In uncertain times, put your money on proven practices and materials. Certain postharvest fertilizer inputs (boron, potassium, and zinc) when needed, are proven to deliver increased yield. At least one nutrient, nitrogen, when fall applied does not deliver yield benefit at this timing in well managed orchards. Understanding effective delivery for each nutrient is key to the health and performance of your orchard.
Boron (B): A fall B spray can increase 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. Recommended rates are 0.2-0.4 lbs. B/acre – equivalent to 1-2 lbs. Solubor® – as a foliar spray applied at 100 gallons/acre spray volume targeting a full, healthy canopy. Lower rates of actual B may not deliver the expected results. Higher rates of B/acre can reduce yield. Note: fall soil-applied B fertilizer doesn’t increase plant B levels until after petal fall the next year.
The key to effective fall B fertilization is getting B into the flowers by early bloom. Fall B foliar applications increase pollen viability the following spring and increase nut set compared to trees with low B levels. ‘Pink’ bud bloom stage is also an effective timing for foliar B to improve almond yield, while B sprayed at full bloom has been shown to actually reduce nut set. Since full bloom timing across an orchard can vary by variety, a fall spray at the right rate increases flower B with no risk to fruit set.
Finally, a fall B spray is at best a maintenance application. 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; that’s equivalent to 2 lbs. Solubor®/acre as a foliar spray if somehow, all the B in the spray tank ends up in the tree*. If hull B levels are really low, work with your CCA to increase orchard B status with spring or summer soil applied B fertilizer. Keep in mind, even when hull levels increase above the adequate threshold, a fall spray at the low end of the standard rate, may help yield.
Zinc (Zn) is critical for 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 often the most effective Zn fertilization method. 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 earlier in October were as effective in getting Zn into the trees as later sprays at higher rates without damaging leaves.
Potassium (K) fertilization in the fall is based on using the soil as a “K bank” for the following years. This practice is a holdover from before micro-irrigation and solution-grade K fertilizers. Some growers continue to use fall application of banded dry potassium fertilizer since it works to get an essential nutrient into trees and it “frees up” the calendar the following year for injections of nitrogen and other nutrients and/or amendments. Orchards with very sandy or gravelly soils having a cation exchange capacity (CEC) of less than 15 meq/100 gm soil – should receive lower rates of dry K fertilizer in the fall than soils with more clay and so higher CECs. This avoids the expensive loss of fertilizer K by leaching with excessive winter rain or irrigation water.
Don’t broadcast fertilizer K in orchards. Banding or applications concentrated in the micro-irrigation zones deliver effective results. Numerous UC potassium fertilization trials, all running for just 2-4 years, have shown that with micro-irrigation, annual rates of 200-250 lbs. K2O/acre, banded in fall, are as effective in increasing summer leaf K and yield in K deficient orchards as higher, more expensive K fertilizer rates. A final note; don’t get behind in your K fertility program. When summer leaf K levels show the orchard is deficient, yield will suffer the following year even if you fertilize in the fall.
Nitrogen (N) is critical to sustained high yield in almonds. However, late fall (October) N applications have not increased yield in 2 years of replicated trials in Colusa County in a high yielding orchard with adequate summer leaf N levels. Leaves in these study trees were not removed by fall zinc sprays. The key timings for N applications are from early leaf out to early June and should not be missed when good to heavy crops are present.
Tank mixes: Improper mixing of Zn and B in the spray tank can reduce flower B levels the following bloom compared to just applying B in the spray. In UC research, when 0.4 lbs. of B as borate and 20 lbs. of zinc sulfate were tank mixed in 100 gallons of water, a beige cloud formed 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 5.0 with organic acid (not phosphoric acid) before adding Zn and B eliminated the haze and produced flower B levels the same as if B alone was applied. In that study, tank mixing B and Zn with the right tank chemistry defoliated trees and increased flower B the next year.
What about mixing phosphorous-acid (phosphite) materials (Phosgard®, Nutri-Phite®, etc.) along with Zn and B to get 3 materials on with one spray? In my experience, different phosphite materials behave differently in the spray tank with Zn and B. A reputable phosphite manufacturer will have tech support who can give you a recipe for mixing Zn, B and their product. I called the chemist at one phosphite manufacturing company and got a careful recipe that he had tested to put all three ingredients into solution for their product. However, this recipe didn’t work with a distributor’s brand of phosphite. Since as Bill Olson, retired UCCE Farm Advisor in Butte Co. used to say, “the most expensive spray is the one that doesn’t work”, paying a little extra for a brand-name phosphite with a tested recipe for a Zn-B-phosphite tank mix from the manufacturer is most likely to deliver the best results. An experienced PCA/CCA will know who to call or may already have a recipe.
Time and/or money can be short in the fall, so spend those resources wisely. When tissue test results show low Zn, B, and/or K, fall application(s) should deliver results the following season. Fall foliar sprays get Zn and B into the tree ahead of the critical bloom timing, while fall, soil-applied dry K fertilizer can be effective and convenient compared to multiple in-season applications. Mid-October N applications to trees with adequate leaf N haven’t shown benefit in recent trials.
*Well-calibrated, dialed-in sprayers deliver about 75% of the spray volume to the tree.