Importance of Hull Sampling for Boron

Boron toxicity of a young almond tree. Photo by Franz Niederholzer

Boron toxicity of a young almond tree. Photo by Franz Niederholzer

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)
Deficiency <80 PPM
Sufficient 100 – 160 PPM
Toxicity >300 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)
1 1 1 31.8 102
2 32.8 85.7
2 1 34.7 117
2 32.3 111
2 1 1 29.3 175
2 31.7 62.1
3 1 1 28.4 122
2 29.7 84
4 1 1 28.8 53
2 27.7 82
2 1 28.7 86
2 29.7 74


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.

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5 thoughts on “Importance of Hull Sampling for Boron

  1. My boron levels have been hoovering in the 80’s ppm, so I broadcasted some boron to the strips of my orchards back in June. My rate was 15#’s of actual boron per acre, with 20 gallons of water per acre. These blocks are flood irrigated, on sandy loam soil, irrigated approximately every 16-18 days after my application. I pulled hull samples on 8/18/14. My results came back in the 80’s again, relatively unchanged from last year.
    Shouldn’t I be seeing results by now? Can the trees take up that amount of boron before it is leached out of the root zone?

    1. Rod,
      I would suspect that the boron will show up in next year’s hulls. There is a good chance that it also showed up in this years too – its just that they would read lower than what they did if boron wasn’t applied. A foliar spray is still recommended even with the application.

      Boron does leach – just as any salt. I am sure, based on your irrigation system, that it has moved into the rootzone, and some of it may move past, depending upon the amount of water applied. Smaller, more frequent doses for sandy soil and flood irrigation may be a better strategy.

      I have to add that applying that high of a rate of boron (15 lbs of actual boron/acre) is a bit risky. At high rates, boron can be toxic and can increase stick tights within the tree. Smaller applications of boron should be made – around 2-4 lbs of actual boron/acre (10-20 lbs of a 20% boron product).

      Hope that helps,

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