If you put old pieces of metal in the ground and then plant over them is it beneficial or harmful to, say, trees?
Certified GKH Gardening Expert
This depends on the metal! Iron, or rust in the soil can help as an acidifier, and to put iron back in the soil. Something like copper, or aluminum however, can be fatal to the plant depending on how much it breaks down in the soil, and how much the plants actually take up through their roots. As a general rule, I would not recommend putting metals into the soil in this fashion.