More farmers are waiting until spring to apply nitrogen fertilizer for corn. That trend has accelerated in recent years, especially the past two years, as spring application is a strongly recommended practice in the Iowa Nutrient Reduction Strategy. That strategy is a voluntary effort based on education and use of sound, scientifically proven best management practices. Introduced in November 2012 it's an on-going education and demonstration effort, aided by funding from the Iowa Department of Agriculture & Land Stewardship with Iowa State University Extension and the Iowa Department of Natural Resources also involved.
Applying N in the spring instead of fall is the route to take to reduce the risk of nitrogen loss. "When you apply nitrogen in the fall, that's way ahead of the time when the corn plant is there to begin to use it next May or June," notes John Sawyer, Iowa State University Extension soil fertility specialist. However, if you are convinced you must apply nitrogen in the fall, you should only apply anhydrous ammonia, he notes. And follow the "Don't go before the soil is 50 degrees or below" guideline.
You need to wait until soil temperatures cool to 50° F or below
"If you are considering applying nitrogen fertilizer in the fall, anhydrous ammonia is the only form of nitrogen fertilizer recommended by Iowa State University agronomists for fall application," says Sawyer. However, even with fall application of anhydrous, you need to remember to wait until soil temperatures cool to 50° F and continue to trend colder. "The same guidance applies to applying manure with high ammonium N content, like liquid swine manure," he points out.
Why wait until the soil is 50-degrees and trending lower? Sawyer provides the following explanation.
Ammonium is not leached or lost by denitrification
The form of nitrogen that can potentially be lost from soils due to wet conditions is nitrate (NO3-). You can see how the nitrogen cycle works and all the factors involved in the accompanying illustration (Figure 1). The form of nitrogen that's applied as anhydrous ammonia is NH3, which converts to ammonium (NH4+) in water. Because ammonium is a positively charged ion, it is attracted by electrostatic forces to the negatively charged soil. Ammonium is not leached or lost by denitrification (conversion to nitrogen gas). Therefore, it will stay in soil even if the soil becomes excessively wet.
Nitrate, which is produced by soil microbes from ammonium in a process called nitrification, is a negatively charged ion, is repelled by the negatively charged soil, and is leachable and subject to denitrification.
Figure 1: An illustration of the soil nitrogen cycle
Biggest influence on nitrate loss is soil temperature
Since nitrification is a microbial-mediated process, the rate is influenced by several factors that affect biological activity, such as ammonia in soil water (inhibits nitrification), temperature, soil aeration (only occurs in aerobic soils), soil pH – range from 4.5 to 10.0 (optimum at pH 8.5), and soil moisture (highest at field capacity); but the largest influence is soil temperature. For that reason, a way to slow conversion of ammonium to nitrate is to have cold soil temperatures (example of soil temperature effect on nitrification shown in Figure 2).
The optimum temperature for nitrification is around 90° F. Below 50° F the rate slows rapidly, but nitrification continues until 32° F. Soil temperature cannot be controlled, but because soils cool in the late fall, then nitrification of late-fall applications will be reduced. The later you wait to apply anhydrous ammonia the better—soils with colder temperatures mean less nitrification and a greater probability that soil temperature will not rebound to warm levels.
Figure 2: Effect of soil temperature on nitrate formation. Adapted from Frederick, L.R. and F.E. Broadbent. 1966. Biological interactions. p. 198-212. In M.H. McVicker et al., Agricultural anhydrous ammonia technology and use. ASA, Madison, WI.
How about using a nitrification inhibitor?
Nitrification inhibitors slow the conversion of ammonium to nitrate, says Sawyer. If more ammonium remains in soil during wet springtime periods, then less nitrate will be present and subject to loss. Nitrification inhibitors are not foolproof. They temporarily slow, but do not stop nitrification and formation of nitrate. They degrade in soil, which lessens effectiveness over time. Warm soils that speed nitrification also speed inhibitor breakdown, which means lower effectiveness and faster nitrification reestablishment.
Also, the impact of the nitrification inhibitor on N loss is solely dependent on substantially more ammonium being present during excessively wet periods (which typically occur in the springtime). If wet soils occur after the inhibitor loses its ability to effectively enhance ammonium remaining, then it will have no real impact.
Waiting for soils to cool and using an inhibitor is no sure bet
Waiting for cold soils and/or using a nitrification inhibitor does not guarantee that fall-applied N will be a completely successful practice, says Sawyer. Even in late fall the weather may turn warm, or warm and wet conditions may occur the next spring. Keep in mind that the time period with historically high potential for wet soils and nitrate loss is May and June. "However, if you decide to make applications of anhydrous ammonia in the fall, then waiting until soils are cold is better than applying early," says Sawyer.
Where to find soil temperature information for your location
Soil temperatures can be found at several websites. One that gives the 3-day, 4-inch depth soil temperature estimates for each county in Iowa is available here.
The site can also be accessed through the Agronomy Extension Soil Fertility website, either from the weather page or nitrogen topic page. You can find the average daily soil temperatures from yesterday, two days ago, and three days ago and you can get the 6-10 day weather forecast. The 4-inch soil temperatures are estimated for each county based on interpolation of observed soil temperatures at multiple locations. The estimates are for soil temperatures on level, bare soil.
Summing up: "Remember, the decision to begin fall N application is not the first day that soil temperatures reach 50° F; rather it's when the trend is for sustained soil temperatures below 50° F and continued cooling," says Sawyer. "The dates in Iowa when soils cool below 50° F will vary considerably, from late October to late November. Therefore, you should watch for temporary cold spells, especially in the early part of the fall. Also, observe the 6 to 10 day weather forecast, where a forecast for above average temperatures may signal soil warming."
John Sawyer is an ISU professor of agronomy with research and extension responsibilities in soil fertility and nutrient management. He can be reached at [email protected] or 515-294-7078.