Traditional thinking suggests that inoculation is usually unnecessary if a well-nodulated soybean crop has been grown in the field within the past three to five years. For example, inoculate the first soybean crop in fields removed from the Conservation Reserve Program (CRP) and placed back into production, since soybean has not been grown in the field in more than ten years.
Several studies conducted over the past decade in Indiana and other Eastern Corn Belt states have evaluated the need to inoculate soybeans grown in fields with a corn-soybean rotation. These experiments have had variable results among states, suggesting that regional differences may exist.
In Indiana, inoculation experiments have generally been conducted in traditional corn-soybean rotation systems.
However, Purdue Extension conducted research in two experimental sites where soybean had not been grown for at least 15 years. At one of these sites, inoculated soybean had a yield of 75 bushels per acre while the uninoculated soybean yielded 63 bushels per acre. The other experiment site showed no yield response to inoculation. The unresponsive field was located downslope from a field where soybean was grown regularly in rotation with corn, so it is likely that water and/or wind moved soil containing enough bacteria to provide adequate nodulation.
The Purdue University Plant and Pest Diagnostic Laboratory routinely receives soybean plant samples that exhibit severe nitrogen deficiency symptoms (very light green color) and a total absence of nodules. In virtually all cases, the fields from which the plants were taken had no history of soybean production.