As long ago as 1927 Pieters, in his comprehensive treatment of green manures, stated that "Green manuring is the practice of enriching the soil by turning under undecomposed plant material (except crop residues) either in place or brought from a distance." He further stated "A cover crop is one planted for the purpose of covering and protecting the soil." The Soil Science Society of America (1987) defined green manure as plant material incorporated into the soil while green or at maturity, for soil improvement.

Cover crops are any crops grown to produce soil cover, regardless of whether they are later incorporated. They are used to cover and protect the soil surface, although they may be turned under as green manures. Further, the term cover crop also refers to crops grown between orchard trees or on fields between cropping seasons to protect the land from leaching and erosion (Martin 1975). Diver and Sullivan (1992) wrote: "Any field or forage crop grown to provide soil cover is a "cover crop." Since a crop grown as a cover crop may later be soil-incorporated as a green manure, the two practices are often referred to interchangeably.

- H David Thurston

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