This past weekend I was at a nursing home visting my mother. She was having a treatment done. While I was waiting I takled with an 86 YO, that had many years experience. We got talking about diseases. He said that most seed/soil borne diseases are in the top 1 in of soil So when he had a problem, He would scrap of the top couple inches and the bury it at least 8 in deep. FWIW Byron