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  • Benny Thompson

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    February 10, 2023 at 10:25 pm
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    Thanks @JohnWarmerdam! We’ve done a lot of trials this way. I definitely appreciate you mentioning it, because the trials always require more patience than I want to give, and I too often convince myself to skip the trials.

    Still begs the question: when you run trials, for each micronutrient, do you err on deficiency or excess? Typically I would do three tests 1) 1/2 the recommended concentration, 2) the recommendation, and 3) 2x the recommendation.

    Let’s talk about Boron as one example…say the sap analysis says you have a boron deficiency.
    Correct me if I’m wrong on any of this…
    A good level for Boron in lettuce sap is about 10ppm (and calcium is about 5000ppm). Calcium and Boron are immobile, which means the deficiency shows up in the new growth. A systemic deficiency should be corrected with weekly foliar applications otherwise the new growth of the following week won’t get the boron.

    For example, when I do calcium foliar sprays, I do it at Ca 1000ppm (mixture of calcium acetate and calcium carbonate). Works perfectly and we’ll do it every week if there’s systemic calcium deficiency. With systemic deficiency, the application won’t last two weeks, we would get leaf tip rot if we didn’t spray every week.

    Seems intuitive to me that you shouldn’t foliar spray a higher concentration of a nutrient than what is optimal in the sap (though I haven’t seen any literature to back that up). So if Boron is deficient, the foliar concentration should not be greater than 10ppm(for lettuce). However, Looking at AEA Rebound Boron and Biomin Boron, the recommended application rates are MUCH higher.

    AEA Rebound B (2% Boron) says 1 gal per acre per week. It’s about 5 gallon solution to cover 1 acre, so that’s 4 gallons RO water + 1 gallon Rebound B… 2%/5gal = .004 = 4,000ppm. That’s a huge dose of Boron! I should note that the salesperson at AEA said that the label application rates are maximums. If I were to run test strips based on their recommendation I would do strips at 2000ppm, 4000ppm and 8000ppm. Seems outrageous to me.

    Biomin B (3% Boron) gives a range in their application. On the low side it’s: 1 pint per acre per 2weeks. It’s about 5 gallon solution to cover 1 acre, so that’s 39 pints RO water + 1 pint Biomin B… 3%/40pints = .00075 = 750ppm. That’s still a huge dose of Boron! If I were to test this just based on their recommendation I would do strips at 400ppm, 800ppm and 1600ppm. Still seems too high.

    If I were to personally design boron foliar spray without looking at literature, I would do a 10ppm Boron foliar spray, that’s only 1gram of Boric acid per 5 gallons water (for 1 acre). (I would add yucca extract as a wetting agent). I’d do strips with foliar of 5ppm, 10ppm and 20ppm. I suppose that would be erring on deficiency.

    For context, a reasonable soil Boron level is about 2ppm. @Justas23 said that they have only overused Boron of all the micronutrients. Lots of literature states that boron has a small window between deficiency and toxicity. (Now I’m starting to wonder how true that is and if the problem is that farmers commonly overdo a foliar application of boron.)

    All of this goes back to the original question. For each micronutrient, would you err on excess or deficiency? And by how much? I typically do 50%, 100% and 200%. For boron, with recommendations all over the map, would a more appropriate test be 10%, 100% and 1000%? possibly 10ppm, 100ppm and 1000pppm?