Bloom is the most important time of the year for adequate boron (B) and zinc (Zn) levels in almonds. Zinc is essential to many processes such as cell division, protein synthesis and auxin synthesis in growing points (flowers and shoots), and bloom is the time of the most growing points in a tree. Boron is essential for cell wall synthesis and division. Boron fertilization has improved fruit or nut set compared to deficient plants in many crops, but only if applied in time to get B into buds at bloom. Both Zn and B can be absorbed into leaves and translocated within almond trees.
Check leaf symptoms, summer leaf analysis or hull analysis results to see if Zn and/or B fertilization is needed. Zinc deficiency produces “little leaf” symptoms (see photo below), with trees showing summer leaf levels of 15 ppm Zn or less considered deficient. Trees with hull B levels below 80 ppm B at harvest are thought to be deficient, but almond yield may benefit from B application if hull levels are below 120 ppm B. Do not fertilizer with B if hull levels are 200 ppm B or greater, as excessive B is toxic to plants.
A fall foliar nutrient spray is a cost effective way to get Zn or B into buds for the following year’s bloom. The return on investment for a fall B spray can be significant. Yield increases of 200-400 kernel pounds per acre have been measured from a foliar nutrient application the previous fall at Nickels Soil Lab of 0.6 lbs. of actual B (the equivalent of 3 lbs. Solubor®/acre applied in 400 gallons/acre). Fall Zn sprays should increase leaf Zn the following year and eliminate Zn deficiency symptoms while reducing the consequences of any phytoxicity resulting from zinc applications at this time compared to earlier in the season. The following are tips that may help maximize the benefit from a fall Zn and/or B spray.
Materials: Zinc sulfate is a good, cost effective source of zinc in postharvest applications. Other sources work, but are more expensive. Most B sources, as long as a similar rate of actual B is applied, are effective foliar fertilizers.
Zn Rates: A moderate rate of Zn, for example, 5 lb. zinc sulfate (ZnSO4)/acre, applied in October, is as effective as higher rates applied in November when natural leaf drop begins. The 5 lb. ZnSO4/acre rate in October will not remove leaves, allowing for continued carbohydrate production and storage for use next growing season. To get defoliation plus fertilization, use higher rates – for example, 20 lbs ZnSO4/acre – once natural leaf drop has begun. In my experience, defoliation with zinc sprays requires moisture in the orchard – either from rain or irrigation.
B Rates: Generally 1-2 lbs Solubor/acre in 100 gallons per acre is recommended as a fall spray. Excess B can be toxic. Consult with an experienced PCA/CCA when selecting a B rate. If the canopy is healthy and green, B sprays should be absorbed into November, even when tank mixed with a high rate of ZnSO4.
Tank mixing: When B fertilizer is tank mixed with 20 lbs. ZnSO4, B uptake occurs before defoliation and subsequent flower B levels equal those from a B spray alone. To reduce application costs, Zn and B can be tank-mixed, but certain steps are needed. Acidify the spray solution to pH 5 before adding zinc using an organic acid based material (for example, Mixwell or Tri-Fol) and not a phosphate buffer (will precipitate with zinc). Then add B. If the solution pH climbs above 5, a light brown haze (precipitation) forms in the tank and lower boron levels in the flowers the following spring can result. Add more acid to eliminate the haze.
Soil applied B and/or Zn: Foliar application is more efficient and faster at getting the nutrient into the target tissues (buds); however, both soil applied Zn or B can be effective in increasing the plant level of the applied nutrient. When B is applied to the soil in the fall, flower B is not increased the next bloom, but hull B at the following harvest is increased. Adequate B at bloom is your target, but soil application may be helpful if the orchard is badly deficient. To correct a very B deficient orchard, a combination of foliar and soil applied B fertilizer may be needed. Almonds export 0.4 lb. B in a 2000 lb. per acre kernel yield crop (in kernels, shell and hulls), so 2 lbs. Solubor®/acre will not change hull B levels unless additional B is added. Solubor is 20.5% B by weight.
Canopy Health: Foliar application in orchards with extensive defoliation is not recommended.