Identifying nitrogen loss factors

GENESEO, Ill. — Nitrogen is often a balancing act between profitability and optimum productivity, and the possibility of N loss further complicates the issue.

Understanding the factors that influence nitrogen loss will help growers determine their specific situation and help them make more informed decisions, according to Eric Wilson, Wyffels Hybrids agronomy manager.

Leaching, denitrification and volatilization are the main pathways for nitrogen loss.

“Leaching is the primary one that we talk about. It’s the movement of nitrate through the soil with water,” said Wilson in a video focusing on factors to help farmers identify at-risk fields.

Nitrate is the most abundant form of nitrogen because typically under warm spring conditions, all forms of nitrogen convert to nitrate. It does have a negative charge, so it’s not held by the cation-exchange capacity, and it readily moves with water.

“Losses are directly dependent on rainfall and temperature. However, it’s not always associated with heavy rainfall events. In fact, a lot of heavy rainfall events you can get surface runoff that doesn’t move through the surface profile,” Wilson said.

“You have to have warm temperatures for ammonia to convert to nitrate in order for it to be leached through the profile.”

Denitrification is the conversion of nitrate to dinitrogen. It typically happens under saturated and warm or flooded conditions.

“Denitrification is the primary loss pathway under flooded conditions. This is very dependent on temperature. Losses per day can range from 2% (50 to 60 degrees) to 5% (65-plus degrees). Significant loss does happen under saturated soil conditions, especially when we have warmer temperatures later in the spring,” Wilson said.

Volatilization is the gaseous loss of ammonia from urea or ammonia-based fertilizers.

“Volatilization is typically not a primary loss pathway for a lot of us, but it can happen. It’s directly associated with surface-applied fertilizers that are left unincorporated,” Wilson noted.

“Keep in mind we have to have a little bit of rainfall to get that nitrogen to move down into the soil profile, unless we’re doing some early spring tillage to incorporate that.”

Management Tips

Wilson gave the following tips to determine if and when to act if nitrogen loss is suspected:

• Above-average spring rainfall, specifically between May and June, usually justifies additional nitrogen applications above what was already done.

• If ponding or saturated soils have existed for extended periods, assume significant nitrogen loss to denitrification. “Keep in mind these are all temperature dependent. If it’s happening early season and we don’t have warm temperatures to get conversion to happen, it’s unlikely that we’ve lose a lot of nitrogen, especially if it’s still in the ammonium form,” Wilson added.

• A low spring nitrate test justifies an additional nitrogen application.