There have been a few questions regarding high boron levels in hull and leaf tissue sampled from this past year. The general consensus is that it is “just one of those odd things that occur every year.” Never-the-less, that prompted me to give Dr. Patrick Brown (UC Davis) a ring and inquirer about environmental influences on boron.
Dr. Brown indicated that boron levels within tissues is dependent upon yield. When yield is high, boron concentrations can be low, leading to subsequent fertilization. When yield is down, boron concentrations can be high, causing concern of toxicity. When heavy boron applications (which may be normal rates for 3000+ lb crops) are made on almond trees yielding less than expected, higher hull and leaf concentrations will most likely be observed.
Toxicity can occur when hull levels increase above 200 ppm, which leads to an increase in stick tights, gummy hulls and twigs, and gummy kernels. To help reduce hull levels to the “normal” range of 120-160 ppm, soil applications of boron should be reduced or eliminated. Since boron is exported out of the orchard with the crop, the soil and tissue concentration will drop over time. Remember to restart applications when hull levels drop below 100-120 ppm. Foliar nutrients can be maintained, especially the critical application timing between bud swell and pink bud.
If boron seems excessively high and well water is being used, perform a water analysis to determine boron concentration.