Managing low hull boron: late season corrective measures before next year's bloom.

Recently, boron hull analysis was provided by an almond farmer. The analysis consisted of two samples from his ~300 acre (~125 ha) third-year farm. The results were 62 and 84 ppm from the cultivars Nonpareil and Monterey, respectively. Both values were under our targeted hull analysis of 95-105 ppm, with the Nonpareil sample also being deficient (<80 ppm).This orchard has followed a boron program since the beginning of its development. Initial soil samples had indicated very low boron (<0.2 ppm, undetectable), high soil pH (7.7-8.2), and a clay loam texture across the operation. Prior to planting, soil sulfur was applied to lower the pH, as well as diammonium phosphate and potassium sulfate. After establishment, boron in the form of disodium octoborate tetrahydrate (20.5% boron, tradename: Solubor®) was applied at 5 lbs/acre (~5.5 kg/ha) twice a year (spring and fall) through the drip system. A foliar application of sodium tetraborate was also made at 1.8 lbs/acre (2 kg/ha) in the fall after the second year of growth and pink bud. As a result, soil boron levels have improved slightly, with some detectable boron within the wetting pattern.Boron deficiency can greatly impact almond yields. Boron improves pollen tube germination and growth, helping to increase the number of flowers fertilized during pollination. Being deficient in boron can lead to very low nut set and yield, and a willowy- look to the trees. More on this can be found in previous articles (here and here are a few).Given that the most recent hull samples indicate deficiency or borderline deficiency in boron, a new plan has been developed. This plan will utilize additional foliar sprays and soil boron applications and will be applied through dormancy and into the next year. The grower has already applied 5 lbs/acre (5.5 kg/ha) and 2 lbs/acre (~2.2 kg/ha) of Solubor® in his post-harvest fertigation and an October foliar spray, respectively. The concern is that this may not be enough, and so, to increase tree boron levels going into next year’s flowering, an additional 4 lbs/acre (4.5 kg/ha) of Solubor® will be applied in late January, with an additional 1-2 lbs/acre (1.2-2 kg/ha) being applied during pink bud/20% flowering. These additional foliar sprays should increase tree boron levels. Since boron is mobile within almonds, it should move from source to sink prior to flowering, hopefully reducing the effect of deficiency.As previously mentioned, soil boron applications will also be increased. Next year, the rate of Solubor® applied by fertigation will be doubled, with 20-25 lbs/acre (22-27 kg/ha) being applied throughout the season. 10-12.5 lbs/acre (11-14 kg/ha) will be applied before harvest with the remaining material being applied in the postharvest. This should provide an additional 5 lbs of actual boron/acre (5.5 kg/ha), increasing soil boron levels. Making soil applications during the winter will not impact the tree’s boron status before flowering, so these applications are timed to improve tree boron status for future years. Additionally, reliance on ammonium-based fertilizers will be continued as this helps acidify the soil profile, which will reduce the “tie-up” of boron in the soil. This plan will be continued until hull boron levels reach 95-100 ppm. After achieving this level, the applied soil rates will be decreased back to 10 lbs/acre (11 kg/ha) of Solubor®. Foliar spray timings will also be reduced to a single timing between postharvest and pink bud.When deficiencies occur, multiple approaches should be made to correct the problem. Foliar sprays provide the quickest relief, but symptoms will re-occur if applications stop. Furthermore, it is very hard to apply large amounts of material by foliar sprays. To reduce reliance on foliar sprays, soil applications should be utilized to build soil levels. Once soil levels are in the ideal range (0.5-1.0 ppm), foliar sprays can be reduced to the critical times (fall-pink bud), reducing operational expenses.