Plant and Soil Health: Soybeans and Nitrogen Cycling

Back in May 2015 I posted a blog titled “N on Beans: A Different Perspective.” I suggested that the reason soybeans don’t consistently respond to supplemental nitrogen is due to the following factors:

• Yield potential is too low (soil and N fixation can meet the plant’s needs)
• Nitrogen fixation is very high
• Soil is capable of delivering greater amounts of nitrate than we think

On March 18 of this year a reader posted a comment based on reading this blog. He stated “I’ve done some tile water testing for N following soybean crops and found levels at times to be higher than those following corn. What can you share with me on this topic and wouldn’t this be likely to be even more of a problem if nitrogen was applied to beans?”

The reader is correct on these two points:

• Nitrogen in tile lines can be greater after soybeans than corn
• Supplemental nitrogen applied to soybeans can draw criticism from environmental quarters

Why is more nitrogen coming through tile after soybeans than corn? Soybean residue has a lower carbon-to-nitrogen ratio (25:1) than corn (70:1). That means more nitrogen is recycled faster, more nitrogen is mineralized and more is put back in the soil in the fall and again in the spring. Add this mineralized nitrogen to residual nitrate and you have significant nitrogen loading into tile lines. Planting cover crops after soybeans can capture this nitrogen and keep it on the land for the next corn crop.

During most years corn will scavenge a lot of nitrate out of the soil during the season. Of course there is the exception: the 2012 drought knocked corn yields back by 75% and a lot of that nitrogen remained in the soil and was detectable in tile lines the next spring. Corn also has a high carbon-to-nitrogen ratio of 70:1, so during decomposition any free nitrate will be taken up by microbes to break down the corn residue. This is a slow process and much of that nitrogen doesn’t become available again till the following summer. Thus, less flowing out tile lines.

There is a lot of discussion about applying supplemental commercial nitrogen on soybeans. Sometimes soybeans respond and sometimes they don’t. It all depends on the three factors listed above. No one wants to add more nitrogen to the system and have it lost to surface waters. Yet 100-bushel soybeans require a lot of nitrogen, over 500 lbs. per acre. If they can’t fix it and the soil can’t supply it, yields will be limited. That is why it’s important to understand the environment, how much nitrogen the soil can provide, how much the plant can biologically fix and what the yield potential is. If yield potential is less than 70 bushels, most agronomists recommend not considering supplemental nitrogen and I agree. Above 70 bushels, you need to look at the soil’s contribution as well as the plant’s ability to fix its own nitrogen.

Your soybean checkoff is funding research on understanding the plant’s nitrogen needs relative to yield, what the soil’s contribution is to total nitrogen demand and what the plant is capable of providing itself. The results from these studies will provide better guidelines for when supplemental nitrogen makes sense. Stay tuned.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com.