How to Deal with Heavy Rainfall and Nitrogen Loss in Your Corn Crops

Are you worried that heavy rainfall caused nitrogen loss in your corn crops?

Nitrogen levels are one of the most critical factors for optimal corn yield. But heavy rainfall can wash away your valuable applications.

May 2019 was the second wettest month in U.S. history, and nation-wide rain totals were 3.32 inches above average between January and May, making it the wettest year-to-date in 125 years.

High levels of rain fell early in the growing season — across many areas of the country — delaying corn planting. As of early June, planting was still 16% behind the 5-year average. 

This one-two punch of heavy rainfall and delayed planting has been challenging for farmers.

Today, I’ll give you an in-depth breakdown of when nitrogen loss happens, what causes it, and the steps you should take to save your corn crop. 

When can nitrogen be lost? 

When we talk about the loss of nitrogen during wet weather, we’re referring to the loss of nitrate-N. Nitrate-N is the form of nitrogen most easily leached with water below the root zone, making it unavailable to the plant. It can also be lost to the air (denitrified) when soil is flooded. 

Eventually, all applied nitrogen fertilizer converts to nitrate-N. 

• Nitrate-containing fertilizers, including UAN solutions and ammonium nitrate, are at risk for loss through denitrification and leaching once they’re applied. 
• Urea can convert to nitrate-N in less than two weeks during late spring and is also at risk for denitrification and leaching loss. 
• Anhydrous ammonia converts more slowly to nitrate-N due to its initial toxic effect on the soil microbes that convert ammonium-N to nitrate-N.

Wet soil leads to nitrate-N loss.

Denitrification occurs when soil bacteria in saturated soil converts nitrate-N to oxygen and nitrogen gases. The nitrogen gas is then lost through evaporation. As a result, fields at greatest risk to denitrification are those that are naturally heavy and poorly drained.

Denitrification rates depend on saturation and temperature.

Think about how much rainfall actually soaks into your soil. Extreme rain, especially on long or steep slopes, is more likely to result in runoff. Saturation occurs when rainfall continues over multiple days rather than in shorter bursts, like passing thunderstorms.

According to research, nitrate-N is lost by denitrification at the following estimates:

• 10% when soil is saturated for 5 days and soil temperatures are between 55 and 60°F
• 25% when soil is saturated for 10 days (2.0 to 2.5% per day)
• Even higher as soil temperatures rise

Additional research indicates approximately 4-5% loss of nitrate-N by denitrification per day in soils that are both saturated and reaching temperatures higher than 65°F. 

Soil texture affects leaching rates.

Leaching occurs when water moves nitrate-N down to depths that plant roots can no longer reach. Soils at greatest risk to leaching loss are coarsely textured, sandy soils. In the trial mentioned above in Illinois, researchers found that almost all nitrate-N was moved out of the root zone of very coarsely textured soil at a rate of approximately 90-100 lb N per acre.

Soil texture also plays a role in how fast water moves down through the soil. In general, with each inch of water running down through the soil, nitrate-N will move down 5-6 inches in silt loam and clay soils and up to 12 inches in sandy soils.

Nitrate-N that moves below the corn crop’s roots can still be available later in the growing season if it hasn’t left through a tile drain. As surface soils dry out, water from below the rooting zone often starts moving back up through the soil, bringing nitrate-N with it to plant roots.

Nitrate-N loss expectations.

Assume the following nitrate-N loss rates under these conditions:

• Well-drained fields, saturated for two days or less = minimal loss
• Poorly drained fields, saturated for two days or more when soil temperatures are higher than 65°F = approximately 4% loss
• When coarsely textured or sandy soils that are already moist receive significant rainfall = all nitrate-N may leach out of the crop rooting zone

What to do when heavy rainfall causes nitrogen loss for your corn crops.

Determine how much nitrate-N you may have lost before you make plans to fix the problem. 

For example, if 90% of a 100 lb N application is converted to nitrate and soils were then saturated for 8 days when warm, the N loss estimate would be (100 lb N per acre x 90% nitrate/100) x (4% per day/100) x (8 days) = 29 lb N per acre. 

Lower or higher losses could happen, depending on the following:

• Warmer or cooler conditions
• Different forms of applied nitrogen
• More or less time from nitrogen application to wet conditions
• More or less time that soils were saturated 

Keep an eye on your corn. If you suspect nitrogen deficiency in your soil, the only rescue option is a top-dress, either flown on or applied via irrigation, due to the current growth stage.