Maybe it was used for giving Iron to a tree's diet, But never ever do that with an oak tree, The oak tree will die from it. We had a rather large red oak fall on it's own on a clear day, no wind. Half of tree was dead, other side had leaves. When it fell, it whacked a couple houses to the south of it. Insurance company claimed it act of God. When the main trunk was being sawed, interesting black marks were found. Sawmill told us that someone had put nails in it at one time, so out came the detector, and I found 3 iron nails evenly spaced every foot, for about 40 feet. Some kid, when the tree was only 40 years old had apparently built a tree house in it, and made steps to climb up and used steel or iron nails to attach said steps. The rings were counted and the tree was 106 years old, it was about 5 ft. in diameter. When an oak tree calls for sap for the leaves from the roots, the sap will travel up and get to the nails, then it will turn around and go back to the roots, allowing that part of the upper part of the tree to die. By the way, when I mentioned the black marks to the sawmill guy, he immediately refused to buy it. The main trunk line was free of branches for the first 50 feet, and would've made lovely furniture.......NGE