Estimating nitrogen losses in wet fields
Some areas of Iowa have recently received heavy rainfall, resulting in soils saturated or with standing water. Following are excerpts from an article published in 2011 when similar conditions occurred. The early spring 2014 season was on the cold side, so conversion of ammonium fertilizers to nitrate should have been slower than normal. This could be helpful for avoidance of nitrate losses, as would recent sidedress application of ammonium-containing fertilizers.
However, wet soils in June are much more conducive to nitrate loss (compared to early spring) as soils are warm and, with prolonged saturation and tile flow, losses mount. Remember, ammonium is held on the soil exchange complex, but nitrate can leach or be denitrified to nitrogen (N) gasses. Also remember that corn plants do not respond well to saturated soils, and therefore can express symptoms similar to N deficiency when they really are showing excess water stress.
One way to determine nitrogen (N) loss is to calculate an estimate. Predicting the exact amount is difficult as many factors affect losses. However, estimates can provide guidance for supplemental N applications.
Research Measurement of Nitrate Loss
Research conducted in Illinois indicated approximately 4 to 5 percent loss of nitrate-N by denitrification per day that soils were saturated. An all-nitrate fertilizer was applied when corn was in the V1 to V3 growth stage (late May to early June). Soils were brought to field capacity and then an excess 4 inches of water (above ambient rainfall) was applied by irrigation evenly over a three-day period (which maintained saturated soils for three to four days on the finer textured soils) or an excess of 6 inches of water was applied over an eight-day period (which saturated soils an additional three to four days).
The excess water application resulted in loss of 60 to 70 lb N/acre on silt loam and clay loam soils, due to denitrification loss. On a very coarse-textured, sandy soil, virtually all nitrate-N was moved out of the root zone by leaching. On the finer textured soils, an addition of 50 lb N/acre after the excess water was sufficient to increase corn yields to approximately the same level where no excess water was applied. This was not the case on the sandy soil because considerably more N was lost due to leaching.
Nitrate loss via tile drainage does increase with above normal rainfall. At the Gilmore City, Iowa, ag-drainage research site, where tile-flow nitrate has been monitored since 1990, nitrate loss is greatest in years with higher precipitation and hence greater tile flow. At N fertilization rates of 150 to 160 lb N/acre, the annual nitrate-N loss per acre was 52 lb in the 1990-93 period, 9 lb in the 1994-99 period, and 39 lb in the 2000-04 period (average nitrate-N losses for the combined corn-soybean sequence). The range in yearly nitrate-N loss for the years studied was 1.0 lb nitrate-N/acre in 1997 to 75 lb nitrate-N/acre in 1990.
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