These plants have long, deep taproots that help to break up hard subsoil.  Their roots accumulate minerals from deep soils and make them available to surrounding plants when they are "chopped and dropped" for use as a mulch.

Here are some good nutrient accumulators to plant:

Dandelion

Yarrow

Chicory

Fennel

Borage

Comfrey

Lambs's Quarters

Alfalfa