Legumes are the best choice, giving you the most bang for your buck. Legumes include crops like beans, peas, clover, vetch and alfalfa. Since nitrogen is readily used and released from the earth, most soils are deficient in this macronutrient at some point.
When inoculated with species-specific Rhizobial bacteria, legumes form nodules on their roots that fix nitrogen from the atmosphere. Where other types of plants and cover crops continue to mine the soil of organic matter and nutrients and take it away, legumes add back, creating a fertile, healthy soil.
Always inoculate your legume seeds, or make sure they are inoculated with the proper strain of bacteria when purchasing. Most seed and feed stores will have the proper bacteria for purchase, or you can buy online. Good choices for winter cover include Austrian Winter Pea, Crimson Clover, Red Clover, Arrowleaf Clover, Hairy Vetch and Fava Bean.
Root crops for winter cover include turnips, radishes and beets. Root crops penetrate deeply into soil, bringing unusable nutrients to the surface for use and breaking up plow pans and compacted soil. When allowed to rot in place, these root crops add an abundant amount of organic matter to the soil.
For heavily compacted soils, winter root crops like tillage radishes are a good option. If you need a ground-buster for summer, try deep-rooted sunflowers to pierce hard pans.
The most popular cereal crops for winter cover include wheat, rye, barley and oats. Cereal grains form massive root systems and top growth, creating an abundance of biomass. These roots can infiltrate a tremendous amount of soil but also may rob it of nutrients. They require a tremendous amount of nitrogen, as opposed to inoculated legumes, which make their own nitrogen.For the gardener practicing modified no-till, I don’t recommend these crops unless planted with an inoculated legume like clover or winter peas.