Similar to calcium, boron is taken up by the plant passively, via transpiration. Dr. Brian Krug, at the University of New Hampshire, in conjunction with Dr. Brian Whipker at NCSU showed that transpiration greatly influences boron uptake and coined the term "evapotranspiration". It is not just the relative humidity of the air that matters, but also having sufficient root aeration. Boron is only taken in through the root cap so an abundance of fresh root hairs is needed for optimum uptake. Sufficient media aeration is needed to produce strong root growth and fresh root hairs. Good air movement and allowing the plants to dry down in between watering is a good strategy to improve uptake.
Boron is not able to be chelated so it is more availalbe at a pH below 6.0. Calcium is antagonistic with boron so watch overapplying calcium.
Solubor (20.5%) 0.18 grams / 100 gallons = 0.1 ppm B
Borax* (11.5%) 0.33 grams / 100 gallons = 0.1 ppm B
Be sure to include any boron in the irrigation water and fertilizer when calculating how much to add. Boron does not cling to soil particles and is readily leached from the media. Therefore, is it necessary to supply boron at each irrigation. Target rate is 0.25 ppm B. Avoid supplying more than 0.5 ppm to avoid toxicity.