Grasses
Grasses perform numerous functions when used as cover crops: preventing erosion, inhibiting weed growth, increasing organic matter in soil, and nitrogen retention. Common species of grasses to use as cover crops include oats, rye, and wheat.
Sustainable Agriculture Research and Education (SARE) warns against the indiscriminate use of grasses, writing:
A problem common to all the grasses is that if you grow the crop to maturity for the maximum amount of residue, you reduce the amount of available nitrogen for the next crop. This is because of the high C:N ratio (low percentage of nitrogen) in grasses near maturity, which ties up nitrogen when decomposing after termination, especially when plowed under. This problem can be avoided by killing the grass early or by adding extra nitrogen in the form of fertilizer or manure. Another way to help with this problem is to supply extra nitrogen by seeding a legume-grass mix.
A wonderful quality of many grasses is their ability to germinate and grow later in the season than nearly any other type of plant. This allows the grasses to improve soil and trap moisture when nearly all else is lying dormant.
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