The upper horizon of the soil is where the most nutrients are liberated. One of the most famous deep-rooted, supposed dynamic accumulators is comfrey (Symphytum officinale). (Dynamic accumulators are those plants thought to gather more of particular minerals than other plants.) When it comes to the accumulation of NPK and silica, the anecdotal opinion is that comfrey is a dynamic accumulator plant with long roots that mine minerals and nutrients from very deep in the soil. (There are reports of comfrey roots reaching as much as ten feet into the ground.) As to comfrey being a dynamic accumulator, it’s hard to find any data. I was able to find a study that showed that the immobilized tannins prepared from lateral roots of comfrey chelated (pulled out) 3.5 times more lead from the soil than those from the taproots. Yikes—comfrey as a lead-accumulator plant! Young plants have more "side-ways" roots than a deep taproot. One drawing done by "mapping" the root system by excavation it I have in my book Understanding Roots shows comfrey roots down only to 24 inches (each square is one foot) and the taproot appears to be withering. Here's another drawing of comfrey roots I have found no documented taproot down to 10 feet.
