why would you do that??
maybe they are hydro grows & you MUST flush with hydro

Ive done both, flushed soil and no flushing.. no difference

flushing soil is a waste of time.. and nearly impossible & plus if you do that..... when you harvest your buds they will be full of even more water
the ripen nute ive used before is a 0-52-32 NPK works really good, 2 weeks before harvest, then I let it go 1 extra week or sometimes not. depends on the plant and if I think its ready or not