Extremely heavy rains May 1-2, and subsequent flooding, have created concern with producers in those fields where fertilizers that were already applied prior to or at planting may be lost.
As long as the topsoil did not wash away from flooding, your phosphorus and potassium should still be in place and available to the crop.
(For information on replanting where needed, see http://southeastfarmpress.com/cotton/tennessee-flooding-0510/).
However, nitrogen is another case all together.
According to University of Tennessee Extension Soils Specialist Hugh Savoy, heavy rains leading to flooding, and saturated soils for several days, contribute to nitrogen loss through denitrification, which occurs when nitrogen in the nitrate (NO3) form is converted to nitrogen gas by bacteria in the soil. The amount of denitrification increases with the amount of time a field is water logged (soil needs to be saturated for at least 2 days in order for the bacteria to start the denitrification process).
In fields where some or all nitrogen fertilizer has already been applied, estimating the amount of nitrogen loss depends on the 1) nitrogen source used, 2) how long the nitrogen has been on the field, and 3) duration of flooding if water is standing in the field. An estimate of denitrification loss is 3-4 percent loss of nitrate nitrogen for each day soil stays saturated beyond 2 days.
Savory says post-emergence or side-dress nitrogen applications to supplement nitrogen that may have been lost should be made before the V6 stage of corn, but can be economical up to tasseling in some situations. If no nitrogen was applied at planting or the majority was lost due to flooding, side-dress nitrogen should be applied as soon as practical after V3 corn.
According to UT Extension Cotton Specialist Chris Main, the best way to determine how much nitrate is available in the soil is by a pre-side-dress nitrate test (PSNT). For more information on this test you can contact your local UT Extension office.
More detailed information on all crop issues can be found at www.UTCrops.com or in the weekly University of Tennessee IPM Newsletter. You can view the complete newsletter by clicking on the following link: http://www.utextension.utk.edu/fieldCrops/cotton/cotton_insects/newsletters/2010/IPM5-2010.pdf.