Monitor Corn Nitrogen Levels during Critical V6 to Silking

Categories: GROWING, CORN
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Nitrogen is essential to the growth and development of corn because of its direct involvement with photosynthesis. According to Purdue University, corn plants begin rapid uptake of nitrogen by V6, and by silking plants take up 70% of the total N they need. If N isn’t available during these critical times, yield potential can decline, so be sure to monitor for signs of nitrogen loss or deficiencies.

Symptoms of nitrogen loss begin on the plant’s lower leaves. One common symptom is a yellow “V” that runs down the midrib of lower leaves to the center of the plant. In severe cases, plants will be stunted, look pale green to yellow and have smaller ears with “tip back” and smaller kernels. 

Nitrogen deficiency also causes poor stalk quality that can lead to late-season standability issues. This is because plants will compensate for low N levels by taking available nutrients from the plant and diverting them to the ear to improve grain fill. 

Types of Nitrogen Loss

  1. Runoff: Movement of nitrogen by water to off-target areas. Nitrogen that was not incorporated either through tillage or a gentle rainfall after application is at risk for loss due to excess rainfall and runoff.
  2. Leaching: Movement of nitrogen by water deeper through the soil profile. Nitrate and urea are both susceptible to leaching. If there is continuous wet weather, nitrogen can move deeper through the soil profile and become unavailable for plant uptake.
  3. Denitrification: Gaseous loss due to reduction of nitrates to nitrous oxide or nitrogen gas.  Saturated soils create an anaerobic environment in which microorganisms obtain oxygen by removing it from the nitrate creating gaseous forms of N, which are unavailable to plants.
  4. Temperature also affects the microbial activity in the soil. Cooler temperatures slow the process, while warmer temperatures increase the rate of nitrogen loss due to denitrification.
  5. Volatilization: Soil surface loss when the N source contains urea forms that were not incorporated into the soil.

Determining Nitrogen Levels in Corn Fields
  1. Take soil samples when the corn is 6-12 inches tall.
  2. Sample similar areas in fields, pulling 15-20 soil cores at a 12” depth. The representative area should be no more than 20 acres. Pull more cores and several samples if the area or field is larger. Remember to note on field maps where you pulled samples for reference after the results are analyzed.
  3. When pulling soil cores, avoid known nitrogen bands in the field such as anhydrous and starter fertilizer bands. Sample between bands if possible.
  4. If test results show that soil nitrate levels are below 25 ppm, add additional N to the field. Iowa State University has a formula for determining the additional N needed based on soil test results. Subtract the soil nitrate level from 25. Then multiply the difference by 8 to get the additional N needed. For example, if the test results showed 20 ppm of nitrate in the soil, the formula would be 25 ppm to 20 ppm = 5 x 8 = 40 pounds per acre of N needed.

Sidedress Nitrogen Options
  1. Anhydrous (NH3) can be used to sidedress corn and usually requires larger equipment to run through the corn. This can result in leaf burn especially on the ends when the toolbar is picked up. The use of a nitrogen stabilizer is recommended when applying NH3. 
  2. Urea can be applied with a high clearance applicator. It should be treated with a nitrogen stabilizer since it has a high risk for volatilization. It is best if rain is expected to incorporate the urea into the soil. Urea can be lost if applied during a long period of dry weather with no rain.
  3. UAN can be injected into the soil or dribbled on between rows. If dribbled, a nitrogen stabilizer should be used since it will need rain to move it into the soil.  
    • Another macro element that is partnered occasionally with UAN is Ammonium Thiosulfate (12-0-0-26s).  Similar to nitrogen, sulfur is a very leachable nutrient. An additional application blended with UAN keeps the nitrogen stable longer in the soil and provides the corn the additional sulfur it needs for the growing season.  
Contact your Golden Harvest Seed Advisor with questions or for additional agronomic insights.

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