Substantial nitrogen loss from fall applied fertilizer can happen under a few key conditions:
- Warm temperatures (especially above 50°) that increase the activity of nitrifying microorganisms
- A large portion of nitrogen in the soil in the form of nitrate nitrogen (NO3-N), usually due to nitrifying microorganisms,
- Significant precipitation.
Nitrogen loss can also result from a lack of fertilizer to soil contact. For example, urea fertilizer applied without incorporation is susceptible to runoff in snowmelt or volatilization. Anhydrous ammonia applied without complete closure of the knife tracks is also vulnerable to volatilization to the air.
Microbes in the soil are critical to nitrogen loss, so let’s look at their part of this equation first. Three forms of nitrogen are very important in the soil: urea, ammonium and nitrate.
Urea will very quickly convert to ammonium in the soil, as will anhydrous ammonia. Then, a small group of microorganisms – nitrifiers or nitrifying organisms – in the soil will convert ammonium to nitrate.
These microorganisms are always present in the soil, but certain chemicals can either kill these organisms or temporarily prevent them from being able to convert ammonium to nitrate.
Warm temperatures and precipitation will contribute to nitrogen loss no matter what the timing. It doesn’t need to happen in the fall after application, just before the period of rapid nitrogen uptake the following year (usually beginning in early- to mid-June). In fact, most nitrogen losses from fall-applied N will occur in the following spring, not that fall.
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Forms of Nitrogen
Some forms of nitrogen will move with water more easily out of the soil than others. Of the three forms of nitrogen (urea, ammonium, and nitrate), nitrate (NO3-N) moves the most with water while ammonium (NH4+) largely stays put where it is. Urea ((NH2)2CO) moves easily with water as well, but usually converts to ammonium fast enough that it doesn’t leach. Keeping nitrogen in the ammonium form helps minimize nitrogen losses.
For N loss to occur due to precipitation, most of the fertilizer needs to be in the nitrate form, and precipitation needs to exceed what the soil can hold. This happens most frequently in the spring, when soils are already holding a lot of water and rainfalls are frequent.
It takes quite a lot of excess rainfall to push nitrate out of the root zone and the soil. In sandy soils, nitrate will typically move down through the soil around 12 inches with each inch of excess precipitation. Silty and clayey soils will move nitrate down five to six inches with each inch of excess precipitation.