According to the herbalist Dr. John R. Christopher: “Yellow dock is considered to be the most medicinal variety of the dock family. The roots of yellow dock have been found to contain as much as 40 percent iron compounds…”
But iron needs to be in the soil for it to end up in a plant. I imagine the area your trees are in, is deficient. Perhaps you may need to grow your dock someplace else and let the dug up roots decompose under the trees, at least for a while. Or bury some rusty nails under the trees where the dock will grow.