Published 13 June 2008 (References updated 6/16/08)
Nitrogen Loss in Wet and Wetter Fields
Jim Camberato, Brad Joern, & R.L. (Bob) Nielsen
Agronomy Dept., Purdue Univ.
West Lafayette, IN 47907-2054
Email address: firstname.lastname@example.org, email@example.com, and rnielsen
Heavy rainfall and ponded soils at this time of the year will result in nitrogen (N) loss. The quantity of N lost is difficult to predict and depends on many factors, including soil type, fertilizer type, placement and timing, soil temperatures since application and the amount of rainfall received. We cannot tell you how much N you lost but we can give you information that will assist you in estimating the loss for your situation.
Conversion of Fertilizer Nitrogen to Nitrate
With the exception of urea just after application, N must be in the nitrate form for N loss to be significant. Urea is just as leachable as nitrate when in the urea form. If rains come within the first 12-24 hours or so of urea or 28% application, the leaching of urea can be significant from sandy soils.
Although urea and anhydrous ammonia contain no nitrate at application and only ¼ of the N in 28% is nitrate, the N present in all of these fertilizers is converted to nitrate by soil bacteria in the process known as nitrification. Therefore we need to estimate how much nitrification has occurred before we can speculate about N loss.
Nitrification begins almost immediately after application with urea and 28%, but is delayed at least 10-14 days with anhydrous ammonia because anhydrous ammonia kills the soil bacteria in the injection zone. Soil temperature has a large influence on nitrification, with about 2 weeks required for complete conversion at 60F and 1 week needed at 70F. Soil temperatures have been in the 60's and 70's since about May 20 th . That means the vast majority of N applied pre-plant or at planting was converted to nitrate before the heavy rains hit. If you were lucky enough to have sidedressed shortly before the rains (or have not yet sidedressed) you have lost little nitrogen.
Loss of Nitrate by Leaching and Denitrification
Soil type, rainfall and temperature control both how and how much N is lost from the soil once it is converted to nitrate. In well-drained sandy soils, much of the nitrate can be lost by leaching as water moves nitrate down through the soil profile. Sandy soils have a low water holding capacity and a high infiltration rate. This means that much of the rainfall moves through a sandy soil and runoff is generally pretty small. Consequently, the depth of nitrate movement in sandy soils is closely related to the amount of rainfall. Since every inch of rain can move nitrate 6 to 8 inches deeper in the soil profile in lighter textured soils, sandy soils that have received more than 8 inches of rain in the past week have likely lost significant amounts of N below the root zone.
In heavier textured soils (silt loam, clay loam, etc.), nitrate can be lost by either denitrification or leaching. Leaching in heavier textured soils is substantially slower than in sandy soils due to the greater water holding capacity and slower infiltration rates of heavier textured soils. That is why ponding is so prevalent in these soils after rainfall, even though the field may be tile drained.
Over time in a ponded area, soil microorganisms quickly deplete oxygen levels in the soil and cause denitrification. Denitrification is the conversion of nitrate to nitrogen gases as soil microorganisms use the oxygen in nitrate to survive. Denitrification losses take place at a much slower rate than nitrate leaching. Average N loss is usually around 2% of the nitrate per day of saturation, but has been reported to be as high as 5% per day. The longer the soil remains saturated and the higher the temperature the more N is lost via denitrification.
Typical N losses from heavier soils in winter and spring have been in the range of 20 to 50 pounds of N per acre in tile drainage studies at Purdue (Brouder et al., 2005). Ohio studies showed an average total N loss of 50 pounds per acre from fall N applied at 160 pounds per acre as anhydrous ammonia, but some years as much as 90 pounds N per acre was lost. The Ohio studies were conducted on a Crosby silt loam soil that is pretty typical of central Indiana . Rainfall in April through June during these studies was around 14 inches. Unfortunately some Indiana fields have had this much rainfall in 1 week although much of it has run off the field or collected in low spots rather than infiltrate into the soil.
