Many years ago when I bought my place I had very little money, but tried this and it worked. A fellow gave me a large spool that wire comes on. Wood had rotted and left the very large galvanized metal center.
I drilled holes in large stumps, poured used motor oil in them. I put spool over stump with a few bricks on bottom for airflow. Then some wood on stump and lit it.
It burned for a week, finally you could see roots burning underground.