Definition of mushroom compost in US English:

mushroom compost

noun

  • A compost prepared as a medium for growing mushrooms, typically a mixture of manure and straw with added nutrients; (also) such compost after mushrooms have been grown in it, used by gardeners as a soil improver (more fully "spent mushroom compost").

Origin

1920s; earliest use found in Annals of the Missouri Botanical Garden.