Soil Nitrate is a Causative Factor in Iron Deficiency Chlorosis in Soybeans

Iron deficiency chlorosis (IDC) is a problem for soybean (Glycine max. L.) production in poorly drained calcareous soils in the northern Great Plains, with reported yield loss of 24%. Solution culture and soil studies show that solution bicarbonate, which increases at higher soil moisture, and soil calcium carbonate content are important factors in causing IDC. Other investigations show soybean IDC increases with addition of nitrate (NO3–) in calcareous soil in pots and with N fertilization in field plots, but the response to NO3– has not been evaluated with IDC tolerant varieties on production fields where severe IDC has been consistently observed. Our research was conducted in two parts. The object of the first part was to identify the soil factors that are associated with the greatly decreased IDC that is often observed in the tractor wheel tracks associated with secondary spring tillage. After finding that decreased soil NO3– was associated with the green wheel tracks we conducted a field plot study to determine the IDC response to N fertilization. This study included an oat (Avena sativa) companion crop to determine if uptake of NO3– by oats decreases IDC. The wheel track study showed that green wheel tracks are correlated with lower soil and leaf tissue NO3–. The N fertilization study showed increased early season IDC (V-3) and decreased grain yield with added N, with and without, the oats. The oats increased yield at 1 of the 4 study sites, but decreased yield at the one site where moisture was limiting. In addition to reactive calcium carbonate and soluble bicarbonate, NO3– must be considered as a very significant factor in the induction of IDC in soybeans and variation in NO3– and bicarbonate likely explains much of the inter annual variation typically seen on growers fields.