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Wiktionary
green manure

n. A crop that is ploughed into the soil to improve its fertility

WordNet
green manure

n. a growing crop that is plowed under to enrich soil

Wikipedia
Green manure

In agriculture, green manure is created by leaving uprooted or sown crop parts to wither on a field so that they serve as a mulch and soil amendment. The plants used for green manure are often cover crops grown primarily for this purpose. Typically, they are ploughed under and incorporated into the soil while green or shortly after flowering. Green manure is commonly associated with organic farming and can play an important role in sustainable annual cropping systems.