Soybean production is expanding. In some situations, the crop will be planted in fields that have never been planted to soybeans or haven't been planted to soybeans for more than four years. This article provides recommendations for planting soybeans under these conditions.
Soybeans obtain up to 70% of their total nitrogen requirement from biological nitrogen fixation conducted by Rhizobia bacteria colonies (nodules) living on the soybean roots. If the nodules fail to form, the plants will become deficient in nitrogen and significant yield reductions will occur. Inoculation (nodulation) failures can occur in first-year soybeans, especially when planting into fields where the previous crop was a forage legume or where manure was applied. Forage legumes and manure may contribute enough nitrogen — 40 pounds per acre — to the soil to reduce nodulation and biological nitrogen fixation.
The first step to preventing inoculation failures is to apply the bacterial inoculant to the seed prior to or at planting and also in the seed furrow. Monitor the plants in these fields closely. If stunted and light green plants are present in late June, suspect poor nodulation and carefully dig up at least 10 representative plants in each field. Wash the soil off the roots and determine the average number of nodules per plant. Count only the nodules that are two millimeters, the width of an unsharpened pencil lead, or larger, and pink or red when split open. If there are less than seven nodules per plant, the plants will be deficient in nitrogen and supplemental nitrogen fertilizer is recommended.
For additional information about supplemental nitrogen fertilizer applications, please see the Michigan State University Extension article on “Identifying and responding to inoculation failures.”
Insects such as wireworms and white grubs are likely to feed on young seedlings when soybeans follow an old pasture or hayfield. Seed corn maggot damage will occur whenever soybeans are planted within two weeks of incorporating green plant material or manure into the soil. Damage from white grubs, wireworms and seed corn maggots can be reduced or prevented by planting seed treated with an insecticide containing one of the following active ingredients: clothianidin, imidacloprid and thiamethoxam. The potential for seed corn maggot damage can be significantly reduced by delaying planting for two weeks after green plant material or manure has been incorporated into the soil.
When planting soybeans following a perennial pasture or hayfield, the perennial forage species and perennial weeds need to be controlled prior to planting. Ideally, the herbicides were applied in the fall as this application timing produces the best results. If no herbicides were applied in the fall or if forage species and perennial weeds are still present, apply systemic herbicides this spring. A tank-mixture consisting of glyphosate applied at 1½ pounds of acid equivalent per acre plus 2,4-D ester applied at 1 pint per acre will provide the best control of most common forage species and perennial weeds this spring. Soybean planting must be delayed for seven days following an application of 2,4-D ester at 1 pint per acre. Always include 17 pounds per 100 gallons of ammonium sulfate with glyphosate applications.