A number of areas, particularly in southern SD, had poor corn yields last year due to drought. As a result, many of these fields have high carryover levels of nitrogen. Carryover nitrate-N levels typically were over 100 lbs/a in two foot soil samples for many fields. Most of that carryover is still in the top 6 inches. Many of these fields will be rotated into soybean for 2013. For many fields or areas within a field, these high carryover N levels are not a problem, but it can aggravate those soil areas that historically have shown iron deficiency chlorosis (IDC) symptoms on soybean. These symptoms include interveinal chlorosis and stunted plant growth. Data from SDSU show soybean yield decreases of 20-22 bu/a with elevated carryover N levels compared to where carryover N is more typical.
The higher carryover N does two things to increase soybean IDC symptoms:
- The soybean has increased nitrate-N uptake which releases more bicarbonate ion at the root surface which can reduce iron availability and uptake by the plant.
- The additional nitrate-N uptake by the plant increases the plant sap pH which decreases available plant iron that is transferred to plant cells.
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Both processes can magnify IDC symptoms for those soils prone to this condition. Symptoms can be particularly extreme when the early growing season is wetter than normal. In addition, plant stresses such as herbicide injury, salinity, disease, and nematodes can intensify IDC symptoms.