Understanding nitrogen in soils

What you should know

  • Numerous nitrogen (N) sources exist. Consider these when evaluating the N budget.
  • Soil type and climate greatly affect nitrogen loss from the soil system.
  • Because Minnesota has such diverse soils and climate, N cycle interpretations should be site-specific.

Here, we’ll cover how nitrogen behaves in Minnesota soil systems and how to manage it for more profitable and environmentally friendly crop production.

Basics of nitrogen

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Inputs of N for plant growth

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Nitrogen transformations

Nitrogen, present or added to the soil, is subject to several changes, or transformations. These dictate the availability of N to plants and influence the potential movement of NO3--N to water supplies.

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Nitrogen loss from the soil system

When developing N programs and evaluating environmental effects, consider nitrogen’s mobility factor in the soil. Sandy soils may lose N through leaching, while heavy, poorly drained soils may lose N through denitrification.

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Key points for crop producers

Considering the many transformations and reactions of N in soils, there are some major points to keep in mind:

  • Although you can add either organic or inorganic N forms to soil, plants only take up inorganic N (that is, NO3--N and NH4+-N).
  • One form isn’t more important than the other and all N sources can be converted to NO3--N. Commercial N fertilizers, legumes, manures and crop residues are all initial sources of NO3--N and NH4+-N.
  • Once it’s in the plant or water supply, it’s impossible to identify the initial source.
  • Nitrate is always present in the soil solution and will move with the soil water.
  • Inhibiting the conversion of NH4+-N to NO3--N can result in less N loss and more plant uptake. While it’s not possible to totally prevent the movement of some NO3--N to water supplies, sound management practices can keep losses within acceptable limits.

Fabian G. Fernandez, Extension nutrient management specialist and Daniel E. Kaiser, Extension nutrient management specialist

Reviewed in 2018

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