Answering fall fertility questions

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What happened to all of the nitrogen applied last fall and this spring to the 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.

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 a soil analysis has been obtained of the nitrogen present, proceed with building and managing  the desired level. But what source of nitrogen should be chosen?

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.


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