Gunz And Rows – What’s Up With Nitrogen in Spring 2019?
June 24, 2019
Introduction & Background
Before digging into the situation this year, let’s take a moment to refresh ourselves on the basics of the Nitrogen Cycle. You may remember from chemistry or agronomy class the nitrogen cycle, with nitrogen moving out of the soil as crop removal and some being returned as biological fixation, synthetic fertilizer and conversions within the soil along the way. Understanding what is going on with nitrogen in the soil is often difficult, especially with the dynamic biological system that our environment is.
We can generally think of nitrogen in two forms – organic and inorganic. Organic nitrogen is in the form of manure, crop residues and soil organic matter and is not plant available in that form. For years this was the only form of nitrogen fertilization for our crops; our forefathers relied on manure from livestock and nitrogen fixed from the air by clover and alfalfa. This organic nitrogen becomes available for plants through the process of Mineralization. Organic N converts to Ammonia, which has the chemical name NH3, and then converts to Ammonium, a positively charged ion designated NH4+, and attaches to the soil. Mineralization mostly takes place when soils are warm, between 68 and 95 degrees Fahrenheit, and are moist but not completely saturated. In general, soils with higher measured Organic Matter (from your soil tests) will mineralize more nitrogen than those with lower Organic Matter, but as we are finding from research, not all Organic Matter mineralizes the same (that’s a subject for a future discussion).
Inorganic nitrogen is typically added through synthetic fertilizers, like ammonium nitrate, anhydrous ammonia, urea, and urea ammonium nitrate (UAN), but it can come along with manure as well. Each of these have different concentrations of nitrogen (28 to 82%) and have different forms of nitrogen within them. Urea is not plant available, but converts to ammonium through hydrolysis in a few days after being placed in the soil. Ammonium and nitrate are both plant available.
Ammonium (NH4+) is stable ion in the soil, attaching itself to the soil colloids. However, ammonium can be converted to nitrate (NO3-) through the process of Nitrification. Bacteria known as nitrosonomas and nitrobacter first converts ammonium to nitrite (NO2-) and then to nitrate. Nitrification occurs when the soil temperatures are between 41 and 122 F, with the greatest amount occurring between 67 and 86 F in moist and well-aerated soils.
Nitrate, being negatively charged (the same as soil particles), stays in the water solution and therefore can be lost through two different paths. Leaching, or downward movement of the nitrate away from the plant can occur. Well drained soils, either naturally or through artificial drainage, can allow nitrate to move to waterways or downward into aquifers. Nitrate can also be lost to Denitrification, a conversion of nitrate to gaseous forms of N (nitric oxide, nitrous oxide, and dinitrogen) when soils are saturated and bacteria use nitrate as a source of oxygen. Denitrification occurs in poorly drained and wet soils.
Nitrification can be slowed in synthetic fertilizers by the use of nitrogen stabilizer products. Corteva produces N-Serve® for anhydrous ammonia and Instinct II® or Instinct HL® for urea, UAN and manure that have been proven to keep nitrogen in the ammonium form longer by temporarily suppressing the bacteria responsible for nitrification conversion. While these stabilizers have often been used in fall nitrogen applications, there are distinct scenarios where their use in the spring can be of benefit as well.
Analysis & Explanations
Now that you have a basic idea of the nitrogen cycle in the soil, let’s get back to what has been going on this year. As mentioned before, we’ve had record rainfall, but along with it, colder than average air temperatures. These conditions have made a miserable environment for the reduced acres of planted corn to emerge and grow, pushing back the typical growth stages by at least a couple weeks.
But what about the nitrogen that got applied before and during the rains? What has happened to it?
Digging into the model used for Encirca Nitrogen Management, I picked out a particular scenario reported by an Encirca CSA in Illinois to investigate. First, I looked at estimated soil nitrogen (both ammonium and nitrate and total) in the top 2 feet (60 centimeters) between an application of incorporated UAN on April 20 and the first days of June.
Total nitrogen stayed at a pretty consistent level for most of this time, starting to turn lower towards the end of May. Ammonium converted slowly to nitrate initially, but started to increase later.
Next, I looked at the soil temperatures for a couple of layers – 2 to 6 and 6 to 12 inches ranges.
From the time of application until the last week of May, the estimated soil temperatures stayed right around 50 F, only starting to rise towards the end of the month, providing some explanation to the increase in nitrification.
What about losses to denitrification and leaching, and are we gaining anything from mineralization?
This chart shows the estimated nitrogen increase from Mineralization and lost through Denitrification, creating an N Balance between the two (Leaching was minimal and not shown here). During this time, extra nitrogen was generated through mineralization (this particular soil had 5.5% organic matter) which more than offset any losses from denitrification.
One would think that during this time the total amount of nitrogen in the top 2 feet should have gone down, but due to A) lower soil temperatures that kept nitrification in check (it occurred, but slower than it could have with warmer temps) and B) additional mineralization from the higher OM soil kept soil nitrogen levels in line. This is a unique situation – if soil temperatures would have been higher, we would most likely seen greater losses through denitrification than we modeled here.
That was for May, but what about as we move into June and beyond? Soil temperatures have increased since that time, converting even more ammonium into nitrate. With delayed crop development and planting, corn isn’t going to be in a position to start pulling that available nitrate up through the roots – corn takes up less than 10% of its total nitrogen needs by V6, with the majority (55-60%) removed between V6 and VT/R1. Depending on soil drainage capacity and rainfall, this nitrogen is at greater risk for loss in June than it was in May.
If you are concerned with the amount of nitrogen left in your fields, I would suggest taking soil samples at 0 to 12 inch depths and sending them to your favorite soil testing lab. Take the samples perpendicular to the application direction with multiple cores in a close distance from one another to minimize a potential bias from concentrated nitrogen at injection points. A general rule of thumb conversion is to multiply the total Ammonium and Nitrate, reported in parts per million, by 4 to get pounds per acre. This will provide you with a general idea of what the nitrogen levels are in your fields.
Better yet, reach out to your local Pioneer Encirca CSA to learn more about Encirca Nitrogen Management. They can set up your fields in the system using a proprietary soils database, localized weather, and your management, to get you an estimate of nitrogen in the soil so far and provide you guidance. This way you’ll have a better understanding if there’s enough nitrogen left to grow out your corn crop.
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