Rotation N & Carried Forward
Plans for legume N credit
A legume only pays off if the next crop claims its nitrogen — so a rotation is really an N budget rolled forward year by year. Build a 3–5 year sequence and the planner totals the fertilizer-N offset (kg & cost), scores diversity and pest-break value, and flags risky back-to-back pairings.
Build your rotation
| Year | Crop | N received | N given to next |
|---|---|---|---|
| Yr 1 | Corn (grain) | +168 | — |
| Yr 2 | Soybean | — | 45 kg |
| Yr 3 | Winter wheat | +45 | — |
| Yr 4 | Alfalfa (good stand, ≥4 plants/ft²) | — | 168 kg |
Next: cut next year's nitrogen rate by the credit each crop receives (see the ledger) — e.g. up to 168 kg N/ha after the best legume — and bank the $10,224 saving.
N credits follow Penn State / University of Minnesota previous-crop tables (converted to kg N/ha). Credits are the maximum a following crop can deduct; soil-test and realistic yield goals still set the final rate. Risk flags follow ISU/UW rotation guidance.
Rotation nitrogen credits — key facts
- Soybean → corn
- ≈ 45 kg N/ha (40 lb/ac)
- Good alfalfa stand
- ≈ 168 kg N/ha (150 lb/ac)
- Red clover
- ≈ 90 kg N/ha (80 lb/ac)
- Hairy vetch cover
- ≈ 100 kg N/ha
- Credit applies to
- the very next crop
- Corn / cereal credit
- 0 kg N/ha
- Corn-on-corn
- high pest + yield penalty
- Offset value
- credit × area × $/kg N
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A rotation is a nitrogen budget rolled forward
A legume fixes nitrogen, but that nitrogen is only worth money if the crop that follows it claims the credit and you cut its fertilizer rate to match. That is the difference between a sequence-only rotation planner and this one: it rolls each crop's previous-crop nitrogen credit forward into the next year's fertilizer offset, totals it across the whole cycle, and prices it at your nitrogen cost. Place a high-credit legume — good alfalfa, hairy vetch or red clover — immediately before a heavy nitrogen feeder like corn, and the credit lands where it pays.
At the same time, a good rotation breaks pest and disease cycles. Growing the same crop family two years running builds rootworm, cyst nematode, take-all or clubroot and forfeits the legume credit, so the planner scores diversity and pest-break value and flags risky back-to-back pairings such as corn-on-corn or soybean-on-soybean. The result is a single view of the two reasons rotations pay: the nitrogen carried forward and the pest cycles broken. Pair it with the Crop Rotation Planner, Green Manure and Crop Calendar tools for a complete plan.
Previous-crop nitrogen credits
Credit the FOLLOWING crop receives, kg N/ha. Source: Penn State Extension & University of Minnesota previous-crop N-credit tables (converted from lb/ac).
| Crop | Family | N credit to next crop | Note |
|---|---|---|---|
| Corn (grain) | grass cereal | — | Heavy N feeder; leaves no N credit. |
| Soybean | legume | 45 kg N/ha | ≈ 40 lb N/ac credit to next corn (UMN/Penn State). |
| Winter wheat | grass cereal | — | Cereal break crop; no N credit but disrupts row-crop pests. |
| Alfalfa (good stand, ≥4 plants/ft²) | legume | 168 kg N/ha | Full stand ≈ 150 lb N/ac credit (Penn State). |
| Alfalfa (fair stand) | legume | 112 kg N/ha | Fair stand ≈ 100 lb N/ac credit. |
| Red clover | legume | 90 kg N/ha | Established red clover ≈ 80 lb N/ac credit. |
| Oats | grass cereal | — | Small-grain break crop; no N credit. |
| Canola | brassica | — | Brassica break crop; strong disease break for cereals. |
| Sunflower | broadleaf row | — | Broadleaf row crop; deep-rooted break. |
| Dry edible bean | legume | 34 kg N/ha | Modest legume credit ≈ 30 lb N/ac. |
| Field pea | legume | 45 kg N/ha | Pulse credit ≈ 40 lb N/ac. |
| Cover: crimson clover | cover legume | 78 kg N/ha | Winter legume cover ≈ 70 lb N/ac if well grown. |
| Cover: hairy vetch | cover legume | 100 kg N/ha | High-biomass legume cover ≈ 90 lb N/ac. |
| Cover: cereal rye | cover grass | — | Scavenges N, suppresses weeds; can tie up N short-term. |
| Fallow / bare | grass cereal | — | No crop; minimal pest break, no N credit. |
Risky back-to-back pairings
Consecutive pairings that build pest, disease or weed pressure. Source: Iowa State & University of Wisconsin rotation guidance.
| Pairing | Risk | Why |
|---|---|---|
| Corn (grain) → Corn (grain) | high | Corn-on-corn builds corn rootworm, gray leaf spot and residue pressure; expect a continuous-corn yield penalty. |
| Soybean → Soybean | high | Soybean-on-soybean raises soybean cyst nematode, sudden death syndrome and white mold. |
| Winter wheat → Winter wheat | high | Wheat-on-wheat increases take-all, Fusarium head blight and Hessian fly. |
| Canola → Canola | high | Brassica-on-brassica builds clubroot and blackleg — keep ≥3 years between canola. |
| Sunflower → Sunflower | high | Sunflower-on-sunflower builds Sclerotinia and downy mildew. |
| Dry edible bean → Soybean | medium | Two legumes back-to-back share white mold and root-rot hosts. |
| Red clover → Alfalfa (good stand, ≥4 plants/ft²) | medium | Legume-on-legume limits the N-credit reset and shares root diseases. |
How to use it — 5 steps
- 1Set the rotation length. Choose a 3, 4 or 5 year cycle to match how you plan your fields.
