Pioneer agronomist Kevin Phillips wrote an article a few years ago that is still valid today regarding our recent weather patterns. It is very good information regarding the fate of nitrogen in corn fields. He is an excellent agronomist that provides great information. He reposted that article and I asked him this morning if I could share his email (directed to Pioneer corn growers) to you. At the end, I have added few comments.
“First, I want to say I hope you are safe and well even through the storms that pushed through the area over the past days and weeks. Several areas have gotten lots of rain. I have gotten a few questions on N Loss. I have put below a previous article I wrote in a wet spring. My best guess is that we have lost N, and will need to add some extra to finish corn grain fill.
How much nitrogen fertilizer have I lost with all the rain? That is a good question; the answer I must give is that it depends on several factors. Factors may include soil type, temperatures, rainfall amounts (or after a point does it matter??), type of fertilizer, timing of application and application method.
For the most part nitrogen exists in the soil in the nitrate form (NO3-) and as the ammonium form (NH4+). Nitrogen can be lost from the soil in either of these forms. Nitrates (NO3) are lost through these three processes
- Leaching – NO3- does not bind to negatively charged soil particles and moves with water flow.
- Runoff – water flowing across the soil surface moves the NO3-.
- Denitrification – saturated soil conditions remove the oxygen from NO3 to produce the N2 which is a gas that is lost to the atmosphere.
Ammonium (NH4+) is lost through volatilization, which is not a problem from excess rainfall. Ammonium (NH4+), a positively charged ion, will bind to soil particles and does not leach as readily from the soil.
- Soil types will determine percolation rates and, in general, clay soils hold more nitrogen than sands.
- Temperatures affect the rate that denitrifying organisms break down nitrogen and release it into the atmosphere.
- Rainfall creates runoff and moves the nitrates down into the soil profile. I am not sure anyone can comprehend how much water has moved through our soils, so this is anyone’s guess.
- Type of fertilizer – it is either applied as NO3 or NH4+. Some common nitrogen-source fertilizers are:
- Urea – converts to NH4+ which would be held to the soil particles and not as susceptible to leaching, but dependent on temperatures as to how fast it is broken down.
- UAN solutions and Ammonium Nitrate granular – half NO3- and half NH4+, so half will readily leach.
- If you applied urea or NH4+ based N sources, the longer it has been applied the more time it has had to change to the nitrate form and therefore more of it has been lost.
No one knows exactly how much fertilizer is lost or if the nitrogen is still there just deeper in the soil than the roots of small plants can reach to utilize. I would advise you monitor plants for color and to apply early N to ensure you will have what the plant needs to encourage early and rapid growth as temperatures are more favorable in April than those we received in March. There are not any good tissue or soil tests that can tell you exactly how much nitrogen you have left.”
Kevin has good information and I certainly agree with his assessment and encourage you to add additional N as you monitor your crop. Our recent rainfall has certainly leached nitrate nitrogen down through the soil profile. Georgia coastal plain soils have an infiltration rate of 1 in. of water per hour per foot of soil. Nitrate-nitrogen enters into solution very easily and therefore moves through the soil profile with rainfall.
I also encourage you to conduct tissue analyses and make sure to maintain levels near the top of sufficiency ranges. If you interested in what UGA says are sufficient ranges, then go to the following url: http://aesl.ces.uga.edu/publications/plant/ and look for corn in the left hand column under agronomic crops. I also encourage you to contact your local county extension agent if you have concerns or questions regarding you sufficiency levels.