Using Soil Testing to Determine Nitrogen Availability
A 1 foot deep soil sample analyzed for nitrate can be used to determine soil N levels. If N application was recent, also request ammonium levels. Consult Purdue Extension publication AY-314-W (Brouder & Mengel, 2003b) for proper sampling guidelines, especially if the fertilizer was banded. Soil samples can be taken once the soil drains and the field can be walked comfortably. Guidelines suggest 25 parts per million nitrate-N in the upper 1 foot is sufficient for optimum grain yield. In sandy soils, take an additional sample from the 1- to 2-foot soil depth to measure nitrate that has moved down, but not out of, the root zone. Each part per million of nitrate- or ammonium-N in 1 foot of soil is equivalent to 4 pounds of N per acre.
Using Reference Strips to Monitor Nitrogen Availability
Consider applying high N rates to several reference strips in several representative fields now if you can sidedress additional N to the rest of the field later in the season with high clearance application equipment. Nitrogen rates for these reference strips should be high enough so that N is not limiting. Consider approximately 150 pounds of N per acre N or at least as much as you would consider applying to the remainder of the field. A chlorophyll meter (Brouder & Mengel, 2003a) can be used to compare the N status of corn in the bulk field to that of the reference strip to determine if additional N is needed.
Grain Yield Response to Fertilizer Nitrogen
Determining the amount of lost N is not an easy task. Determining what to do from here on out is even more difficult. Corn in ponded areas may be damaged and incapable of responding to additional N. Corn replanted this late in the season may have a different N requirement than timely planted corn. Otherwise for most remaining corn fields, the decision to add N should be based on an estimate of how much N was lost, the initial application rate, the projected optimum N rate for the field under minimal N loss conditions, N cost, and grain value.
The projected economic optimum N rate (EONR) differs from field to field. For corn grown after soybean, the EONR was 150 pounds of N per acre (calculated at $6.50/bushel and $0.75/pound N) in 31 studies conducted in the past 2 seasons (Camberato et al., 2008). Nitrogen was sidedressed in many of these trials and N loss was considered minimal. Although the average EONR was 150 pounds of N per acre, some fields required as little as 100 pounds of N per acre to achieve the economic optimum yield while others needed as much as 200 pounds of N per acre. Soils in northeastern Indiana tended to require more N than average whereas silt loam soils in northwestern and west central Indiana required less N than average.
The average yield at 150 pounds of N per acre was 184 bushels per acre. Yield at 105 pounds of N per acre was 173 bushels per acre, representing an 11 bushel per acre yield decrease with 50 pounds of N per acre less. As the N rate approaches the EONR, it takes 7 to 8 pounds of N to produce the last additional bushel of yield with $0.75 per pound N and $6.50 per bushel grain. Yield may continue to increase above the EONR, but the cost of N exceeds the value of grain produced. Corn grown after corn required about 30 pounds of N per acre more than corn after soybeans to achieve economic optimum yield, but the yield decrease with 50 pounds less N per acre was the same 11 bushels per acre.
Figure 1 . Average grain yield response of corn grown after soybean to fertilizer N in Indiana in 2006 and 2007. The line represents the average of 31 trials on a number of different soils with limited N loss conditions.
To Fertilizer or Not to Fertilize, That is the Question
These are our opinions based on our knowledge of N loss mechanisms and experience. Consider your experience and specific field and weather conditions in evaluating our suggestions.
Additional N Likely Required:
Consider 60 to 120 pounds of N per acre for fields where urea or 28% UAN was applied more than 2 weeks prior, or anhydrous ammonia applied more than 4 weeks prior, to excessive rain, especially if:
- Silt loam and poorly drained field areas were saturated long enough to kill the crop and you will be replanting those areas to corn. Also consider starter fertilizer to get the replant corn off to a good start.
- Sandy fields received more than 8 inches of rainfall that infiltrated the soil.
Additional N May Be Required:
Consider 30 to 60 pounds of N per acre for fields where urea or 28% UAN was applied 1 to 2 weeks prior, or anhydrous ammonia was applied 3 to 4 weeks prior, to excessive rain, especially if:
- Silt loam field areas were saturated for more than 3 days and the crop survived.
- Poorly drained field areas were ponded for more than 3 days and the crop survived.
- Sandy fields received 4 to 8 inches of rainfall that infiltrated the soil.
