Boron is a micronutrient for almonds. It is required in the process of flower fertilization, being involved in directing the germinating pollen tube. Deficiencies lead to a reduced nut set, and in severe cases, an appearance of a “nonproductive symptom.” Within the tree, boron deficiency can lead to a dieback of small twigs and a “weeping branch” like look. Boron is also toxic at too high of concentrations. Often being found in areas with high soil boron or in blocks irrigated with water containing boron, toxicity symptoms appear as gummy nuts that may form “stick tights” or presence of gum exuding from pruning wounds, bud and leaf scars, and spurs.
Critical threshold levels for boron hull values are as follows:
|Hull Boron Level (PPM)|
|Sufficient||100 – 160 PPM|
To determine if an orchard is deficient in or showing toxicity from boron, hull samples must be collected. Since the hulls serve as the primary boron sink, they tend to accumulate the nutrient, and provide a consistent reading. Leaf values are not effective in determining adequate levels. This is because they are not able to detect toxicities and are inconsistent at the lower end of the sufficiency levels. This is demonstrated in table 1, which compares boron mid-July leaf and hull values from several different orchards. In the orchards, leaf values are all very similar, but hull analysis revealed some orchards were deficient.
|Orchard||Block||Sample||Boron Leaf (PPM)||Boron Hull (PPM)|
Boron deficiency can be alleviated through applications of boron to the soil. Caution should be used when determining the rate of material applied as high rates of application may cause toxicity. There have been some reports that boron applied in the summer may lead to stick-tights at harvest; suggesting that is may be better to apply boron in the postharvest or earlier in the season. A foliar spray of boron should always be considered between postharvest and pinkbud. More on boron can be found here and here.