Mapping Manure-Hungry Soybean and Alfalfa Fields

Midwestern farmers and the Gulf of Mexico might both be better off if the farmers applied manure to soybean fields, rather than to corn, Agricultural Research Service (ARS) scientists report.

Applying manure to soybean or other nitrogen-making legumes such as alfalfa could provide an ideal way to ensure that extra nitrogen from manure doesn't end up in the Gulf of Mexico, where it can contribute to the periodic formation of an oxygen-depleted "dead zone."

Not all legume fields are equal, though. Soil scientist Michael P. Russelle, in the ARS Plant Science Research Unit at St. Paul, Minn., and a colleague have created the first large-scale maps to help locate the best possible fields, those with the highest nitrogen-making, or fixation, rates. Their maps show the varying rates in soybean and alfalfa fields across the Mississippi River basin, which drains into the Gulf of Mexico.

Plants with the highest nitrogen-fixation rates are those that are hungriest for the easy supply of nitrogen that manure offers.

Farmers usually apply manure to non-legume fields such as corn, believing that since legumes always fix their own nitrogen, why fertilize the legumes? But fixing nitrogen is optional for legumes. If you give them enough fertilizer, their nitrogen-making factories shut down.

This makes them self-regulating. If farmers put too little manure on a field, the legumes will start making their own nitrogen again, after using up the nitrogen in the manure. This eliminates the need for "insurance fertilizer," the extra commercial fertilizer farmers tend to use to prevent the possibility of nitrogen deficiencies that non-legume crops can't compensate for. And it offers farmers a safe way to dispose of abundant livestock manure.

Legume nitrogen-fixation rates vary widely, rising as soils provide less nitrogen. Russelle estimated that rates vary from 0 to 96 percent in soybeans, and from less than 20 percent to 99 percent in alfalfa.

Read more about the research in the August 2005 issue of Agricultural Research magazine.

ARS is the U.S. Department of Agriculture's chief scientific research agency.