Renate, boron deficiency is indeed a difficult problem to combat. With the small window between what is needed and what is toxic, I find that the best thing to do is add a little boric acid to all of my compost teas,
root drenches, foliar feedings, etc. I bought a one pound bottle three years ago (roach killer from the dollar store), and I still have half of it left, so I'm pretty sure it's not going on too heavy. Just a squirt of the powder, not even a gram, into a 5 gallon
bucket of compost tea, and it will get incorporated into the garden environment.
I had a poor fruit set this year, so maybe that is boron deficiency, maybe something else, but I don't have a chemistry lab at my disposal to make that determination. I will continue to mulch and add manures and sprinkle the compost tea around hope that the chemistry of my garden environment is improving. As I think back to how it was since I first moved in, it seems a lot is improving: more spiders, more anoles and skinks, more toads, less loss to
Fusarium diseases. Qualitatively, it looks better, but it's frustrating my scientific side not to quantify it.