Fall fertility issues: Answers for your questions

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What happened to all of the nitrogen you applied last fall and this spring to your 2012 corn with the intent of producing a bumper crop? The corn plants certainly did not use it, so where is it, and is it available for use by the 2013 crop? About every corn grower is asking that question, wanting to save money by not having to buy more nitrogen for the crop next year. Let’s tackle this hot topic with the help of University of Illinois fertility specialist Fabian Fernandez.

Subscribers to Corn Belt Update (inquire at Stu@farmgateblog.com for subscription information) learned in early September what to consider about nutrient resources that may be available after the poor 2012 crop. There are ways to assess the nutrient reserves that might be available, albeit with some difficulty:  

  • Knowing your P & K resources is difficult, says Illinois fertility specialist Fabian FernandezUptake was hard for crops because of the lack of moisture, but measuring what remains is also hard. He says, “During the season, plants have extracted P & K out of the more easily available pools, but because of the lack of moisture, the soil has to a large extent been unable to replenish those pools from less available nutrient pools--what could be called "nutrient reserve pools." While some of the moisture we are beginning to receive will certainly help the replenishment process from the nutrient reserves in the soil, those processes take time. The longer you wait in the fall to collect samples, the more reliable K test values will be.
  • Knowing how much N to apply is not easy either, says Fernandez. The indirect method is easiest, which he says is a function of calculating how much grain and biomass was produced and removed, and subtracting that from the amount of the last soil test. He says the direct method is more reliable, and that is determining the soil nitrate-N level. He says, “A composite of at least 12 cores should be collected, taken to a depth of two to three feet from representative portions of the field and at different positions with respect to the crop-row. Each foot of sampling depth should be kept as a separate sample for analysis.” But some of that will be lost over the winter, so he suggests sample and apply nitrogen in the spring.

Once you have obtained a soil analysis of the nitrogen present, you can proceed with building and managing  your desired level. But what source of nitrogen will you choose?

Nitrogen Sources:

The only sources of nitrogen for fall application that are recommended by fertility specialists are anhydrous ammonia and ammonium sulfate. Fernandez says ammonia quickly converts to ammonium, and ammonium sulfate is already there. Ammonium quickly attaches to soil particles and is protected from leaching. But other forms of nitrogen which are in the nitrate form do not attach and can be leached away with soil moisture heading toward field tiles and waterways. Those include ammonium nitrate and urea ammonium nitrate (UAN). Urea converts to the desired NH4 overtime but has a greater potential for loss before it can be utilized by the spring crop. The same can be said for the polymer coated urea products, and they are lost before anhydrous ammonia is lost.

Fernandez says a desired benefit of anhydrous ammonia is that it kills nitrifying bacteria, preventing its conversion into a nitrate. To lengthen and enhance that process is the purpose of a nitrification inhibitor, such as N-serve, which is the most valuable when used while bacteria are active above 50 degrees F. Fernandez says, “The use of a nitrification inhibitor might not pay every year. For example, if the following spring is dry and cool, the inhibitor might not be as beneficial to enhancing ammonium recovery. However, the practice will overall ensure the greatest chance to protect your N investment and at the same time enhance environmental protection.”

He says ammonium sulfate is a good choice for no-till fields and is always best to apply it before soils freeze so it has the chance to dissolve and be absorbed into the soil. However, it will require a higher amount of lime because it is more acidifying than other nitrogen sources.

Application Timing

Since harvest was finished earlier than normal due to early planting and a smaller crop, Fernandez, many farmers may want to get fall tillage and fertility application completed  early as well. That means nitrogen may be applied at warmer soil temperatures where nitrifying bacteria are lying in wait to convert your nitrogen into a nitrate and send it down the river. Consequently, either nitrogen applications have to wait or be combined with an inhibitor such as dicyandiamide (DCD) or N-Serve.

While bacterial activity slows below 50 degrees, they are still active above 32 degrees. He says apply nitrogen by the temperature and not the calendar. So, if the daily temperature does not exceed 50 degrees, is it acceptable to apply nitrogen? No, the soil temperature is the key, and Fernandez says, “Since soil temperatures can be influenced by multiple factors (including residue cover, soil color, and drainage), it is always best to monitor soil temperatures in individual fields prior to N application.”

Application Methods

Soil characteristics should be a major determinant of application. A light, sandy soil should not get a fall application because it just won’t remain there until next spring. Heavier organic soils can hold it through the fall, winter and spring, so keep the soil type at the top of the list on issues to consider. Fernandez says ensure soil conditions are fit for the application, and the means not too dry or not too wet. Either will allow the liquid to volatilize and either escape between dry soil particles or up through the unsealed knife track.

Amount to apply

Your ultimate decision on the nitrogen available to the crop should be based on an optimal rate determined by the cost of the nitrogen and the price of corn, which is the Maximum Return to Nitrogen or MRTN.  MRTN can easily be determined by the corn nitrogen rate calculator that is available for IA, IL, IN, MI, MN, OH, and WI soil conditions. The only entries needed are your state (and location), whether you have a C-C or C-S rotation, the type of nitrogen source you plan to use, its cost per ton, and the price of corn. Yes, it is simple. You will be provided with an optimal rate in pounds per acre, along with a profitability range, and the actual N cost per acre when applied at the MRTN rate.

Fernandez reminds you that you do not need to apply all of it in the fall, and should consider applying some in the spring. He says that is because, “Many fields will likely have high nitrate levels this fall because of the drought, and it is uncertain how much of that N will be present for the next crop. If a good portion is available, that should be all the plant needs to get started until sidedress time, which would reduce the need to supply additional N in the fall. If N is not present because of excessively wet conditions in the spring, chances are that a fall application of N could suffer similar losses.”

While many farmers will apply nitrogen in the fall to take advantage of the time availability, Fernandez says that is an expensive decision. “An ongoing study over three years showed that fall applications reduced yield 17% relative to preplant applications done within three weeks of planting. The difference in yield, averaged across N rates, was 23 bushels per acre less with fall than with preplant applications.” Based on 2013 harvest prices, that is a $150 per acre loss, with a fall application, compared to a spring application.

Source: FarmGate blog


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