More and more farmers are looking at cover crops as a way to improve their soils and reduce erosion. There are many published studies on how cover crops can take up excessive nitrogen from the soil, as well. However, adopting this technology first requires a clear understanding of your fields and growing conditions, as well as the impact cover crops have on nutrients in the soil.
Perhaps one of the first things to consider is local precipitation levels. Cover crops require water to grow, and water they use from the soil is water that is unavailable for the next crop you grow. If you live in a dry area, it is important to determine if groundwater levels can support another crop other than your cash crop.
Another point of consideration is how much nitrogen remains in your field once harvest of your cash crops is complete. Several types of cover crops uptake excess nitrogen and combine it with carbon, replenishing the soil with needed nutrients as it decomposes. However, some types of cover crops leave a residue with a high carbon/nitrogen ration, which can cause microorganisms in the soil to increase rapidly. Those microorganisms then use the nitrogen in the soil, which can cause a shortage of the nutrient for a young cash crop plant. A Farm Journal field agronomist conducted a study about this carbon penalty, the results of which can be found here.
The type of cover crop that you plant is also important. If your intent is to scavenge excess nitrogen, grasses are the best choice. However, if you choose ryegrass, be aware that it will overwinter and will have to be killed early in the spring to manage it well once cash crops are in the ground. If your intent in planting a cover crop is to supply nitrogen, choose a legume. Be aware, though, that many legumes require adequate moisture to ensure good growth, so your geography and precipitation averages become critical with this choice. If you are just starting to use cover crops in your soil management regime, there are good choices for you to experiment with, such as radish.
Clearly, there are many considerations when choosing a cover crop. A great resource to use in determining what may work best for your farm is the Midwest Cover Crop Council. Cover crops certainly have their place in nutrient and soil management. Aside from reducing erosion and improving soil quality, they can help retain nutrients in the soil, help combat weeds that may spread during the winter, and assist in breaking disease cycles.