Fertilizer is essential for successful crops; the nitrogen in fertilizer is a basic building block of plant growth and yield.

The critical importance of nitrogen

Since development of the Haber-Bosch process in 1914, increased use of nitrogen fertilizer has contributed significantly to increases in crop production. Since that time, there has been a six-fold increase in U.S corn yields. When applied to fields in commercial fertilizers, such as ammonia, urea, and ammonium nitrate, much of the applied nitrogen converts to water-soluble nitrate (NO3) very quickly via a process called “nitrification,” mediated by soil microbes. Nitrate is the form of nitrogen that plants take up from soil. Here’s how it works, in detail.

Flooding and heavy rain can rob your fields of nitrogen

But when heavy spring rains and flooding overtake our fields, water-soluble nitrate can be lost, via the processes of leaching, denitrification, and runoff.

Leaching.

When excess water infiltrates through the root zone, during periods with excess rainfall, the water takes the nitrate with it.

Denitrification.

During periods when soils are covered with water, or fully saturated, soil oxygen levels fall to zero such that soil microbes “steal” the oxygen from the nitrate (NO3) to stay alive and release gaseous nitrous oxide gas (N2O) to the atmosphere.

Runoff.

When rainfall exceeds the infiltration rate into soil, the excess water either ponds on the surface or departs field boundaries as runoff, carrying topsoil and nitrate with it.

Nitrogen is the “Achilles heel” of modern agriculture: too little or too much can be harmful. It must be managed carefully, with pro-active nutrient management that achieves “just-in-time delivery” and synchronizes the available N supply with crop demand. Because flooding increases the risk of Nitrogen losses, and thus N deficiency, it may be necessary to adjust N fertilizer plans accordingly.

Farm management choices when flooding causes substantial nitrogen loss

Determine if the crop is nitrogen deficient. Scout fields to identify areas where plants have nitrogen deficiency symptoms, which show as leaf yellowing (youngest leaves first, see picture). If you are not sure the yellowing is due to nitrogen deficiency, take a leaf sample and send to a lab for nutrient analysis (ask for rapid turnaround). Taking a soil test for nitrate is also an option to determine if N losses have reduced nitrate below recommended levels.

Re-apply fertilizer. If nitrogen deficiency affects a significant part of the field, consider re-applying a portion of the previous nitrogen application amount to affected areas. The amount to re-apply depends on severity and extent of the yellowing. Details about re-application decisions are provided here.

Wait and see. If N deficiency symptoms are not severe, wait to see if they go away or get worse. With good growing conditions, symptoms will worsen within a day or two if nitrogen is deficient. Don’t wait too long, because reduction of early growth due to nitrogen deficiency can reduce yields.

“Nitrogen is an essential part of plant biomass building blocks, such as amino acids, proteins and enzymes, nucleic acids (DNA, RNA)
and chromosomes—which are the genetic foundation of all life forms.” – Ken Cassman

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