St. Paul, Minnesota
February 12, 2007
By George Rehm, University of
Minnesota Extension
For many years, an explanation of
the severity of Iron Deficiency Chlorosis (IDC) in soybeans has
evaded those who have worked to solve the problem. That
situation changed in 2005 when there was an effort to get an
explanation for green soybeans that were growing in wheel tracks
through otherwise yellow soybeans. These yellow plants were
affected by IDC.
A search of published literature combined with the results of a
survey produced a theory that the severity of IDC was linked to
high levels of nitrate-nitrogen in the soil that subsequently
increased the concentration of nitrate-nitrogen in the soybean
plant. If this theory was true, a competition crop that would
take up nitrogen from the soil should be effective in reducing
the severity of IDC. Soybean producers provided observations
that supported the concept of the competition crop.
With the financial support from the soybean check-off in 2006,
trials were established in west central Minnesota to test the
accuracy of the theory proposed at the end of the 2005 growing
season. At three sites (Chippewa County, Yellow Medicine County,
West Central Research and Outreach Center) soybeans were planted
with and without oats as a competition crop. Three rates of
nitrogen (0, 100, 200 lb. of nitrogen per acre supplied as
46-0-0) were broadcast and incorporated before the planting of
soybeans in both planting systems (oats, no oats).
Glyphosate was applied to the soybean crop when the oats reached
a height of 12 to 14 inches. Oat samples were collected at this
time and analyzed for nitrogen. Nitrogen uptake by the oat crop
was calculated from the nitrogen concentration and dry weight
data.
In evaluating the information collected, it appears that the
theory developed from the survey conducted in 2005 was correct.
The severity of IDC was increased by adding nitrogen to the soil
system. The additional nitrogen in the soil resulted in more
nitrogen in the soybean plant as well as additional
nitrate-nitrogen.
Now the challenge is to develop management practices that keep
concentrations of nitrate-nitrogen in the soil to a minimum. One
approach would be to avoid using excessive amounts of fertilizer
nitrogen for the preceding corn crop. Another approach is to use
a competition crop such as oats. Obviously, more research is
needed to develop management practices for this competition
crop.
A more detailed article is available in the Jan. 4, 2007,
edition of Minnesota Crop eNews at
http://www.extension.umn.edu/cropenews/2007/07MNCN03.htm. It
is published by the University of Minnesota Extension.
George Rehm recently retired as a soil scientist with the
University of Minnesota Extension. |
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