foliar zinc product was applied before summer leaf samples were taken, leaf Zn values will show Zn in and on the leaf and will not give an accurate measure of Zn status.

If the leaf samples were contaminated in one of these ways, growers must then decide if a fall zinc treatment will be worth the cost (roughly $5- 20/acre for zinc sulfate) without knowing if the orchard is really deficient or not. Waiting for foliar Zn deficiency symptoms after bud break means any corrective spray will be too late.

If a decision to spray is made, carefully consider the rate to use. A fall Zn spray at a high rate (for example, 20+ lbs. zinc sulfate/acre) can defoliate trees while delivering needed Zn. This has been a standard late fall (November) practice for many years in California almonds. Recent research in stone fruit showed that lower rates of zinc sulfate (5 lbs/acre) applied in October were as effective in getting Zn into the trees as later sprays (November) at higher rates without damaging leaves.

Trees benefit from natural leaf drop (without a high zinc rate) through recycling of leaf N that stay in the tree and is stored for use at bloom. If you choose to use a high rate of zinc sulfate plus boron, research shows that the boron gets into the tree before the leaves drop.

Boron (B): A fall B spray can increase almond yield by hundreds of kernel pounds per acre when hull boron (B) analysis show low to adequate orchard B. This yield bump has been documented in multiple studies by UC researchers. The decision to use a fall foliar B and at what rate be based on harvest hull analysis (see table below).

Orchard Boron Status Hull Boron (at harvest)
Deficient <80 ppm
Adequate 80-150 ppm
Toxic >300 ppm

 

When harvest hull B levels are less than 150 ppm, the recommended rates are 0.2-0.4 lbs. B/acre – equivalent to 1-2 lbs. Solubor®/acre – as a foliar spray targeting a full, healthy canopy either during the fall or at pink bud.

Note: fall soil-applied B fertilizer doesn’t increase plant B levels until after petal fall the next year but can correct deficiencies for several years. Growers with <120 ppm B in hulls on a regular basis may want talk with their CCA to look at soil application(s) of B in the growing season.

Don’t expect to change orchard B status with a fall spray. A 2000 kernel lb. /acre almond crop removes

0.4 lbs. of B from the orchard; equivalent to 2 lbs. Solubor®/acre.

Tank mixes: In UC research, 0.4 lbs. of B as sodium borate and 20 lbs. of zinc sulfate mixed in 100 gallons of water produced a beige cloud in the spray solution. This cloud didn’t clog spray filters or nozzles, but reduced flower B levels compared to just B in the spray tank. Lowering the solution to pH with organic acid (not phosphoric acid) before adding Zn and B eliminated the haze and produced flower B levels the same as when B alone was applied. Tank mixing the same rates of B and Zn with the right tank chemistry defoliated trees and increased flower B the next

A fall foliar nutrient spray in most growing regions of the Sacramento Valley should include zinc and/or boron to make sure the nutritional status of buds allows the best nut set that the bees and the weather will allow. It could mean several hundred more pounds of kernels per acre in a low B orchard. This is the critical post-harvest nutrient task in Sacramento Valley almond orchards.


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