Fall Nitrogen Management Following Prevent Plant

In Southwest Minnesota and Eastern South Dakota, there were many prevent plant acres in 2019. This puts many growers in a situation they have never experienced. Making decisions on where to plant which crop and how to manage crop inputs can be extremely variable. Understanding the basics of crop production will be a key when determining which management strategy will best fit your operation's goals. This article discusses the basics of nitrogen (N) management and how prevent plant may alter your normal fertility plan.

There are two forms of plant-available N in the soil: nitrate and ammonium. Nitrate is negatively charged and is mobile in the soil with water movement, aka leaching, or in waterlogged soils susceptible to denitrification (conversion of nitrate to N gas lost as a vapor). N in the ammonium form is positively charged and is stable in soil with minimal risk of loss except soil erosion.

Ammonium is converted to nitrate in the soil by soil bacteria, which is called nitrification. Since this process is mediated by a living organism, there are several factors that affect how quickly the process occurs. The main factor is soil temperature. Once soil temperatures drop below 50° F, the rate at which ammonium is converted to nitrate slows significantly. A few of the other factors that impact this process are soil pH (optimal at pH 8.5), soil moisture (optimal at field capacity), and soil aeration as nitrification only occurs in aerobic soils.

The soil gets N from several sources:

  • Organic plant residues
  • Animal manure
  • Inorganic sources like commercial fertilizers

Let’s discuss the implications of these three sources as it relates to prevent plant acres.

When plant residues are broken down by microorganisms, it adds to organic matter in the soil. When organic matter breaks down through mineralization, one of the compounds created is ammonium. This process takes time and is highly variable based on many factors.

Cover crops used as green manure can introduce large amounts of residue to the soil, which causes microbe populations that break the residue down to rise. These microbes need N to live and break down the plant residue. Nitrogen can be tied up if the residue being broke down does not have enough N in it for the microbes to use. If N-fixing legumes were used in a prevent plant cover crop, the microbes may not need to draw N from the soil to break down the residue, versus a small grain that is low in N, causing microbes to pull it from the soil.

Nitrogen from the cover crop and microbes will eventually end up in the soil; however, if the microbes are breaking down the residue at the same time your crop needs N, the crop could end up in an N-deficient state. The type, quantity, and timing of how you manage your cover crop can impact how you manage your N program.

The nitrate and ammonium portions of animal manures and commercial fertilizers are similar when it comes to N. One difference is animal manures contain plant residues that break down over time (see the previous couple of paragraphs). Some growers are applying manure to prevent plant fields now. Nitrate N from animal manure is the same as commercial fertilizer and is subject to potential loss as soon as it is applied. The organic portion or ammonium when applied early converts to nitrate quickly with warm soil temperatures. Broadcast animal manure and commercial fertilizers need to be incorporated to minimize losses due to volatilization as well. Applications of animal manure and commercial fertilizers should be done after the soil temperatures are below 50° F or plant a cover crop that can scavenge the nitrate N holding that value in the field.

Key nitrogen management strategies when following prevent plant acres with corn:

  • Good seedbed to plant into.
  • Plan to split-apply N if rainfall and high soil moisture levels persist.
  • If weather has been dry, a 2-foot-deep nitrate N soil test should be taken to see how much N has mineralized through the season.
  • Depending on density and type of residue, consider applying additional N to reduce the risk of microbes tying up the N.
  • Depending on timing, temperature, method, and formula of N application, consider using a nitrification inhibitor, N stabilizer, or urease inhibitor to help minimize losses.
 

Tony Weis

Agronomist

Sources:

Sawyer, J. 2014. Fall nitrogen applications and soil temperature. Integrated Crop Management. Iowa State University. https://crops.extension.iastate.edu/.

Web site verified 9/16/19.

Performance may vary, from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields.

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