If you can run a disc across it and then get a spike tooth chain drag and run over it several times it will do a lot of good. It needs to rain on the area and let it settle before you are finished.
A football field is over an acre and it takes a lot of work to clear and get into planting shape. I'm working with an old family cemetary expanding the burial area. We cleared a little over two acres of trees, brush, etc and had it burned in a pit. It cost $10,000 just to get that part done, and that was the cheapest bid we could get.