Im no expert at all not even close so your mileage may vary.........
However from the information I have read dark and/or chunky ashes are due to the dry and cure, not flushing.
I think the ash is about the moisture content and getting a "complete" burn.
I have seen this myself. If I smoke a sample flower while its still a bit on the wet side I get an incomplete burn and dark chunky ash.
If I smoke the same flower after a full proper dry and cure the burn is more complete. The ash is soft and crumbly, grey to white in color.
I don't believe it's possible to flush nutrient content out of a plant. The way I understand it there's not raw NPK just sitting around inside a plant.
Do they flush the higly valued tobacco varietals used to make high end expensive cigars?