As you drive across the countryside, field corn is not the only yellow crop in our Wisconsin fields. Many of our soybean acres have a pale green hue to them as well. Developmentally soybean does not start fixing nitrogen until the the V2/V3 (two to three trifoliates) crop growth stage, therefore they must rely on soil reserves for their early season nitrogen needs. Cool weather coupled with excessive rainfall has hampered nitrogen uptake by the soybean plant. Once soybean reaches the V2/V3 growth stage and nitrogen fixation begins the the soybean crop will quickly greenup. When inspecting a soybean root for nodulation carefully dig up a soybean plant and wash off the roots. Then with your knife split the nodule in half. If the nodule is firm and white the nodule is alive however nitrogen fixation has not yet begun. If the nodule is bright red as seen below in Image 1 then the nodule is actively fixing nitrogen. If the nodule is soft or black the nodule is dead. In flooded or excessively saturated soils nodulation may be delayed. Remember it takes 10 to 14 days from rhyzobial infection for nodulation to begin.
For more information on nitrogen fixation please refer to my publication entitled: Utilizing Inoculants in a Corn-Soybean Rotation