Inoculate or Not?

How do I sort out whether to inoculate my soybean seed?

Today when you decide to treat your soybean seed you have the opportunity to add an inoculant to improve nitrogen fixation. Growers don’t routinely inoculate their soybean seed in their corn-soybean rotation for a couple of reasons. In the past, treating seed with inoculum was problematic, and growers usually believed that there was not enough yield benefit to justify the cost.

While we know that rhizobia exist in the soil and supposedly in plentiful numbers in a corn and soybean rotation, farmers are still encouraged to treat soybeans by the marketplace. So how do you know if you should actually inoculate soybeans every year you plant? Maybe you see it as a small investment in risk management to ensure that every acre of soybeans has an ample population of active rhizobia to infect soybean roots. That in itself may be good reason to treat soybeans seed because every acre is not the same. But then again if you don’t test and measure, how will you know for sure?

Generally, inoculation is recommended for soils with no previous history of soybean production, are in fields that were in alfalfa or corn for five years or CRP for a decade or more. Even back in 2012 when soils were very hot and dry, some agronomists speculated that rhizobia didn’t survive the harsh climate, and soybeans would respond to being inoculated in 2013. And then salesmen pointed out that growers can realize a three to five or more bushel-yield bump if they inoculate seed at a cost of maybe $4 to $8 per acre–if that is true, then you are earning a profitable return of several bushels per acre.

Sufficient numbers of rhizobium bacteria must be present to ensure that plants form nodules. Since many soils may not contain sufficient numbers of rhizobium, inoculation is recommended to assure early formation of functioning nodules. Or, though the rhizobium bacteria may exist in sufficient numbers, they may no longer be efficient in fixing nitrogen.

Today, our best guess on whether soybeans are fixing nitrogen is digging up plants, counting nodules and cutting them open to see if they are pinkish red on the inside, which indicates they are active. In virgin fields, nodules will be located on the taproot. In fields with a soybean history, nodules also will be found along lateral roots. Eight to 14 nodules per plant indicates adequate nodulation.

The argument we hear from the marketplace is that native rhizobia strains have adapted to survive in their natural soil environment and have lost some of their ability to fix nitrogen compared to newer strains that were selected particularly for their ability to fix more nitrogen or withstand less than ideal soil conditions like low pH, wet soils, high nitrate concentrations, salts, etc.

Today commercial inoculants are composed of either native rhizobium strains or new strains selected for maximum fixation potential. However, when these new and more efficient strains are introduced into the soil, there is no guarantee they will compete well with native strains for entry into plant roots or even survive from year to year. Thus the need to inoculate every soybean crop.

Over the past couple decades, growers were told that inoculants did not work. Most inoculants in the past were applied through planter boxes and comprised of native rhizobia that were less efficient at fixing nitrogen. Today, new public and private rhizobium strains are available that are more efficient at fixing nitrogen than the native strains and under less than ideal conditions.

So what is your approach to inoculating soybean seed? Do you do it? If not, are you confident your soybeans are nodulating and fixing ample nitrogen?

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