Insatiable curiosity drove me to perform cover crop biomass tests on standing soybeans at the R6.5 growth stage in an effort to determine how much nitrogen (N) is captured in the above ground biomass just prior to harvest. What I found was staggering.

R6.5 is the equivalent of black layer for soybeans. In this growth stage, the soybean plant is physiologically mature and the membrane inside the pod separates from the grain. As a result, no more nutrients are being allocated from the plant to the mature soybean.

Sampling when the beans are finished and before the foliage drops, allowed me to capture the maximum nutrient measurement in the stover. In two fields, I captured the plant biomass within a 4-square foot area, removed all the pods and sent the leaves/petioles/stems to the lab for analysis. You could argue that I should have included the pods, but not the beans, for a more accurate analysis.

For a long time, I've questioned the rationale for what is described as the "soybean rotational credit." The experts have told us that corn following soybeans earns a 30-40-lb. N credit. The University of Wisconsin and other reputable institutions claim that this credit is not N left over from the previous season. Instead, it is rather the absence of a corn-on-corn penalty whereby additional N (30-40 lbs.) is needed to overcome the "tie-up" of corn residue on the growing corn crop.

I've struggled to accept this rationale for many reasons, the greatest of which is recognizing just how much N remains in the soybean stover after harvest. The Ag-PhD FertRemoval app, which sources its information from the International Plant Nutrition Institute, reports that a 50-bu. soybean crop leaves 55 lbs. of N in the residue.

What happens to that 55 pounds of nitrogen between harvest and planting?

The experts have concluded that those 55 lbs. are not to be counted as a N credit. While this concept is a head scratcher for me, wait until you see what's next.

Plant samples taken in the two soybean fields in October of 2024 showed there were 138.9 lbs. per acre and 127.3 lbs.. per acre of N in the stover. Those numbers aren't typos and the yields were 59 and 45 bu., which are average to above average yields for our region.

Besides opening up a can of worms, here are a few logical takeaways.

1 Many researchers have assumed the N from the soybean credit is hanging out in the dead nodules of the root system. Not so. Instead, the N manufactured by the nodules moved up into the plant a long time ago and very little remains below ground. In fact, research suggests that the C:N ratio of the soybean root system is highly skewed in carbon's favor. It may actually take N from the soil for microbes to break down the roots.

2 Consider the soil surface the next year's corn ground in early May. How much soybean residue is typically left? Depending on the biological activity of the soil, moisture conditions, etc., there may be little more than a few stems from last year's soybean crop there.

Again, where does it go? These are the kind of questions that keep me up at night.