Sidedress nitrogen on corn: timing, rates and tools

What the decision depends on

Four things set the sidedress rate and timing:

1. How much N is already in the field. Preplant, starter, herbicide-carrier UAN and manure all count toward the total planned rate from the Corn Nitrogen Rate Calculator or your provincial calculator.
2. How much of that N was nitrate when the rain came. Ammonium holds on the soil exchange; nitrate leaches or denitrifies. Fall anhydrous and manure ammonium are nearly all nitrate by June, early-April preplant N is mostly converted, anhydrous applied within the last two weeks is still mostly ammonium, and UAN starts out about one-quarter nitrate.
3. Soil texture and drainage. On silt loam and clay loam soils the loss pathway is denitrification; on sands it is leaching, and the losses are larger.
4. Crop stage and equipment. A conventional toolbar fits until about V8. Beyond that you need high-clearance equipment with drop tubes or coulters.

Splitting N is a hedge against item 2: apply part of the rate near planting and set the rest at sidedress, using the test or the loss estimate below. Ontario's corn N calculator goes further and reduces the total when N is sidedressed at V4 to V6 on most textures in southwestern and central Ontario.

Step-by-step

1. Decide whether to test or to estimate. The LSNT is most useful on manured fields, corn after alfalfa and fields with little or no preplant N. On fields that received a full preplant rate followed by heavy rain, a loss estimate is often more practical.

2. If testing, sample correctly. Pull cores to 12 inches when corn is 6 to 12 inches tall (about V5 to V6, usually late May to early June). In a late-planted year, Iowa State advised not delaying past about June 10 to 15 even if corn is still short. Where N was banded or injected, take cores systematically across and between bands, because nitrate moves little sideways.

3. Interpret the number. Above 25 ppm nitrate-N, the probability of a yield response to more N is very low. Below it, apply about 8 lb N/ac per ppm of shortfall. If rainfall from April 1 to sampling was about 20% above normal, lower the critical value by 3 to 5 ppm (to 20 to 22 ppm), because cold, wet soil delays mineralization and the test reads low. The test measures nitrate only, so it undercounts recently applied anhydrous or inhibitor-treated N.

4. If estimating loss, use Iowa State's two-step method. Multiply the N applied by the fraction that had nitrified, then by a daily loss rate and the number of days the soil was saturated. Illinois work on silt loam and clay loam soils found 4 to 5% of nitrate lost per day of saturation when soil temperature was above 65°F (18°C); Nebraska work at 55 to 60°F (13 to 16°C) found roughly 2 to 2.5% per day. On sandy soils, 4 to 6 inches (100 to 150 mm) of rain on already-moist soil can move essentially all nitrate below the root zone, so the estimate is closer to total loss.

5. Choose the method and product. Iowa State's order of preference: injected anhydrous, UAN or urea; then surface-banded (dribbled) UAN; then broadcast urea with a urease inhibitor; then untreated urea; and last, broadcast UAN over the crop. Y-drop style tubes that lay UAN at the row base are surface banding and rely on rain or soil moisture to move N down. Knifed UAN in a closed slot does not need a urease inhibitor.

6. Watch the forecast for surface applications. Rain of 0.25 to 0.5 inch (6 to 12 mm) within two to three days moves urea into the soil and stops volatile loss. Wisconsin adds that in warm weather rain is needed within a day, that losses become significant with no rain in five days, and that one Wisconsin study lost 19% of surface urea N as ammonia within two days. Urease inhibitors (NBPT, NPPT, Duromide) extend the window; UAN needs about half the inhibitor rate of dry urea because only half its N is urea.

7. Respect the stage limits. Do not broadcast UAN over the canopy beyond V7; leaf burn gets worse with size, heat and rates above about 60 lb N/ac. For a rescue application, Iowa State advises getting it on as soon as possible and not later than about tassel.

Worked example with numbers

Field A, planned split. Corn after soybean, planned total 150 lb N/ac (168 kg N/ha). 50 lb N/ac went on in mid-April as UAN with the preemergence herbicide. In late May a 4-inch (100 mm) rain left the silt loam saturated for five days with soil above 65°F. Assume 95% of the April UAN had nitrified: 50 × 0.95 × 0.045 × 5 ≈ 11 lb N/ac lost. The sidedress plan becomes 100 + 11 ≈ 110 lb N/ac (123 kg N/ha). As UAN 32 knifed at V6: 110 ÷ 0.32 = 344 lb of product, or about 31 gal/ac (290 L/ha) at 11.06 lb/gal.

Field B, manured, no preplant N. Corn 8 inches tall on June 3, LSNT result 16 ppm. April 1 to sampling rainfall was 25% above normal, so use a 21 ppm critical value: (21 − 16) × 8 = 40 lb N/ac (45 kg N/ha). Results below 25 ppm predict the exact rate less reliably than results above it, so treat this as an estimate; reference strips with 25% more N help confirm it.

Common mistakes

• Replacing the whole rate after rain. Loss is a fraction of the nitrate present, not of everything applied. A 120 lb N/ac preplant application with ten warm saturated days loses about 45 lb N/ac in Iowa State's example, not 120.
• Testing in a band without a plan. Cores that hit an anhydrous band read very high; cores that miss it read very low.
• Sidedressing deficient corn late and expecting full recovery. In Purdue trials, N at V15 produced 100 bu/ac more than starter alone, but 13 bu/ac less than the same N at V7; the gap was kernel number. Earlier is better; later is still far better than nothing.
• Using UAN as a post-emergence herbicide carrier. Most herbicide labels prohibit N solution carriers after crop emergence. Check the label registered in your state or province.
• Ignoring waterlogged roots. Ponded corn with damaged roots responds less to late N; Purdue's older guidance caps late rescue N at about 60 lb N/ac for severely stressed corn.

Regional differences

In the US Corn Belt, the LSNT interpretation above is Iowa State's calibration. Wisconsin and other states run a pre-sidedress nitrate test (PSNT) with their own sampling and critical values; use the interpretation from the state that calibrated the test. On sandy soils the loss arithmetic above understates leaching; Minnesota and other states publish separate guidelines for irrigated sands, and those, not the MRTN tables for medium and fine soils, are the starting point.

In Ontario, the Ontario Corn Nitrogen Calculator already reduces the recommended rate for V4 to V6 sidedress on most textures in the southwest and central regions, so do not apply a second "efficiency" discount on top of it. Eastern Ontario receives no sidedress credit. If you use a pre-sidedress nitrate test in Ontario, interpret it with the current OMAFRA guidance rather than the Iowa numbers.

For Canadian readers, stabilizer products sold in the United States are not automatically available in Canada; buy products registered for sale in Canada and follow the Canadian label.