By: Greg Roth

V1 soybean collected 6/9/14 with good nodulation on taproot.

Soybean acres continue to grow and that means that we will likely have more acres of soybeans planted into “virgin” soils, where there has been no recent history of soybeans. Sometimes achieving good nodulation is tricky, even if growers try the multiple inoculant or triple inoculation strategies that we have recommended on these sites. A key management practice on these fields would be to monitor nodulation now and determine whether nodulation has occurred on these fields.

Lack of nodulation is due to a lack of viable bacteria in the inoculant or environmental conditions that are not conducive to nodulation. Very wet or very dry conditions can inhibit nodulation. High nitrogen fertility can also limit nodulation. Sandy or low pH levels in soils can reduced bacteria levels and cause parts of fields to show less than ideal nodulation. 

Viability of the inoculant bacteria can also be hampered by exposing seed or inoculants to high temperatures prior to planting. Inoculants’ viability can also be reduced through extended exposure to the insecticide or fungicide seed treatment. Inoculant manufacturers have guidelines for the interval between treatment and planting.

We should begin to see nodulation occurring in many May-planted fields now in early June, and ideally these plants should have 8-20 nodules by early flowering. Nodules will form on both the taproot and the lateral roots. Those on the taproot are more likely associated with the inoculant applied.

Eventually fields that are non-nodulated will appear light green and likely have patches or patterns of nodulated and greener areas of the field. Note that these differ from potassium-deficient fields that have yellow leaf margins. 

The goal of early scouting is to identify non-nodulated fields prior to the onset of severe deficiency. Ideally, it is best to apply at least 60 pounds of nitrogen as a dry product that will not burn the foliage on a non-nodulated field prior to flowering. This will maximize the potential response. 

Take time now and look at some of the roots of soybeans planted in these virgin fields to assess nodulation.