I agree about amending the soil, and that the fertilizer you first mentioned seem really strong. BGI has a "Hibisgain" formula for hibiscus ats way more balanced I thnk something like 12-6-8. Since hibiscus like to be feed and your soil is sandy,regular feeding is a good idea, since frequent irrigation will wash fertilizer through the soil. BGI's formulas are slow release too, so that's a plus. I've not actually used their hibiscus formula, but have been a regular user of their signature product "Bougain" on my potted bougainvilleas here in TX(I'm an OC transplant, trging to grow SoCal tropicals in zone 8 north TX).Good luck!
