Comfrey is a dynamic accumulator that mines subsoil minerals and makes them available.
Contested · none if ignored → inert
What is actually known
Symphytum has deep roots and produces large potassium-rich biomass that decomposes fast. That is a good mulch plant. The specific mineral-mining claim has very little behind it.
Comfrey makes excellent chop-and-drop mulch. The mineral-mining story is unsupported.
The evidence
Evidence: confirmed against 1 source in the research-based literature.
the source
Cornell Small Farms Program (Cornell Coop. Extension), 'New Findings Further the Study of Dynamic Accumulators' (Tyler & Zarro, 2022), smallfarms.cornell.edu/2022/04/new-findings-further-the-study-of-dynamic-accumulators (read 2026-07-11): an on-farm study 'has shown Russian comfrey to be a dynamic accumulator of both potassium and silicon' (tissue concentrations above the set thresholds). NOTE: this establishes high nutrient CONCENTRATION in comfrey biomass (supporting the 'potassium-rich biomass / good mulch' point), not the deep-root subsoil-MINING mechanism the rule refutes — accumulation reflects uptake from available soil, not sourcing of otherwise-inaccessible subsoil.