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The Bulletin

Fertilizer Decisions, Fall 2021

Emerson Nafziger

Department of Crop Sciences
University of Illinois

October 6, 2021
Recommended citation format: Nafziger, E.. "Fertilizer Decisions, Fall 2021." Department of Crop Sciences, University of Illinois, October 6, 2021. Permalink

With warm and dry weather prevailing over the past month, harvest of the corn and soybean crops has proceeded at a brisk pace in Illinois. By October 1, 41% of the corn crop and 32% of the soybean crop had been harvested; both were ahead of the average over the past five years.

Early harvest as dry conditions continue gets many producers and dealers thinking about fall fertilizer. This has taken on a greater urgency in 2021 due to large increases in fertilizer prices over the past year, and also to the uncertainty about prices and supplies of fertilizer, especially N fertilizer, both this fall and into next spring.

This is not the first time that P and K fertilizer prices have been high relative to the price of corn, but that doesn’t make it easier to decide whether and how much to apply for next year’s crop. Fields with soil test levels above 30 ppm P and 175 ppm K could almost certainly go a year without additions with little or no risk of yield loss. At some point, however, the P and K that have left the field in harvested grain will need to be replaced, and there’s no certainty about when and by how much prices of these nutrients, relative to grain prices, will moderate. If soils tests are low, or if less than full replacement amounts have been applied routinely in recent years, then some P and K may be needed to maintain full yields. Applications can be made either this fall or next spring.

Questions about nitrogen are far more immediate than questions about other nutrients. Neither the price nor supply of UAN is very stable at the moment, but UAN-28 at $500 per ton is $0.89 per pound of N. UAN is not suitable for fall application of course, but getting tanks filled for next spring is a concern.

A more immediate issue is the price of anhydrous ammonia for those who plan to apply ammonia this fall. Ammonia is currently priced at $800 to $850 per ton, or $0.49 to $0.52 per pound of N. Natural gas prices are currently about double their level in early 2021, and because natural gas is the major feedstock for ammonia production, it appears likely that ammonia prices will continue to climb. As the major feedstock for other forms of N fertilizer, the ammonia price is likely to continue to be lower, on a per-pound-of-N basis, than prices of most other N fertilizers.

The immediate effect of increases in N prices relative to corn prices is a reduction in the MRTN rate, which is the rate predicted to be most profitable by the N rate calculator. With corn at $5.00 per bushel, the MRTN rate for central Illinois, corn following soybeans, is 193 lb N/acre with anhydrous ammonia at $600 per ton; 182 lb N/acre at $800 per ton; 173 lb N/acre at $1,000 per ton; and 166 lb N/acre at $1,200 per ton of 82-0-0.

In this example, with corn at $5.00 per bushel, increasing ammonia prices from $800 to $1,200 per ton and lowering the MRTN from 182 to 166 lb N/acre decreases the net return to N (yield added by N times price per bushel minus N cost) by $41.48 per acre, of which $32.00 comes from the higher cost of the N, and only $9.50 from the lower yield of corn—1.9 bushels less yield times $5.00 per bushel. That’s averaged over 284 N rate trials, and we really can be confident that lowering the N rate by 15-20 pounds, even when starting at a moderate rate, is not going to make yields fall off the table.

A more pressing concern at the moment is not N price, but how to react to possible shortages and further price increases in terms of when we apply N this fall, or whether we should wait until next spring. These uncertainties, along with good soil conditions for application now, create heavy pressure to start applying anhydrous ammonia early—before soil temperatures get to 50 degrees or less. Reaching 50 degrees normally happens around November 1; when this happens earlier than that, it’s OK to begin, if the forecast is for air temperatures to continue to drop into November.

Unfortunately, there is no indication at the present that soil temperatures during October are going to drop ahead of, or even on, schedule. September was warmer than normal, and that trend has continued into the first week of October. Soil temperature 4 inches deep in bare soil at 10 AM on October 5 averaged 66.4 degrees across 19 Illinois sites, reported from the WARM network of the Illinois State Water Survey. They were not cooler in northern Illinois than in central or southern Illinois. The forecast for the next week is for temperatures to remain at or above normal, so while we can’t rule out a sudden change to below-normal temperatures that would be necessary to get soil temperatures to below 50 degrees by late October, there is nothing to suggest that this will happen.

Is applying anhydrous ammonia in the fall when the soil temperature is 60 or 65 degrees really that problematic, especially if we use a nitrification inhibitor? In a word, yes. The nitrification rate—the rate at which ammonium from ammonia is converted to nitrate, which can move in soils and be lost—does not reach zero until the soil temperature is at freezing. This rate is estimated to be about 20% of the maximum rate at 50 degrees; in other words, having soils at 50 degrees does not mean that there is no nitrification, it only means that there might not be a lot before soil temperatures drop further, to lower the rate of nitrification further. The rate does not change with temperature as a straight line, but accelerates from zero at 32 degrees, to 20% at 50 degrees, to 50% at 60 degrees, and to nearly 70% by 65 degrees. Even if soil temperatures drop by 5 degrees per week from now to November 1, they will average about 58 degrees during that time. And, because the nitrification rate drops more slowly than temperature drops, the average nitrification rate over those three weeks would be more than 40% of maximum.

Nitrification is carried out by microbes, and since ammonia kills microbes where it’s at high concentration, ammonia application delays the start of nitrification some. But that delay is less when soils are warmer, and applying ammonia into soils at 60 degrees is almost certain to result in conversion of a substantial percentage of the ammonium to nitrate by the time soil reach 35 degrees, sometime (if we’re lucky) in December. The only way having half or more of the N as nitrate by January 1 “works” is for the rest of the winter and the first half of the spring to remain so dry that nitrate stays where it is in the soil, without moving down to the tile lines and out with drainage water. That is not only unlikely, but a spring that dry (as last seen in 2012) would bring much larger problems than loss of N.

As difficult as it might be, it makes sense for everyone to resist the urge to apply anhydrous ammonia during the month of October. Not only would an early start increase greatly the chances of large losses of N, but some of that nitrate ends up in municipal water supplies, and that will bring scrutiny of where the N came from. Postponing application to November or to the spring carries price and availability risks, but those risks may well be lower than the risk of losing a great deal of expensive N by applying too early.

I did hear a report that some anhydrous ammonia was going on (with 15” knife spacing) as preplant N for wheat. That’s not a new practice, but it does carry some risk that N will convert quickly to nitrate and could be lost before root uptake begins in the spring. It will be important not to come back with topdress N in the spring at rates that could result in lodging and yield loss. We have found that testing soils at soil temperatures in the 40s in early spring to see how much fall-applied N remains is not very reliable, so would not suggest doing that to see how much is there.

Finally, some who do not apply N this fall may be anxious to apply N in whatever form and whenever it’s available over the winter. This is often very risky, especially if the soil is frozen or snow-covered. Any form of N that’s not injected ammonia can dissolve and run off the surface if it rains after application, and the risk of this may exceed the risk of waiting to apply N closer to the time the next crop will need it.

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