"For vegetable crops with a high demand for B like cauliflower or broccoli grown on sandy soils testing low in B, 2 to 4 lbs of B per acre should be broadcast and incorporated before planting.
Boron is mobile in soils and should be applied to annual crops each year when needed. This nutrient should not be applied directly to actively growing green tissue because serious plant injury could occur.
Boron fertilizers should never be applied directly in contact with the seed. Broadcast application of B is recommended over the use of an in-row treatment. Broadcast applications are safer when applied one to two weeks before seeding.
Boron applied to a perennial crop such as alfalfa will usually last for more than one year. A common practice, on known B-deficient soils, is to use a borated fertilizer mixture once every three years.
It is better to withhold B from a new seeding of alfalfa until after the first year of production, if oat is the companion crop. Oat is sensitive to rates of B needed for alfalfa."