castings n.pl.— «The worm’s waste materials, delicately referred to as “castings,” or, indelicately, as “poop,” are eaten by microbes and fungi, allowing them to grow and reproduce and sequester minerals from the soil...
fairy ring n. a naturally occurring ground circle caused by fungi or other biological agents; figuratively, a fanciful ring or circle of mystical or unusual behavior or action. (source: Double-Tongued Dictionary)