Soil Nitrate Increases Iron Deficiency Chlorosis in Poorly Drained Mollisols of Minnesota.

Iron deficiency chlorosis (IDC) is a problem in soybean (Glycine max. L.) in poorly drained calcareous soils in the northern Great Plains, with reported yield loss of 24%. Solution culture and soil studies have shown that bicarbonate is a key factor in causing IDC but field surveys have shown that calcium carbonate equivalent, DTPA extractable iron and electrical conductivity are also associated with IDC. The observations of high annual variability in IDC intensity and the fact that soybean plants growing in the wheel tracks of the tractor used for secondary tillage can be green while other areas are yellow have long puzzled researchers and growers. Nitrate is known to intensify IDC in bicarbonate solution culture and soybeans have been shown to respond negatively to addition of nitrate to calcareous soil in pots, but the response to nitrate has not been evaluated under field conditions. In the first part of our research we did a survey of the soil factors that are associated with the green wheel tracks. In the second part we conducted a field plot study of the effects of N fertilizer on IDC, with and without a companion crop of oats. The results show that green wheel tracks are correlated with lower soil and leaf tissue nitrate. The N fertilization study showed increased chlorosis with added N, with and without, a companion crop of oats and that if soil moisture is not limiting oats increases yield. We conclude that soil nitrate is an important factor in the observed intensity of chlorosis under field conditions and it appears to explain the green wheel tracks.