Potassium bioaccumulator plants in the Pacific Northwest

I'm reading Steve Solomon's book "Growing Vegetables West of the Cascades".  I'm in the southern end of the Willamette Valley.
He says our soils are unbalanced---with too much potassium.  Apparently, I've been making my situation even worse by using the native grass hay in my compost.  And adding comfrey---a bioaccumulator for potassium..
Would it work to plant lots more comfrey and totally remove the biomass from the system?
Any other ideas?