When considering a good source of nitrogen to use, urea is often the most tempting due to its competitive price compared to other nitrogen sources. It is also easy to handle and its high analysis means less bulk to transport and distribute. Despite t he competitive price of urea, many farmers are concerned about nitrogen loss by its conversion to ammonia and subsequent loss to the atmosphere. Several factors play a role in the amount of nitrogen lost from urea. If a farmer is aware of these factors and how they affect nitrogen volatilization, the broadcasting of urea can be timed to minimize loss.
Acidic soils, especially those with clay on the surface and subsequently a large reservoir of stored acidity, are less likely to lose nitrogen from urea. Most soils in the Southeast tend to be acidic (pH value less than 7.0) and in the Piedmont where clay is present at the soil surface, there is a large reservoir of stored acidity. Therefore, unless a soil is overlimed, it is safe to assume that there will be minimal nitrogen loss due to high soil pH values. The higher cation exchange capacity (CEC) of clay soils also helps retain the ammonium produced from urea and keeps it from escaping as ammonia gas. Sandy soils which have a lesser reservoir of stored acidity are more prone to nitrogen loss from urea.
Urease, an enzyme which can be found in amounts proportional to the organic matter content of a soil, is most effective in converting urea to ammonia. Soils in the Southeast tend to be low in organic matter but higher than normal levels can be found i n soils in Piedmont pastures. Urease activity in Piedmont soils could potentially be a problem but the following environmental factors will play an even more important role.
If urea is broadcast when the soil is very dry, there can be no significant enzymatic conversion to the more volatile forms since the urea must first dissolve before it can changed. If the urea is broadcast on damp or wet soil which then slowly dries over several days, nitrogen loss will be significant. This occurs because the urea can now dissolve, be in contact with the soil for conversion to volatile nitrogen, and easily escape to the atmosphere due to its proximity to the soil surface. If rainfa ll moves the urea into the soil, nitrogen loss ceases. One-half to 3/4 inch of rainfall is sufficient for most soils.
As the soil temperature increases above 50 F, so too the loss of nitrogen from urea will increase. As the temperature rises, the enzymatic reactions that breakdown the urea speed up and the urease activity will increase with temperature. Higher tempe ratures also allow more of the urea that has been converted to ammonia to escape as a gas.
The key to minimizing nitrogen loss from urea is to apply it soon before rain is anticipated. If the urea moves into the soil below the soil surface, there will be minimal nitrogen loss. If the urea must be applied and no rainfall is expected soon, t he loss of nitrogen will be in proportion to the length of time it stays on the surface of a wet soil.
These factors should also be considered when using N-30 solution (which contains 41.5% ammonium nitrate and 33% urea) and N-32 solution (which contains 44% ammonium nitrate and 35% urea). Research shows that liquid solutions of urea are slightly more susceptible to nitrogen loss than the solid form when broadcast on the soil.