I'm going with 'Figure of Speech'. I use similar wording myself and even had the guy I buy my garden soil use it today. We were talking about Huegelkulture [[a form of gardening in raised beds where you fill the bottom of the bed with logs, wood, limbs and yard waste-it composts in place and is turned every year or two.]] Anyhoo, he mentioned it will break down and sink. Told him I knew and was planning on having a stockpile of dirt to fill in as it happned and he said "Yeah, but the larger stuff will likely be breaking down long after we are both gone."