story to find out more.
Legumes have tiny (2 mm) round organs called
"root nodules," filled with specialized bacteria that capture
nitrogen from the air and trade it to the legume plant for sugars the plant
produces. The pink color comes from a compound similar to hemoglobin in animal
red blood cells. Click the image for more information about it.
Mapping Manure-Hungry Soybean and Alfalfa Fields
By Don Comis
August 8, 2005
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
P. Russelle, in the ARS
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
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.
more about the research in the August 2005 issue of Agricultural
ARS is the U.S. Department of
Agriculture's chief scientific research agency.