How much nitrogen for corn after soybeans (MRTN explained)

What MRTN is and why it replaced yield goals

MRTN is the N rate at the top of an average economic-return curve built from many N-rate trials in a state or region. For each trial, researchers fit a yield response curve and calculate return to N (yield gained from N times corn price, minus N applied times N price). Those curves are averaged by rotation and region, and the rate at the peak is the MRTN. Because the curve is flat near the top, the calculator also reports a range of rates whose return is within $1.00/ac of the maximum: "typically about 15 pounds of N on either side" in Illinois' description, 10 to 25 lb N/ac in Iowa State's, with yields within about 2% of maximum.

The approach grew out of a regional effort by Iowa, Illinois, Indiana, Michigan, Minnesota, Ohio and Wisconsin; each state maintains its own trial database inside the calculator, and the state menu shows the current participants. It replaced yield-goal formulas, which Illinois notes had been in use for about three decades, with a direct economic answer from measured yield responses.

Typical profitable ranges

The table shows published MRTN values and ranges for corn after soybean and corn after corn. They differ by state because the trial databases differ, and they move with prices, so treat them as orientation rather than a prescription.

Illinois notes that the gap between the two rotations is not constant: a little more than 40 lb N in northern Illinois, a little less than 30 lb N in central Illinois and only 10 to 12 lb N in the south.

The price ratio

The calculator asks for an N price and a corn price and works with their ratio: dollars per pound of actual N divided by dollars per bushel. N at $0.40/lb and corn at $4.00/bu gives 0.10, which is close to the long-run norm. A higher ratio (expensive N, cheap corn) lowers the MRTN; a lower ratio raises it.

The effect is moderate. In Minnesota's corn-after-soybean table, moving from a 0.075 ratio to 0.15 drops the MRTN from 155 to 135 lb N/ac. A University of Minnesota example from October 2025 used anhydrous at $850/ton ($0.518/lb N) and corn at $3.76/bu; with a nitrification inhibitor added the ratio reached 0.164 and the calculator returned 133 lb N/ac (121 to 143) at roughly 98% of maximum yield.

To get a price per pound of N, divide price per ton by the pounds of N in a ton: 920 lb for urea (46% N), 560 lb for UAN 28, 640 lb for UAN 32 and 1,640 lb for anhydrous ammonia (82% N).

The soybean credit is already inside the number

Older yield-goal recommendations subtracted a legume credit from a base rate. MRTN rates do not work that way. Trials for corn after soybean and corn after corn were run in separate fields, so the rotation effect is built into each database. Iowa State's guidance is direct: the calculator's rate for corn after soybean "should not be further modified to account for having a legume in the rotation." Subtracting a credit on top of an MRTN rate double-counts the rotation and leaves the crop short. The benefit also does not scale with soybean yield; what matters is that soybean was the previous crop.

Adjustments: manure, sandy soils and wet springs

Manure. Run the calculator with commercial fertilizer prices, then subtract the first-year available N from manure, which comes from a manure analysis and your state or provincial availability factors. Minnesota publishes plant-available N targets for manured fields (for example, 150 lb N/ac total for non-irrigated corn after soybean) and advises not exceeding them when manure and fertilizer are combined.

Sandy and irrigated soils. Coarse soils leach nitrate and usually have separate guidelines. Minnesota's irrigated sandy-soil table gives 210 lb N/ac (190 to 225) for corn after corn at a 0.10 ratio, with 30 lb N/ac less after soybean. Non-irrigated loamy fine sand with low organic matter gets much lower rates because yield potential is low. Check your state or provincial guide for the right table.

Wet springs. The MRTN is a preplant estimate; loss after application is a separate problem. Iowa State treats preplant rates as approximations that may need in-season adjustment, with soil nitrate testing to guide it. Where losses vary a lot with weather, Minnesota suggests limiting N applied before and near planting and holding back 30 to 40 lb N/ac for an in-season application if needed.

Ontario's approach

Ontario does not use the MRTN databases. OMAFRA's Ontario Corn Nitrogen Calculator, available as a spreadsheet through Field Crop News, builds a rate from a base value set by soil texture and region (southwestern and central Ontario versus eastern Ontario), then adjusts for expected yield (0.76 lb N/ac per bushel, or 13.7 kg N/ha per tonne), heat units (3.7 lb N/ac, or 4.1 kg N/ha, per 100 CHU above or below 2,800) and the N-to-corn price ratio. The previous crop is an explicit deduction: soybeans and dry edible beans carry a credit of 30 kg N/ha (27 lb N/ac), clover and legume forages much more. Starter and manure N are deducted from the total, with manure credits estimated separately through OMAFRA's AgriSuite organic amendment tool. The calculator also reduces the rate when N is sidedressed at V4 to V6 in southwestern and central Ontario (100% of the preplant rate on sands and loamy sands, 90% on other sandy textures, 80% on all other textures), but gives no sidedress credit in eastern Ontario.

The practical difference for a Canadian reader is that the Ontario tool asks you to subtract the soybean credit yourself, while the US calculator has already done it.

Worked example: lb N to product

Suppose the calculator returns 140 lb N/ac (157 kg N/ha) for corn after soybean. Converting to product:

For the UAN gallons, divide pounds of product by the weight per gallon (about 2.98 lb N per gallon of UAN 28 and 3.54 lb N per gallon of UAN 32). Starter N, herbicide-carrier UAN and first-year manure N all count against the total before you convert.

The range matters more than the point. If the calculator shows 128 to 152 lb N/ac, well-drained ground with low loss risk can reasonably sit near the lower end, while poorly drained ground getting everything preplant may justify the upper end. Either choice is inside the data; a rate well above the range is not.