you can flush with soil, the process just takes 2 weeks to do. you dont fert for last 2 weeks, and only water 3x a day. u should also spritz them with the right temps outside/inside and they will absorbe the water and flush the neturts out.
You can fertilize till 2 weeks prior to harvest, I would recommend fertilizing up to this point. 2 weeks before harvest I water with a bit of molasses and homemade strawberry or pineapple concentrate.
Keep in mind if you add something with your flush you must let your buds cure.. to let the sugars and such breakdown..NO additives to the flush less to breakdown and will taste good after 4 to 5 days of drying.