We consider green manure crops and cover crops as two distinctly different things with different purposes. Green manure is a crop, usually with a high percentage of legumes, that is grown specifically to improve the soil’s structure and fertility. Cover crops are planted to hold soil, retain nutrients and control weeds in between annual crops. Green manure is planted, usually for a period of one year or more, to replenish the soil’s reserve of nutrients through the fixation of nitrogen from the atmosphere and the scavenging of mineral from soil resources. Experiments have shown that some legumes, such as hairy vetch, can produce a significant amount of nitrogen on a short-term, cover crop basis. However, most green manures require a longer period of time to produce the amount of organic matter lost to row crop cultivation. Both of the following mixes can be grazed (by ruminants) or mowed and should be before plowing down.
Green manure mixes
For spring planting: Canadian Field Peas, Hairy Vetch, and Cover Crop Oats
Plant this mixture in the spring and allow it to grow through the entire season. All three varieties should winter-kill. Plow down the following spring.
For fall planting: Altaswede and Yellow Blossom Clovers
Plant this mixture late summer/early fall and allow it to grow for twelve months. Plow down the next fall.