Tricking the Soybean to Fix more Nitrogen

Growers and agronomists know that soybeans are legumes and fix most of the nitrogen they need in the nodules they carry and support on the roots. The lesser known fact is that soybeans probably fix (capture) only enough nitrogen to produce 50 or 60 bushels, with the rest of the nitrogen coming from the soil.

Growers producing 80 bushel or 90 bushel beans, or even 100 bushels, know that they probably need to apply some nitrogen to get there. But what is unknown is how much, when to apply and what form is best to apply. If we could predict nitrogen fixation and soil nitrogen mineralization as well as yield, we could suggest how much nitrogen to apply. But that is a lot of unknowns and a real mixed bag when it comes to making a nitrogen recommendation for soybeans.

Perhaps someday we will put all the science together to develop a model for recommending nitrogen that looks at growth and yield potential, nitrogen fixation and the soil’s ability to supply nitrogen (nitrate and mineralized amino-N). But perhaps in the short term we can focus more on increasing nitrogen fixation rates.

The first step is to make sure you have the right enviroment for the rhizobia bacteria to thrive— which includes a near neutral pH (6.2 to 6.8), EC of < 1 dS/m, good soil structure and porosity and a well-aerated profile—since the bacteria need to breathe.

The second step many growers adopt is treating seed with a rhizobia inoculant each time they plant soybeans. Sure—in a corn and soybean rotation the soil is generally teeming with rhizobia bacteria and we may question whether inoculating seed provides any payback in yield. But these ”more native” bacteria may not be the most productive or efficient at fixing nitrogen. Companies that sell these inoculum often claim their rhizobia bacteria can fix more nitrogen or are more adapted to field conditions, will thrive better than the natives and thus fix more nitrogen.

But a report published last fall suggests that a new breakthrough will allow us to significantly increase nitrogen fixation by the soybean plant itself. The scientists discovered how to activate a special transporter gene in soybeans that speeds the flow of the essential nutrient (such as nitrogen) from the roots to the shoots (and thus pods and seeds). They theorize that by exporting nitrogen away from the nodules faster, they can increase the amount of nitrogen fixed. By activating this gene, soybean plants produced 14 percent to 41 percent more pods along with an increase in the number of pods that produced three seeds, versus just one or two. The overall seed yield increased as much as 36 percent. So by moving nitrogen away from the nodules faster, more nitrogen was fixed and transported and this resulted in greater yield.

This discovery sounds promising. But how does one permanently activate the gene and do those lines become part of the breeding pool for future varieties? Also many soybean experts, including myself, raise the question whether the plant can actually sustain higher nitrogen fixation rates because it is an energy intensive process that will require the plant to produce and ship more sugar to the roots.

While this theory has been confirmed in a laboratory environment, it still has to be taken to the field for validation. And we all know that nature throws a lot of curveballs that may impact that response. Click here to learn more.

Agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.