PURPOSES OF COVER CROPS
Cover crops take up water (transpiration) when they grow, but prevent water losses from the soil surface (evaporation) after they are killed. The live or dead cover crop will also improve infiltration by reducing sealing and crusting of an otherwise bare soil surface. Evaporation reduction after a cover crop is killed only takes place if the cover crop is left as a mulch at the soil surface, not if it is incorporated by tillage. Commonly, there is an excess of precipitation over evapotranspiration (evaporation plus transpiration) in the winter months in Pennsylvania. In most years, therefore, a winter cover crop can be safely grown without causing water shortage for the spring-planted crop. To avoid most problems with water shortage in spring, the cover crop should be killed two weeks before the main crop is planted (see Table 1.10-5). On poorly and somewhat poorly drained soils, a heavy mulch can be detrimental to the main crop because of excess water. On the other hand, live cover crops can help remove excess spring moisture from the soil. It requires careful management to make use of this principle on poorly and somewhat poorly drained soils. After killing the cover crop, row cleaners and in-row tillage methods such as zone-till and strip-till will help to dry out the soil in the seed zone, but the benefits of crop residue mulch will be maintained between rows. The need for mulch from cover crop will be greatest if the preceding crop did not leave any crop residue. Examples of such situations include spring crops planted after fall-killed alfalfa, soybeans, corn silage, or small grains that had straw removed. If a cover crop is grown in the summer, kill the cover crop in a timely manner to avoid water shortage for the following crop. To conserve water, plant cover crops that produce the most residue cover that lasts for the longest period of time.