More isn't necessarily better when it comes to nitrogen fertilizer.

That's the conclusion from a 10-year study conducted by Agricultural Research Service scientists based in Fort Collins, Colo., and colleagues at Colorado State University.

From 1998 to 2008, the researchers compared potential management strategies for reducing nitrogen and nitrate nitrogen levels in soil and groundwater.

The research, led by ARS soil scientist Ardell Halvorson, focused on irrigated cropping systems in the Arkansas River Valley of Colorado, according to a news release.

The area has high levels of nitrate nitrogen in the fields and groundwater, due, in part, to heavy nitrogen fertilizer applications and shallow-rooted crops, such as onions.

The first study showed that onions used only 12 percent to 15 percent of the nitrogen applied to the crop. Much of the remainder stayed in the top 6 feet of soil.

The next year, Halvorson and his colleagues planted corn on the same land and found that it recovered about 24 percent of the nitrogen that had been applied to the previous onion crop.

Following the study, the scientists grew alfalfa on the land for five years, then followed it with a watermelon crop and a corn crop.

In the first year that the corn was grown, an unfertilized control plot yielded about 250 bushels of corn.

By comparison, a plot fertilized with 250 pounds of nitrogen per acre yielded about 260 bushels.

Additional corn studies following onions in rotation showed corn was a good residual nitrogen scavenger.

The results suggest that when managing fields with relatively high nitrogen levels, farmers could benefit economically from reduced nitrogen fertilizer rates.

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