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The Bulletin

Notes on Nitrogen as Planting Gets Underway

Emerson Nafziger

and Giovani Preza Fontes
Department of Crop Sciences
University of Illinois

April 12, 2024
Recommended citation format: Nafziger, E., G. Fontes "Notes on Nitrogen as Planting Gets Underway." Department of Crop Sciences, University of Illinois at Urbana-Champaign, April 12, 2024. Permalink

March was relatively warm and dry in Illinois, and corn planting started early: NASS reported that 1% of the Illinois corn crop was planted by March 31, and 2% by April 7. Those are no record-early starts, but it’s rare to have enough planted acreage to report by April 1. Most of the state has had above-normal rainfall for far in April, so we expect limited planting progress in the coming days.

In this article we’ll consider a few nitrogen-related issues to keep in mind as planting progresses in 2024:

Due perhaps to an article in the Ohio State C.O.R.N newsletter this week, questions have been raised about how much of the N applied last fall might be in danger of loss (if not already lost) with the rain that fell over much of central and northern Illinois. The authors cited some information from a 2014 University of Nebraska publication to the effect that 10 days of saturated soil at 55-60 degrees can lose 25% of applied N to denitrification, while 3 days at 75-80 degrees can mean loss of 60% of applied N. The OSU authors added that temperatures had been above 55 degrees for several days already at one Ohio site, suggesting that such losses were already underway.

The Nebraska publication referred to soil, not air, temperature, while the OSU authors were using air temperature. This is an important difference: air temperature has fluctuated in Illinois, reaching 70 degrees or above several days during the first half of March and into the upper 60s several days so far in April. Soil temperatures have remained much lower – the temperatures 4 inches deep under bare soil have been mostly in the 40s over the past month, with some drops into the 30s during cool weather in March, and some temporary increases in the 50s in central Illinois during warmer periods. The average soil temperature on April 9 was 49 degrees in central Illinois and 47 degrees in northern Illinois.

A second issue is that most parts of most fields in Illinois, even where 3 inches of rain fell over the past week, are wet but not saturated. One reason for this is that water moves off sloped surfaces, and soil without standing water do not become saturated. Also, it was dry enough through February and March to allow soils to dry out some, making it possible for them to take in some of the rain that fell over the past week.

While there has been some nitrification—conversion of ammonium to nitrate—during periods of higher soil temperatures since last fall, nitrate only moves downward if water moves through the soil, and soils need to be saturated and soil temperatures need to be high enough to cause N loss through denitrification. We have not had such conditions so far, and we do not believe there has been more N loss that usual sine application last fall.

While we do not have many samples at this point to test that statement, data from a study we ran at four Crop Sciences research centers for four years can help put this into perspective. In that study, we applied 200 lb N as NH3 with N-Serve in November, and as NH3 without N-Serve in March or April. We sampled to 2 ft for soil inorganic N once or twice in April, and twice or three times in May at each location. Table 1 shows the average temperature and precipitation for November through April, the amount of N recovered in April-May, and the percentage of the recovered N that was still in the ammonium (NH4+) form, which is not subject to denitrification.

Table 1. November-April weather and soil inorganic N recovered by sampling the next spring (April-May) following application of 200 lb N/acre with anhydrous ammonia in the fall (with N-Serve) or early spring (without N-Serve. Amount of N recovered is in lb per acre of nitrate-N + ammonium-N; % NH4+ is the proportion of ammonium to total N recovered. Data are averages over four sites each year.

The combination of temperature and precipitation varied among years, ranging from cold and dry (2014-15) to warm and wet (2015-16). But fall/winter/early spring conditions had little effect on how much N remained in the soil the next spring after soil temperatures had begun to warm up, but before much crop uptake. In fact, the coldest and driest “preseason” conditions (2014-15) had the lowest amount of N recovered the next spring, while the warm/wet preseason in 2015-16 had soil N amounts only a little lower than those in 2017 and 2018.

Subtracting soil N without fertilizer from the amount recovered from fertilized plots shows net soil N available from fertilizer of 100 to 140 lb, or 50 to 70% of the amount of N applied. The amount recovered differed by year, and more of the recovered N was still in the ammonium form following spring compared to fall application. But most importantly, we recovered the same amount of N from fall-applied as from spring-applied N. Yields backed this up: averaged over sites and years, fall-applied N with N-Serve produced an average of 225 bu/acre, and spring-applied N without N-Serve averaged 224 bu/acre. Yield without N averaged 152 bu/acre. A three-way split –fall NH3 (100 lb N) + 50 lb N (as UAN) at planting + 50 lb (as UAN) as sidedress averaged 224 bu/acre, and a treatment with 50 lb N as UAN at planting and 150 lb as UAN as sidedress averaged 222 bu/acre.

While the weather since last fall has been somewhat warmer and a little drier than normal through March is year (April rainfall will eliminate the “drier”), we don’t see anything in the previous research that suggests substantial loss of fall-applied N. Tile flow has not been substantial over the winter, but will likely increase with recent rains. But leaching potential is limited in most of the medium- and heavy-textured soils in Illinois, and denitrification, which accelerates under warm soil temperatures and requires saturated (low-oxygen) conditions, is unlikely to be a problem until soil temperatures move above 60 degrees. Once soil temperatures rise, microbes that mineralize N from soil organic matter will become active, and mineralized N will add to the soil N supply. If the weather turns warm and wet in May and June, then we’ll revisit this issue.

Early and in-season N for the crop

As we’ve often said, corn plants need access to some N as they get established, and as the nodal root systems begin to develop. If warming continues as planting progresses, mineralized N will help minimize any concerns we may have about this. But applying some N with the planter, as broadcast with herbicide, or as bands of dry or liquid fertilizer applied over the planted row will eliminate any concern. The closer to the row this is applied, the lower the amount needed: 10 lb of N is enough if UAN or other N fertilizer is applied in (or next to) the furrow or 2 x 2 (2 inches below and 2 inches to the side of the seed). It doesn’t take much N; it just needs to be in the right place by the time the plant needs it.

Although there aren’t data pointing to routine need for sulfur fertilizer in Illinois soils, many producers have started to use some form of S on corn and on soybean crops. Ammonium thiosulfate (ATS) is a commonly-used source of S, and one that can be mixed with UAN for planter or sidedress placement. With some S also from microbial breakdown of soil organic matter, S fertilizer rates needn’t be high: 10-15 lb is generally enough, with less need for S in higher-OM soils.

Once the early-season N need has been met for corn, timing the application of the N still needed can be somewhat flexible. Corn plants take up N slowly—roughly 1 lb N for every inch of height—through V5, after which the uptake rate accelerates. Roots grow out into the row middles as plants grow, so N applied between rows should be available to the plants. Dry weather and soils around crop stage V6 in both 2022 and 2023 caused some delays in availability of surface-applied N, and may have led to some N loss. In the unlikely event that this happens again in 2024, finding a way to inject the N might improve availability.

With the lower corn price in 2024 compared to previous years, this might be a good time to consider managing N based more on economics than on using “plenty of N” in case high yields raise the need for N. As we have seen routinely, productive soils with higher organic matter tend to yield more, but also to provide more of the N the crop needs. So we don’t need to use enough fertilizer to provide all of the N the plant needs; we can trust the soil for the first half of the N the crop will take up, especially in higher-organic matter soils.

Those who have not done so might check MRTN N rates using the N rate calculator at https://www.cornnratecalc.org/ to see what previous research suggests using at current corn and N prices.

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