Spring Nitrogen Management

High fertilizer nitrogen prices are providing an incentive to manage N this spring with as much efficiency as possible. We’ll consider here some ways to work towards that.

Nitrogen rate: Higher corn prices are helping to counter the effect of higher N prices on N rate: if N and corn sale prices hold at current levels, MRTN rates for corn following soybean are 154 lb/acre in northern IL, 167 in central IL, and 185 in southern Illinois. While some may choose not to lower rates from what they were a year ago, those who have in the past used more than 200 lb of N per acre might want to consider lowering them this year.

Weather: The weather this winter was fairly normal, with no extended wet and warm periods that would allow N applied as ammonia last fall to convert to nitrate and move down in the soil. That could change as we move into April, but most of the N should still be present, and at least a typical percentage of it should still be in ammonium form.

The current US drought map shows the northwestern quarter of Illinois as being in mild or moderate drought. This means that much of the N not used by the 2021 crop, including N mineralized after crop maturity, remains in the soil. All of this N is likely in nitrate form now, so will move down with rainfall, but it won’t move out of fields unless tiles flow, which will take several of inches of rain. At least some of this N should be available for this year’s crop unless the weather turns wet. It’s not clear how to adjust fertilizer N rates based on N in the soil, but it makes sense to use moderate N rates (such as the MRTN rate) without adding extra N as “insurance” that the crop probably won’t need.

N near the row—a top placement priority:  Corn plants need to have some N near their nodal roots as soon as those roots start to develop, at growth stage V2-V3. The earlier the planting and the cooler the soil at and after planting, the less N will be available from mineralization, and so supplying this N with fertilizer becomes more important.

Practices likely to work or that may not work consistently to do this include:

• Applying 20 to 30 lb of N as UAN in a 2 x 2 band (2 inches away from and 2 inches below the seed row) should work well.
• In-furrow application of 10-15 lb N (3-4 gallons of UAN) should work without damaging seed, but does not place the N as well as a 2 x 2 band. Knives that stream a little more UAN near the row may work better.
• Ten gallons of 28% or 32% UAN (30 to 35 lb N) broadcast with herbicide may not end up with enough N near the roots soon enough.
• Injected UAN or urea subsurface-banded midway between the rows at or before planting places the N too far away from the row for early root access. Moving it to within 4-6 inches of the row should help.
• Applying at least half the crop’s N as UAN or urea followed by tillage should work, although there could be streakiness in the amount that ends up near the row.

Applying the bulk of spring-applied N: Ammonia is likely to persist in the soil longer than other forms if the weather turns wet, so is the best form to apply in advance of planting. Applying ammonia several weeks before planting, or using RTK to apply ammonia between rows to be planted should minimize the potential for seedling damage. If the ammonia is placed deep enough (5-6 inches) and into moist soil, the field can be field-cultivated after application to prepare for planting.

We have not seen an advantage to applying ammonia after planting compared to applying it several weeks before planting, but both should perform equally well, as long as some N is applied near the row. Ammonia can be injected anytime between planting and when plants are 18 to 24 inches tall. Shallower application with a high-speed applicator can work, but if the surface soil is dry during application there can be some release of ammonia that may damage lower leaves. Urea broadcast over the top of corn plants might cause browning of leaf edges, but this has not been found to lower yield. UAN can be injected between rows or applied using attachments that place it near the rows.

Split N: The term “split N” means applying N fertilizer two or more times to the crop. This applies to nearly every field when we include all of the N forms used: MAP or DAP; application with the planter or as broadcast N; and N applications after planting. The term “split application” implies that application of some of the N was deliberately delayed in order to better synchronize fertilizer N supply with crop demand. This limits the time that N spends in or on the soil, where it’s subject to loss, before the crop takes it up. With the development of equipment that can apply N to tall corn, some have taken this concept farther, and may apply in-season N two or three times to produce a steady N supply for the crop.