Additional N Likely NOT Required:
Fields where N loss is estimated to be less than 30 pounds of N per acre and the projected optimum N rate or higher was used initially. For example:
- Fields where N was applied 2 to 7 days (urea or 28%) or 3 weeks (anhydrous ammonia) before excessive rain.
- Silt loam field areas that were saturated for less than 3 days.
- Poorly drained field areas that were ponded for less than 3 days.
- Sandy fields receiving less than 4 inches of rainfall.
If you sidedressed with urea or 28% within 24 hours of the heavy rains on sandy soils, urea may have leached below the root zone. The amount of additional N required is dependent on how much rain fell on the field.
- Sandy fields that received 4 to 8 inches of rainfall that infiltrated the soil.
- Apply 60 to 120 pounds of N per acre
- Sandy fields that received 4 to 8 inches of rainfall that infiltrated the soil.
- Apply 30 to 60 pounds of N per acre
- Sandy fields that received less than 4 inches of rainfall
- Additional N likely not required
Nitrogen Loss Worksheet
Peter Scharf (2008) at the University of Missouri created an easy-to-use N loss worksheet that may be of additional assistance in deciding whether to fertilize and how much N to apply. http://ppp.missouri.edu/newsletters/ipcm/archives/v18n10/v18n10.pdf
Brouder, Sylvie and David Mengel. 2003a. Determining Nitrogen Fertilizer Sidedress Application Needs in Corn Using a Chlorophyll Meter (AY-317-W). [On-line]. Available at http://www.agry.purdue.edu/ext/pubs/AY-317-W.pdf [URL accessed 6/13/08 ].
Brouder, Sylvie and David Mengel. 2003b. The Presidedress Soil Nitrate Test for Improving Soil N Management in Soil (AY-314-W). Purdue Extension. [On-line]. Available at http://www.agry.purdue.edu/ext/pubs/AY-314-W.pdf [URL accessed 6/13/08 ].
Brouder, Sylvie, Brenda Hofmann, Eileen Kladivko, Ron Turco, Andrea Bongen, and Jane Frankenberger. 2005. Interpreting Nitrate Concentration in Tile Drainage Water (AY-318-W). Purdue Extension. [On-line]. Available at http://www.extension.purdue.edu/extmedia/AY/AY-318-W.pdf [URL accessed 6/13/08 ].
Camberato, Jim, R.L. (Bob) Nielsen, Dan Emmert, and Brad Joern. 2008. Nitrogen Management Update 2008. Corny News Network, Purdue Univ. [On-line]. Available at http://www.kingcorn.org/news/articles.07/NMgmtUpdate-1206.pdf [URL accessed 6/13/08 ].
Laboski, Carrie. 2008. Potential for Nitrogen Loss from Heavy Rainfalls. Wisconsin Crop Manager Newsletter. Univ. of Wisconsin. [On-line]. Available at http://ipcm.wisc.edu/Portals/0/Blog/Files/17/564/WCM_15(14)b.pdf [URL accessed 6/13/08].
Sawyer, John. 2008a. Estimating Nitrogen Losses. Integrated Crop Management News, Iowa State Extension. [On-line]. Available at http://www.extension.iastate.edu/CropNews/2008/0611JohnSawyer.htm [URL accessed 6/16/08].
Sawyer, John. 2008b. Measuring the Nitrogen Status. Integrated Crop Management News, Iowa State Extension. [On-line]. Available at http://www.extension.iastate.edu/CropNews/2008/0612JohnSawyer.htm [URL accessed 6/16/08].
Sawyer, John. 2008c. Nitrogen Loss – How Does it Happen? Integrated Crop Management News, Iowa State Extension. [On-line]. Available at http://www.extension.iastate.edu/CropNews/2008/0610JohnSawyer1.htm [URL accessed 6/16/08].
Scharf, Peter. 2008. Nitrogen loss scoresheet. Integrated Pest & Crop Management Newsletter, Univ. of Missouri . [On-line]. Available at http://ppp.missouri.edu/newsletters/ipcm/archives/v18n10/v18n10.pdf [URL accessed 6/13/08 ].