Do Comfrey and Other Plants Grab Nutrients Deep in Our Soil?

In permaculture circles, they're called dynamic accumulators -- plants that accumulate above average levels of particular nutrients in their leaves, which we can then add to compost piles, use as mulch or in other ways make the nutrients available to our plants. Do they work? NEW FROM Empress of Dirt: How to Grow Leafy Greens Indoors at Home (Simple & Fast Method)