Besides limiting the time between application and uptake, split application is also promoted as a way to increase the efficiency of N use. This would need to come from higher yield at the same N rate, the same yield at a lower N rate, or both. In theory, waiting to apply N in-season could allow us to adjust the N rate based on crop condition at the time of application, perhaps with guidance from canopy color sensing. This has not worked very well in practice. For example, a crop growing in dry soils may have dark green leaves at the time of application, but may also have difficulty in getting access to soil N, so may end up needing more, not less, N. On the other hand, corn growing under very good conditions often has access to more mineralized N, and so may need less fertilizer N. This makes it difficult to know how to make in-season rate adjustments, and there is little indication that such adjustments can consistently lower the amount of fertilizer N applied per bushel of yield.

While split-applying N often makes logistical sense, it also carries risk. If the weather turns wet, it can be difficult to get the N applied on time, and roots may not function well to take up N. If the weather is dry, fertilizer will sit in or on the soil with little chance of being taken up. This has the same effect as delaying application, and it can lower yield if most of the N is applied in this way. This, along with the high capacity of most Illinois soils to store early-applied N safely until it plants take it up, helps explain why “spoon-feeding”—applying N several times during vegetative growth of the crop—often shows no yield advantage over a single (or no) in-season application, and so fails to cover its cost.

Timing of in-season N: Although we have found little yield benefit from splitting N compared to applying all of the N at planting, we can lower risks of in-season application by how we divide N between early and later applications, and by how we apply in-season N. In medium-textured soils with more than 2% organic matter, it is better to apply more than half of the N early and the rest in-season than it is to wait until sidedress time to apply most of the N. In a study at Urbana, we found that applying half of the N at planting meant that we could delay application of the rest of the N to as late as pollination with no loss in yield. If we applied no N at planting and delayed application of all of the N, however, yield began to decrease with application as early as V6. As long as we apply half or more of the N at or soon after planting, including some near the row, the rest can go on as late as V10 to V12 or even (with high-clearance equipment) as late as tasseling with little danger of losing yield.

The idea that some of the N should be applied as late as (or even later than) pollination in order to assure having enough N during grainfilling got a lot of attention several years ago, with high-clearance equipment purchased in some cases in order to do this. This idea gets less attention today, partly because it was found to have less effect on yields than hoped, and also because of the logistics and expense required to do this on large acreages. We conducted fourteen N rate studies to compare early application of N to “late-split” N—keeping back 50 lb N to dribble near the rows at tasseling. Late-split N neither increased yield nor lowered the amount of N needed; in other words, it did not cover any of its cost. Because N uptake by corn plants slows considerably after pollination, N from early fertilizer application and from ongoing mineralization appears to be enough to meet the needs of the crop during the second half of the season.

In soils with lighter texture and lower organic matter, where we expect less mineralized N to become available, there may be more benefit from splitting N, especially if it’s wet after early application. On irrigated, sandy soils where N moves readily, many producers apply some early and the rest through the irrigation system several times during the growing season. In such soils without irrigation, applying half or more of the N as ammonia at planting and the rest in-season can work, but rainfall is needed to help move N to the roots for uptake. In silt loam soils with lower organic matter, common in southern Illinois, soils warm earlier in the spring and so may begin to mineralize N earlier, although mineralization rates may be lower with less organic matter. If it’s wet following early application, sidedressing can be beneficial in such soils, although at least half of the N should still be applied early.

Trusting the crop’s N supply: Recent research on N timing has pointed towards the conclusion that most Illinois soils are better “reservoirs” for nitrogen than we give them credit for. Soils are “active” N reservoirs—they both store N, and, through mineralization, produce N that is available to the crop throughout the season. Today’s higher-yielding hybrids take up more N, but this is more than met by faster-growing plants with root systems that are better at taking up N from the soil. We can have confidence that modest fertilizer rates, applied properly, will provide a safe N supply for the corn crop, with less cause for concern about whether the crop will get enough N.