compost UK /ˈkɒm.pɒst/US /ˈkɑm.poʊst/noun 3verb 1forms 3 noun3 •The decayed remains of organic matter that has rotted into a natural fertilizer.Dig plenty of compost into clay or sandy soil to improve its structure. •(UK) A medium in which one can cultivate plants.Once the seed tray is filled with compost, insert the seeds spaced 3 cm apart from one another. •(no longer used) A mixture; a compound.A sad compost of more bitter than sweet. — 1660 verb1 •To produce compost, let organic matter decay into fertilizer.If you compost your grass clippings, you can improve your soil. formscomposted · composting · composts