Rotation N & Carried Forward

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.