Dumping Charcoal in my Garden

Wood ashes raise pH because of the potassium hydroxide in them. Blueberries require low pH (equals acid soil), so using wood ashes on blueberries pushes the pH in whe wrong direction unless you are starting with extraordinarily acid soil.

The big problem with using charcoal ashes from a grill is the likelihood of meat grease being in the ashes. Animal fat in gardens causes problems in microbial balance and can attract pests from ants to four-legged mammals.

Wood ashes are beneficial for onions provided the ashes are worked into the ground well before the onions are planted. Onions like a fairly high pH and like high potassium level in the soil.

Tropical soils are very different from soils in temperate climates. Purdue was involved is swap programs of students and faculty with a university in Minas Garais, Brazil when I was a grad student. Some of the differences reported were eye-opening. The biggest soil difference is that temperate-zone soils contain most of the nutrients with a fairly small proportion in the active biomass. In contrast tropical soils have very few nutrients while a very high percentage of the nutrients are in the living biomass. This is why slash and burn works but only for a short time.