Small grains or legumes are commonly planted as cover crops in Delaware. While small grains are good at scavenging left over soil nitrogen (N) in the fall, they are not as likely as a legume cover crop to release that N early in the spring. It is helpful to remember that the C:N ratio of a cover crop can predict N availability to crops in the spring. A cover crop with a C:N < 20 at termination will be easily broken down by soil microbes and release N to the soil. In contrast, a cover crop with a C:N >30 at termination could result in tie up (immobilization) of soil N. If N is tied up by microbes, it will not be available for the emerging crop in the spring but may be available later in the season as that residue breaks down.
The timing of cover crop termination will directly affect the C:N ratio and N availability to the emerging crop. For small grain cover crops, the C:N ratio will increase past the boot stage, as more N is moved from the stem to the developing head. Therefore, earlier termination of small grain cover crops will result in more N released from the cover crop residue in the early spring. However, early termination may reduce the ability of small grain cover crops to provide other benefits, like weed suppression. So even though the C:N ratio will increase as the small grain cover crop develops, you may choose to terminate late to get these other benefits. Leaving high C:N ratio biomass on the surface (no-till) will reduce the N tie up by microbes. However, if you decide to incorporate residues with tillage, you may increase early season N tie up because the residue has more contact with soil microbes following incorporation.
Later termination of a legume cover crop has the opposite effect on soil N availability. Terminating a legume cover crop prior to bud stage (March to early April) will result in little to no N contribution from the cover crop. This is because N fixing bacteria do not have enough time to form nodules on the roots of the legume cover crop prior to bud stage. Legumes with more biomass will contribute more soil N in the spring. Termination at flowering stage will result in the most available N. A red clover or crimson clover cover crop terminated at flower stage could contribute 40-80 lb N/acre depending on the quality of the stand. If the stand is poor (<2 ton/ac), you can expect available N to be on the lower end of the scale. In contrast, a good stand (>3 ton/ac) will provide N toward the upper end of the scale. Similarly, hairy vetch, while tricky to manage, could contribute between 50-100 lb N/acre if terminated late. If termination occurs at bud stage, you can expect available N to be approximately 50% of what you could get with termination at flowering.