After the drought – fertilizing soybeans

Farm Forum

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; 1) 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, and 2) the additional nitrate-N uptake by the plant increases the plant sap pH which decreases available plant iron that is transferred to plant cells. 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.

What to do? First, select an IDC tolerant variety for those fields or areas that are susceptible to IDC. Other practices that can help include using a seed placement of an iron chelate product with most of its iron as the ortho-ortho form. A higher seeding rate of up to 250,000 for 30 inch rows and 300,000 for narrower rows has lessened IDC symptoms. In addition, a companion crop such as oats seeded at 1.5 bu/a with the soybean can help dry out the rooting area and utilize the carryover nitrate-N. Oat must be killed by the time it is 10-12 inches tall. Data from SDSU has shown an average of 6 bu increase to adding oats to these areas.

Minnesota results have shown increases of 30 bu/a by use of an oat companion crop. Minnesota studies also indicate that multiple management treatments such as variety, chelate, and companion crops can be combined for even greater yields than single treatments alone.

For more information, refer to the University of Minnesota’s guide “Managing Iron Deficiency Chlorosis in Soybean” at