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Agronomy Update

Too Much Rain? What Now?

Published on Monday, May 01, 2017

Many parts of Indiana, Missouri, and Kentucky have received over 4 in. of rain in a very short amount of time which has caused severe flooding in some areas. Because of this, I have received a number of questions from farmers wondering how long corn can survive under water and how much of their nitrogen (N) will still be there when the water finally recedes.


There are several factors that will affect how long corn will survive underwater. These include the stage of the crop at the time of flooding, how long water remains on the field, if the plants are covered in mud when the water recedes, and air and ground temperature at the time of the flood event. Young corn plants are more susceptible to floods because the growing point is still located below ground. Typically, oxygen will be depleted from the soil within 48 to 72 hours after the field becomes super-saturated. This restricts the plant from obtaining nutrients and water and it will also begin to kill the root system. Cooler air temperatures will actually help keep the plants alive longer. Typically, young corn (between the VE to V5 growth stage) can survive approximately four days if the field stays ponded and if the air temperatures are in the mid-50°F to mid-60°F range. However, if the air temperatures are between the mid-70°F to low 80°F range, I would expect a high mortality rate if the field stays ponded for more than three days.

Estimating how much N has been lost can somewhat tricky. Again, there are several factors that influence how much N has been lost. These include what form it was applied in, how long ago it was applied, and the cation exchange capacity of the soil in question. Nitrogen can be leached out of the soil when heavy rains occur, but the main cause of loss in our southern geography is denitrification. This happens when fields stay saturated long enough to completely deplete all of the available oxygen from the soil. This usually occurs within two to three days for most soils. 

Once denitrification occurs, the microorganisms begin to suffocate and begin to pull the nitrate (NO3-) molecules apart to get the oxygen. This leaves the N portion of the molecule vulnerable, causing it to escape the soil as a gas. Soils in lower lying areas are typically more susceptible to denitrification, therefore if you are applying supplemental N later in the season, they should receive an increased rate. You can calculate how much N has been lost by estimating three to four percent loss of total nitrate per day after the field has been flooded for more than two days. Dr. Lloyd Murdock, Extension Soil Scientist for the University of Kentucky, developed the table below to determine how much nitrate N was in the soil based on the form used for application. Use these totals to calculate how much N you’ve lost and need to account for at a later date.

To summarize, small corn can survive four to five days if air temperatures are cool (55° to 65°F) and two to three days if air temperatures are warm (70° to 80°F). It is best to wait three to five days after flood waters have receded and then examine the growing points of affected plants before deciding whether or not to replant. If your fields stay super-saturated for more than two days, then be conservative and estimate that you will lose four percent of total nitrate-N from the soil, and replace that N or adjust your yield goals accordingly.

As always, if you have questions regarding your crop reach out to your local Beck’s seed advisor or dealer.


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Author: Austin Scott

Categories: Agronomy, Kentucky, Tennessee


Austin Scott

Austin Scott

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