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New research links soybean iron deficiency to high nitrogen levels

Agriculture.com Staff 02/21/2007 @ 8:57am

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 checkoff 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, soybeans were planted with and without oats as a competition crop. Three rates of nitrogen (0, 100 and 200 pounds 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.

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.

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