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Watching for fertilizer burn

Some plants don’t have much of a root system while others show a beautiful root system with no shoot.  What is going on? In each case I have looked at this year, each farmer was using more than 30 pounds of total nitrogen (N) (28% plus 10-34-0) and/or sulfur in a 2×2 system. These reduced stands appear to be caused by fertilizer injury burn.
Urea ammonium nitrate (28% UAN) is made up of 50% urea, 25% ammonium, and 25% nitrate. When urea volatilizes, it turns into ammonia (the same type of ammonia in anhydrous ammonia) and is lost to the air. That’s why we need to work in urea (or stabilize it) within a few days after application if we are using it as a N source. As a starter, stabilizing N is not recommended and is not normally a problem.
So what made urea volatilize faster this year?
  1. Volatility of urea is microbial driven, so warmer temperatures make this reaction occur more quickly. Urea can volatilize within two days in warmer temperatures. If the corn plant took up water slower (due to drying conditions, soil type, or planting depth), then the root system looks burned. If the plant took up water more quickly, then the root system developed and the coleoptile was burned off.
  2. Soils that are drying out cause the ammonia gas to move away from the 2×2 band, so moist soils followed by drying conditions cause more volatility. This is the same reason why anhydrous ammonia is injected 7 to 8 inches deep, because the ammonia gas will move up from the point of injection until it hits water. As the soil dries the ammonia will move up faster.
  3. Urea volatility is enhanced by higher pH. Unfortunately, the conditions that cause us to have higher yields can also increase volatility (both are due to enhanced microbial activity).
Higher rates of N in a 2×2 band has the potential to volatilize more anhydrous, thus causing more burn. This, along with higher temperatures and drying soils, is the reason why farmers who are applying higher rates of a starter may see more damage.

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