Cool soil in most of the Corn Belt means a low risk of nitrogen loss for now, says Emerson Nafziger, University of Illinois Crop Sciences professor. Advising farmers on best nitrogen practices for this spring, Nafziger offers these observations and pointers.
- With the cold winter, fall-applied anhydrous ammonia is mostly still in the more stable ammonium form (NH4+) and has not yet mineralized into the more easily lost form of nitrate (NO3-). It will stay that way as long as soils temperatures stay below 50° F.
- Once ammonium turns into nitrate as soils warm, we only worry about losing it if it turns wet. If the tile lines aren’t running, there won’t be nitrogen loss even if the nitrogen is nitrate.
- Nitrogen stabilizers used last fall also remain in force due to low soil temps.
- Cool soils will dry slowly, and wet soils after planting could slow root access to nitrogen uptake.
- Delayed planting and/or slow growth would mean that nitrogen uptake will start later, so loss potential in May and June could still be a factor.
- Wet spring soils could make soil compaction more damaging to roots, slowing nitrogen intake.
- What appears to be nitrogen loss in wet soils can partly result from damaged roots.
Starter fertilizer value
- Yield response to starter fertilizer has not been consistently positive in academic trials. We do know that a seed has plenty of N and P to support growth past V1 stage, but some nitrogen in the soil can boost early growth sometimes.
- Rapid nitrogen uptake doesn’t begin until about June 1 (in Illinois).
- We can expect more response to starter when planting into cold soils, in corn following corn (residue issues), and when corn’s planted early.
- “It doesn’t make sense to put starter on your planter just for this year’s conditions, but consider using it if you are already set up for it, if soil temps are below 50 degrees at planting, soil-test P is low (less than 10-12 ppm), and no other nitrogen as been applied to surface soil,” Nafziger says.
- If soil test phosphorus is low but you don’t use starter, applying DAP before the final tillage will help.
- Iowa research shows more response to starter fertilizer in soils that are low in phosphorus and potassium to start with (see bar chart).
- UAN applied on the surface at or after planting can often provide some of the starter effect.
What's different in 2014? Late planting?
What might be different this year with nitrogen?
- Cool soils in much of the northern Corn Belt mean slow mineralization, so that means low soil N levels in surface soils without (surface-applied) nitrogen.
- Cool soils will dry slowly, with possible planting delays and/or planting into wet soils that could slow root access to and uptake of soil nitrogen.
- Nitrogen will have a long “dwell time” in the soil, which means slow conversion to nitrate and low nitrogen loss potential as long as soils stay cool and rainfall is low.
- But delayed planting and/or slow growth means that nitrogen uptake won’t start early, so loss potential in May and June will be important.
- Weather patterns could make soil compaction more damaging to roots and nitrogen uptake in 2014 than in 2013.
What if we plant late?
- "If planting is really delayed into the second week of May, yield losses approach 1 bu./day, so plant first and worry about fertility later
- If soils have warmed to average (early May) temperatures by planting, some mineralized N will be available, especially in corn following soybeans.
- If we do this, we should follow planting with broadcast UAN if possible, especially in corn following corn (without starter).
- Nafziger sees no reason to change N rates based on soil conditions or planting date.
- While we don’t at this point expect high N loss conditions, “at present corn and anhydrous ammonia prices, I see so reason to change N rates based on soil conditions or planting date,” Nafziger says.
Anhydrous, UAN and urea application
Spring-applied anhydrous ammonia
- When anhydrous ammonia is applied on wet soils, as is often the case in some areas now, it may not seal very well
- It may result in more than normal soil compaction.
- It could damage roots that enter the zone of high ammonia.
- If soils are still cool, nitrification rates are slow, limiting the benefit of a nitrification inhibitor.
- With 25% of the N in UAN as nitrate (NO3), loss potential exists starting the day of application, regardless of temperature. But this also means that N can move into the soil quickly to supply the seedlings.
- Half the N in UAN is in the urea form, so is subject to volatilization loss (as ammonia (NH3)) assisted by the urease enzyme. Urease inhibitors (sold as Agrotain, SuperU, etc.) decrease urease activity, helping keep urea intact until it can be released in the soil, where ammonium (NH4+) gets trapped
- Application to cool soils slows urea volatilization, reducing the need for urease inhibitors.
- Broadcasting UAN before final tillage is safe.
- Broadcasting UAN right after planting is also safe as long as we don’t get a prolonged warm, dry period; a urease inhibitor might help then.
- UAN is a good sidedress material (injected or dribbled) – keep it off the plant.
- Little expected benefit to nitrification inhibitor at sidedress: there is little need to slow nitrification when uptake is already underway.
- Subject to volatilization loss like UAN, but granules may limit exposure to urease. You can urease inhibitor if there’s risk; cool soils lower risk, and rainfall moves it into the soil. Spread patterns are often less consistent than UAN, may be better than anhydrous ammonia.
- Urea can be broadcast over the crop without much damage.
- Slow-release urea such as polymer coated and sometimes sulfur-caoted can be used to slow the release. Example: ESN
- Release can sometimes be too slow, limiting amount of N reaching roots
- Slow release is more appropriate for early applications than for sidedress
- All urea materials release N to nitrate form (NH4+) first, then nitrification begins. Since nitrate is in the soil, N won’t be lost until it’s in nitrate form (NO3-).