Boron is a critical micronutrient for fruit set. Work has shown that yield can be reduced when boron hull concentrations fall below 80 ppm. In order to determine deficiency, a hull analysis should be taken to determine tree boron status. Leaf tissue levels are not consistent in providing boron status. Applications of boron should occur to the ground if hull levels are below 80-100 ppm. Boron foliar applications, however, should be considered for nearly all orchards except ones approaching toxicity (hull boron greater than 200 ppm).
Boron foliar applications have been shown in several studies to increase yields in orchards deemed boron sufficient. A study from 1999 compared the application of foliar boron at two locations – one in Fresno County and the other in Glenn County – for two years. The Fresno County location applied the foliar sprays either in September, December, or February at three rates (0 lbs/acre, 0.71 lbs/acre, and 1.5 lbs/acre of boron) using Solubor® (a 20.5% boron product). The Glenn County site applied the foliar spray either in August, September, or February at four rates (0 lbs/acre, 0.71 lbs/acre, 1.11 lbs/acre, 1.50 lbs/acre, and 1.91 lbs/acre of boron) using Borosol® (a 10% boron product).
At the Fresno Location, the September application was the most effective in increasing tree boron levels, fruit set and yield. All rates out-performed the control. At the Glenn County location, the increasing rates increased tree boron tissue levels almost linearly for the August and February timings. February application timings at both sites increased initial fruit set, but the greatest increase in tissue concentration, nut set, and yield response came from the September timing. The increases observed are most likely is due to the increased efficiency in nutrient uptake from the active leaves. If a September application cannot be made, a February/early pink bud spray should be considered.
Ground applied boron should be considered if hull boron levels are under 100 ppm. Since boron is regularly exported out of the field, this application should be made consistently (e.g. annually or every other year). Solubor® or boric acid is typically applied by fertigation at a rate of 2-4 lbs of actual boron. Ground applications should not occur if hull boron levels are over 150 ppm as boron can be toxic if in too high of a concentration.
Many operations fail to apply boron to the soil. Since boron is exported with the crop, the tree can become deficient. This is especially true if the water and soil have very low levels. The concentration of boron in the kernels is around 20 ppm, while the hulls can vary widely (and why they are tested). The table below indicates the amount of boron that is exported by the kernel and hull for the respective kernel lbs/acre. For an orchard with a hull boron concentration of 100 ppm, an estimated 3.5 ozs of boron would be exported for every 1000 kernel lbs.
|Boron exported for every 1000 kernel lbs||Hull boron at 10 ppm||Hull boron at 50 ppm||Hull boron at 100 ppm||Hull boron at 150 ppm||Hull boron at 200 ppm|
|Hull||0.32 oz||1.6 oz||3.2 oz||4.8 oz||6.4 oz|
|Kernel (assumed 20 ppm)||0.32 oz||0.32 oz||0.32 oz||0.32 oz||0.32 oz|
|Total||0.64 oz||1.92 oz||3.52 oz||5.12 oz||6.72 oz|
As with any nutrient application, the application of boron is not 100% efficient. Time the foliar application for postharvest through pinkbud if possible, and monitor hull levels to determine if ground applications are needed. Orchards approaching toxic boron levels should not apply any boron and consider a leaching program to assist in reducing soil concentrations.