- 2Pick a crop per year. Select a cash crop, legume or cover crop for each year from the catalogue.
- 3Enter area and N price. Add your field area in hectares and your fertilizer-N cost per kilogram.
- 4Read the credit and cost. See the fertilizer-N offset in kg N/ha and the dollar saving per cycle, plus the diversity score.
- 5Fix the flags. Break any high-risk back-to-back pairing, then deduct each year's credit from next season's fertilizer plan.
Frequently Asked Questions
How much nitrogen credit does soybean give the following corn?+
About 45 kg N/ha — roughly 40 lb N/ac — is the standard previous-crop credit for corn following soybean used by University of Minnesota and Penn State. The credit reflects the more available residual N and the rotation effect, not biological fixation alone. In the planner, place soybean before a non-legume to see that credit roll forward and reduce the next crop's fertilizer-N rate.
What is a previous-crop nitrogen credit?+
It is the amount of fertilizer nitrogen you can subtract for next year's crop because of what grew this year. Legumes such as soybean, alfalfa and clover leave residual and biologically fixed N, so the following crop needs less purchased N. The credit is the maximum deduction; your final rate still comes from a soil test and a realistic yield goal.
How much nitrogen credit does a good alfalfa stand give?+
A full alfalfa stand — about four or more plants per square foot — credits roughly 168 kg N/ha (around 150 lb N/ac) to the first crop after termination, with a smaller residual credit in the second year. A fair stand gives about 112 kg N/ha. The planner uses the Penn State stand-based figures and rolls them into the next year's offset and cost.
Which rotation maximizes legume nitrogen credit?+
Sequences that place a high-credit legume — good alfalfa, hairy vetch or red clover — immediately before a heavy nitrogen feeder like corn capture the most credit, because the credit only counts for the very next crop. Spreading legumes through the rotation and avoiding two legumes back-to-back keeps both the N credit and the pest break high. Build a few sequences in the planner and compare the total kg N/ha per cycle.
Why is corn-on-corn flagged as risky?+
Growing corn after corn builds corn rootworm, gray leaf spot and a heavy residue load, and typically carries a continuous-corn yield penalty versus rotated corn. It also forgoes the soybean or legume nitrogen credit. The planner flags any corn-to-corn step as high risk and suggests breaking it with a different crop family.
How does the planner score rotation diversity?+
It combines how many different crop families appear in the sequence, the pest-break value of each crop, and whether legumes are present, then subtracts a penalty for risky back-to-back pairings such as corn-on-corn or soybean-on-soybean. The result is a 0–100 score: above 80 is excellent, 55–80 good, 30–55 fair, and below 30 a near-monoculture that will build pest and disease pressure.
Do cover crops count toward the nitrogen credit?+
Yes — a well-grown winter legume cover crop credits the cash crop that follows it. Hairy vetch can credit around 100 kg N/ha and crimson clover around 78 kg N/ha when biomass is high and it is terminated near flowering. A cereal-rye cover gives no N credit and can briefly tie up nitrogen, but it still scores for weed suppression and erosion control in the diversity score.
Does the credit only apply to the very next crop?+
The largest part of a legume credit applies to the first crop after the legume, which is why the planner assigns each crop the credit given by the crop immediately before it in the sequence. Long-lived legumes like alfalfa do leave a smaller second-year residual, but for planning the first-year credit is where the savings are, so place your nitrogen feeder right after the legume.
How is the dollar saving calculated?+
The planner sums the nitrogen credit each crop receives across the cycle to get the total fertilizer-N offset in kg N/ha, multiplies by your field area in hectares, and multiplies by your fertilizer-N price per kilogram. So a 213 kg N/ha cycle offset over 40 ha at $1.20/kg N saves about $10,224 per cycle. Adjust the price to your fertilizer cost for a current figure.
Can I plan a 3, 4 or 5 year rotation?+
Yes. Choose the number of years, then pick a crop for each year from the catalogue of cash crops, legumes and cover crops. The planner treats the rotation as a repeating cycle, so the last year's credit also feeds back into year one — exactly how a real rotation behaves once it is established.
Why avoid two legumes back-to-back?+
Two legumes in a row share root diseases such as white mold and root rots, and the second legume cannot use much of the first one's nitrogen credit, so the credit is partly wasted. The planner flags pairings like soybean followed by dry bean as medium risk. Separating legumes with a cereal or broadleaf break crop preserves both the disease break and the usable credit.
How accurate are the nitrogen credits?+
They are solid planning figures from published Penn State and University of Minnesota previous-crop tables, converted to kg N/ha. Real credits vary with legume stand, biomass, termination timing, soil and weather, so treat the result as a budgeting estimate. Confirm with a soil nitrate test where one is available, and use the planner to compare rotations rather than to set an exact rate.