Bougainvillea Leaves Yellow and Drop

Has developed nice blooms and looks pretty healthy, BUT ... every day now I'm picking off half a dozen or so "yellowed" leaves, and also finding some just laying on the ground at the base of the plant. They come right off with no resistance.

This yellowing indicates a micronutrient deficiency. Micronutrient problems often occur not because the soil lacks nutrients, but because, even if they are present, the soil's PH is too high or too low, making it more difficult for the roots to utilize the nutrients.

Bob Zone 4 MN