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Issue No. 2, Article 6/April 8, 2011

Early-Spring Nitrogen Application

Every spring corn growers have to make many important decisions in order to produce the best possible crop. Two of those decisions are when and how much nitrogen (N) to apply.

Let's talk first about when. Early preplant applications are a good way to get ahead of the busy planting season. At this time of the year, soil and air temperatures are typically too cool to start planting, but moisture conditions in the field are sometimes adequate to apply nitrogen. In general, preplant applications can be as effective as sidedress applications. My concern with early preplant applications is the potential for N loss if there are warm temperatures followed by wet soil conditions. Warm soil temperatures (above 50°F) when soils are not saturated with water enhance bacterial activity that converts ammonium (NH4+) to nitrate (NO3-) in the process called nitrification.

Once nitrification starts on applications made in early April, it is not unusual to have excessively wet conditions in May and June. When soils become saturated, the potential for N loss is directly related to the amount of N present in the nitrate form. Under water-saturated conditions, nitrate is most likely to be lost through denitrification in fine-textured soils and through leaching below the root zone in coarse-textured soils. Last year, for example, was one of those years when soil conditions were ideal for early preplant N applications, but after a few weeks of warm and dry weather, soils became saturated with water. Under those conditions we observed that using a nitrification inhibitor, such as N-Serve, or polymer-coated urea, such as ESN, helped protect N from loss compared to urea or anhydrous ammonia without N-Serve. Other applications later in the growing season, either close to planting or at sidedress, did not benefit from the use of inhibitors or polymer coatings.

These findings do not necessarily mean you must use a nitrification inhibitor or polymer-coated urea for early-spring applications, but it is good to keep in mind that in most years, the longer the time elapsed between application and when the crop starts using N, the greater the chance for potential N loss. Another important point to keep in mind (all the time, but especially early in spring when soils tend to be wet) is that if soil conditions are too wet you can risk N loss with anhydrous ammonia applications as the injection knife may cause sidewall smearing. When that occurs, ammonia gas tends to move preferentially back up the knife slot and can escape to the atmosphere.

How much nitrogen to apply, then? Corn plants do not need large amounts of N early in the season. If you apply a total of 30 pounds per acre or so, it is typically enough to get the crop growing until sidedress-N application time. In some years the supply of N from the soil alone, through the process of mineralization, can be sufficient for early growth, but we normally prefer not to take chances. While split applications do not save as much time as single applications, they are becoming more popular because they can reduce the risk of N loss should soil conditions become conducive to it. For those who applied N in the fall, so far this year conditions have been favorable to preserve it, and I suspect that as long as fall applications were done as recommended, there is little need to worry about N loss to this point.

To determine the total amount of N needed for the corn crop we recommend using the maximum return to N (MRTN) approach. This method determines application rates using data from many yield response to N-rate trials conducted under conditions of minimal N loss potential and integrates economics using corn and N prices. Information on the approach can be found at www.extension.iastate.edu/Publications/2015.pdf (Adobe PDF); the corn nitrogen rate calculator to determine application rates can be accessed at extension.agron.iastate.edu/soilfertility/nrate.aspx.

A final point I would like to make regarding application rate has to do with a false concept that has been circulating this winter: that "anhydrous ammonia applications should not exceed 10 pounds of N per unit of CEC." This concept has no scientific foundation. Cation exchange capacity (CEC) is a soil property that is important to determining liming application rates to correct pH or to determining the capacity of a soil to supply plant nutrients such as Ca++, Mg++, K+, and NH4+. When anhydrous ammonia (NH3) is applied in the soil, ammonia reacts with organic matter and with clay, and--most importantly--it dissolves in water. Anhydrous means "no water"--when anhydrous ammonia is applied in the soil, NH3 (note there is no "+" charge) reacts with water to form ammonium (NH4+).

In essence, NH3 takes a hydrogen ion (H+) from water (H2O) to form the positively charged NH4+ ion. Once the NH4+ ion is formed, it is held on the soil exchange complex and kept from moving with water. The initial reactions with water, organic matter, and clays limit the mobility of ammonia and help retain N that otherwise could be lost by ammonia volatilizing to the atmosphere. However, CEC has no direct relationship to how much ammonia a soil can hold at the time of application. Factors that are important in ammonia retention (in addition to organic matter, clay, and water content) include soil texture, soil structure, and application method (including depth of injection and proper closure of the knife track). In some of our trials, after ensuring adequate soil moisture conditions and proper application depth, we have successfully applied more than 200 lb N/acre in sandy-textured soils. The bottom line is that you should not worry about the CEC of the soil in determining how much anhydrous ammonia to apply. If you want to read more on this subject, there is a very informative article at www.extension.iastate.edu/CropNews/2011/0223sawyer.htm.--Fabián G. Fernández

Fabián Fernández

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