PURPOSES OF COVER CROPS
When crops have been fertilized with artificial fertilizer, manure, or compost, or when a leguminous crop has been killed in the fall, large amounts of nitrate are released in the fall and winter. Nitrates released in the winter can leach below the root zone and pollute groundwater resources. In the summer, nitrate leaching is less likely because little precipitation percolates through the soil profile. For farmers, leached nitrate represents a loss of plant nutrients for the following crop. Cover crops can be used to capture the nitrogen released in fall and winter, protecting it from leaching and making it available to the following crop when the cover crop decomposes. It is best to plant a winter-hardy cover crop to capture nitrates from the root zone. In Pennsylvania, a cover crop that dies in the winter (such as oats) does not adequately protect nitrates from leaching. The more growth produced by the cover crop, the higher the amount of nitrate it takes up. The best crops for this purpose are rye, wheat, barley, and ryegrass. Little nitrogen will be released by these cover crops in the following season if they are killed late because decomposition is too slow. If the C:N ratio is above 30, the dead cover crop may actually cause nitrogen deficiencies in the following corn crop. This is because microbes need extra nitrogen to be able to decompose the cover crop residue. The nitrogen deficiency can be overcome by applying extra nitrogen fertilizer or manure.