Cover Crop Nitrogen Credit

Legume cover crops fix atmospheric nitrogen into a form the next cash crop can use, and this calculator estimates how much of that nitrogen credit you can bank against your fertilizer plan. Each species carries a maximum nitrogen credit at full, typical biomass — hairy vetch tops the list at around 100 lb N/acre, cereal rye, a non-legume scavenger rather than a fixer, caps out much lower at 20 lb/acre since its main contribution is recycling nitrogen already in the soil rather than adding new nitrogen from the atmosphere. Your actual biomass relative to that species' typical biomass scales the credit up or down, and termination timing applies its own multiplier, since terminating early (pre-bloom) captures only about half the eventual credit while letting the crop grow to post-bloom maturity captures the full amount.

The calculator then splits that total credit into what becomes available in the first season versus what carries over into year two, since legume residue doesn't release all its nitrogen at once — the model assumes roughly half of a legume's credit and about a fifth of a non-legume's shows up in year one. C:N ratio flags immobilization risk: residue with a carbon-to-nitrogen ratio above roughly 25:1, like mature cereal rye, can temporarily tie up soil nitrogen as microbes decompose the high-carbon material, working against the crop that follows rather than for it. Because these figures are drawn from typical species behavior rather than a tissue or soil test of your specific field, treat the nitrogen credit as a planning estimate to adjust fertilizer rates against, not a substitute for direct soil nitrate testing before planting.