Those brown blotches under an oak tree aren't dirt — they're tannins leaching out of the leaves

Tannins are naturally occurring compounds in bark, leaves, and acorns, and they don't stay put once the plant material gets wet. Leave a scatter of oak leaves or acorns on a driveway after rain and the tannins leach out into that puddled water, which then soaks into the concrete and leaves a brown blotch behind once it dries — right where the leaves sat.

It's an easy stain to misdiagnose as plain dirt, because it looks like a dark patch and a regular rinse does lighten it a little. But the tannin has actually bonded into the porous surface of the concrete, not just settled on top of it, so soap, a garden hose, or a standard pressure wash mostly just clears the loose debris around it and leaves the discoloration underneath. Lifting it back out takes an acid-based treatment made for tannin staining, applied correctly to the surface.

That's also why it's a different fix than the other brown and white stains a Florida driveway picks up — rust streaks come from dissolved iron in irrigation water, and efflorescence is salts migrating out of the concrete itself. All three can look similar at a glance, but each needs its own chemistry to actually come out.