Scientific evidence that nitrogen (N) fertilizer is polluting the northern Gulf of Mexico is not conclusive. Hypoxia in the gulf has been recognized since 1935, long before commercial fertilizer use became widespread in the 1960s. Fertilizer N use in the Mississippi River Basin has remained relatively stable in the last two decades (see graph), but the size of the hypoxic zone has fluctuated markedly, especially since 1988.

According to the U.S. Geological Survey (USGS), the annual discharge of nitrate-N from the Mississippi River has tripled in the last 30 years, with most of the increase occurring from 1970 to 1983. However, since 1980, river nitrate-N discharge has changed very little, while N fertilizer use has grown almost 10%. Using an approach similar to that used by the USGS involving a two-year lag in fertilizer N sales, from 1980 to 1999, fertilizer N sales in the Mississippi River Basin explained only 8% of the variation in river nitrate-N discharge. The Mississippi River average annual flow explained nearly 80% of variation in the size of the hypoxic area.

Recent data suggest that the molar ratio of inorganic N to inorganic phosphorus (P) is the principal indicator of phytoplankton blooms that cause hypoxia, and that the molar ratio of inorganic N to inorganic P in fresh water entering the gulf is high enough that any increase in inorganic P loading will increase phytoplankton blooms and the hypoxic area. The blame appears to be shifting from N to P fertilizer, or to both N and P. However, P fertilizer sales in the Mississippi River Basin have declined by 17% since 1980.

These facts question whether hypoxia can be blamed on fertilizer N or P use in the Mississippi River Basin. Numerous nutrient sources contribute to loading in the gulf. Atmospheric deposition, crop residue and soil organic matter decomposition, legumes, animal manure, municipal sewage sludge and effluent, and composted household wastes all contribute nutrients to the gulf.

Hypoxia results from complex interactions of chemical, biological, and physical factors, as stated in the 2001 Mississippi River/Gulf of Mexico Watershed Nutrient Task Force Action Plan. Fertilizer is a potential pollutant if used improperly. But used correctly, it increases food production and helps protect the environment.

As responsible members of the fertilizer industry and agricultural community, we need to do everything possible within our management abilities to optimize crop uptake of applied nutrients, including appropriate timing of N and P fertilizers. Application timing to avoid rainfall events that highly accelerate the risk of N and P runoff and nitrate leaching can be very helpful toward reducing potential losses of these nutrients from farm fields.

Dr. Clifford S. Snyder is southeast region director of the Potash & Phosphate Institute (PPI) in Conway, AR, csnyder@ppi-far.org.

Dr. Terry L. Roberts is senior vice president of PPI and vice president of the Foundation for Agronomic Research (FAR), in Norcross, GA, troberts@ppi-far.org.

Both men are Certified Crop Advisers and Fellows of the American Society of Agronomy.

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