Cover Crop Nitrogen & Release or Tie-Up, Week by Week

It depends on biomass, %N and especially the C:N ratio. The residue N is biomass × %N — for example 4,000 kg DM/ha of hairy vetch at 3.5% N holds about 140 kg N/ha. Of that, a low-C:N legume nets roughly 40–70% as plant-available N over a season, while a high-C:N grass can net negative (it ties up N) before any release. This tool computes the net for your exact species, biomass and soil temperature.

About 24:1. Below it the residue is N-rich relative to what decomposer microbes need, so they release surplus N as plant-available nitrate and ammonium (net mineralization). Above it the microbes must scavenge extra mineral N from the soil to build their own biomass, so soil N drops first (net immobilization, or "tie-up"). That single number decides whether your crop gets the N early or has to wait.

For high-C:N residue (mature cereal rye, sorghum-sudangrass) the early weeks are dominated by immobilization — microbes pull soil nitrate down to decompose all that carbon — so the cumulative net N goes negative before it climbs back. The chart shows that dip in red. If your cash crop needs N during the dip, it will go short unless you add a small starter N.

Yes, when terminated late. At heading, rye runs a C:N near 36:1 with low %N, so it net-immobilizes for several weeks — this is the classic 'rye nitrogen penalty' in corn. Terminate earlier (lower C:N), plant into it with a starter N of about 20–30 kg N/ha, or pair rye with a legume to lower the blend C:N.

Terminate near early bloom when biomass and N are high but the residue is still soft, then plant the cash crop so its peak N uptake lands a few weeks later as the residue mineralizes. The tool's uptake-week input shades that window over the release curve so you can match the two — that synchrony is what turns residue N into a real fertilizer credit.

Mineralization is microbial, so it roughly doubles per 10 °C (a Q10 of about 2). Warm soil releases the N — and resolves any tie-up — faster; cold spring soil slows everything down, which is why an early-killed cover may not have released much by planting. The calculator applies that temperature factor to the release rate.

It overlaps but is richer. A flat legume-N-credit gives one number; this tool models the C:N-driven timing — how much, and crucially when — including the immobilization dip a grass or grass-legume mix can cause. Use it when termination timing and synchrony matter, not just the season total.

Often 30–100 kg N/ha for a vigorous, well-terminated legume, depending on biomass. Read the 'credit at crop uptake' figure here, then side-dress only the remaining gap to your crop's N requirement. Confirm with an in-season soil nitrate test before cutting deeply.

Yes — tillage radish and mustard have low-to-moderate C:N (about 18–22:1) and winter-kill, so they decompose and release N early in spring, often well-timed for an early cash crop. They also scavenge deep nitrate and recycle it, reducing leaching losses.

More biomass means more residue N, but the C:N still governs the fraction released. A legume at 3,000–5,000 kg DM/ha typically supplies a meaningful credit; a grass at the same biomass may tie up N instead. Enter your measured or estimated biomass — the residue N scales directly with it.

It means that over the window the residue, on balance, removed more mineral N from the soil than it released — immobilization dominated. Your crop will be short of N during that period. Either lower the residue C:N (terminate earlier, add a legume) or supply starter N to bridge the tie-up.

It is a sound planning model built on published biomass, %N and C:N ranges (SARE, Penn State, USDA-NRCS) and a first-order temperature-driven release. Real release varies with rainfall, soil biology and residue placement, so treat the figures as a working estimate and verify with a soil nitrate test before large reductions.