Due to an expected jump in Mid-South soybean acreage this year, inoculants and their role in the crop's production are currently of special interest.

“Everyone is looking for an extra edge in increasing yields,” said Jeremy Ross, Arkansas Extension soybean specialist, at the recent Tri-State Soybean Forum in Oak Grove, La. “In the right circumstances, inoculants provide that edge.”

Why should growers use inoculants? “Soybean is a legume and capable of fixing nitrogen through a relationship with the soybean bacteria, Bradyrhizobium japonicum. There are different strains of the bacteria showing different characteristics: some are hardier than others, some fit certain environments better.”

The bacteria set up on the soybean roots and then get to work forming nodules and nitrogen. That nitrogen is then available to the plant and helps growers avoid costly fertilizer applications.

For a 50-bushel-per-acre soybean crop, around 225 pounds of nitrogen is required. Without nodulation, “that means a nitrogen application later in the season. In 2008, with the urea price through the roof, (such an application) would have been a very expensive and potentially unnecessary.”

Growers should check that proper nodulation is occurring. “Typically, nodulation begins around the V2 stage. It takes about 14 days for the nodules to begin forming. After that, it's another two weeks before they produce nitrogen.

“This will continue for a while — the life expectancy for nodules is something like six to seven weeks. Once they begin to decline, additional nitrogen will be formed by the plants. We begin seeing a decline after pod-fill but, at that point, the crop is past the point of needing a lot of nitrogen.”

How do soybeans form nodules? “It's a symbiotic relationship between the plant and bacteria. The bacteria are specific to the host — they only infect soybeans. The bacteria recognize compounds that are released by the soybean roots and are attracted to those compounds. Once they're in close proximity to the roots, the bacteria will, in turn, begin releasing compounds. So, it's a complicated process: root hairs will begin to turn over and envelop, or capture, some of the bacteria. The bacteria then infect the roots and, once inside, they begin to multiply and form a gall.”

Once the nodules are formed, nitrogenase, an enzyme, takes nitrogen from the air and converts it to a form the soybeans can use. This is done under anaerobic conditions, so it is “fairly unique. Even though soybeans need oxygen, the actual process for the nodules become effective, conditions need to be anaerobic — a lack of oxygen.”

The way soybean plants accomplish that is to produce a leghemoglobin that binds to an oxygen molecule. “It's kind of like the hemoglobin in your blood that binds oxygen and transports it from your lungs through your body before being expelled as carbon dioxide. That's also why, when you see active growing nitrogen-fixing nodules, they're pinkish in color.”

There are many strains of Bradyrhizobium japonicum. USDA has a collection of strains and reports claim there are about 4,000 currently in stock. The one most used in the last several years is USDA 110 and USDA 532C. Recently, there have been some new strains, the latest being TA-11.

Some strains are hardier and will survive in the soil better than others. However, this often comes at a tradeoff, as those less hardy may provide a much stronger nitrogen fixation process. There are many factors and relationships between the strains, said Ross.

A 1979 study looking at bacteria in the soil found between 100 and 1,000 indigenous Bradyrhizobium japonicum cells per gram of soil. To convert that on a per-acre basis, “you're talking about 544.8 billion cells — a bunch! Once soybeans were planted in that field, the bacterial population tripled within 16 days.

“So, there is an indigenous population of these bacteria pretty much anywhere soybeans can be grown. There are numerous bacteria out there and it only takes one to cause the infection and start the nodulation process.”

Environmental factors can cause bacterial populations to decline or increase. Among the most significant: optimum soil pH between 6.0 and 7.0. If a field's soil pH is too low, the current recommendation is to apply lime.

In the past, molybdenum was recommended frequently. A micronutrient, molybdenum is used by Bradyrhizobium japonicum in the nitrogen-fixation process. When pH is extremely high or low, molybdenum becomes unavailable in the soil matrix. That's one more reason an optimum pH level is essential for soybean health.

Soil temperatures also affect nitrogen fixation. The optimum temperature is between 77 and 86 degrees. In Arkansas, especially in the northern part of the state, “we often plant as quickly as possible in late March or first of April. A lot of times, the soil temperature isn't nearly 77 degrees. Studies have shown that soybeans that are planted fairly early in cool conditions, it extends the time for infection and nitrogen fixation to occur.”

Soil texture — especially as it pertains to soil moisture — plays a role. Populations tend to decrease rapidly in soils that have very high sand content. That's because a lack of organic matter to hold moisture means sand dries out quickly causing the bacteria to desiccate and die.

Residual soil nitrogen also plays a role and can inhibit the formation of nodules. Typically, a soybean plant will get 35 percent of its nitrogen from the soil with the balance coming from the symbiotic relationship with Bradyrhizobium japonicum. That 35 percent of nitrogen “usually comes from organic matter, fertilizer that has been used in the past.”

If the soil-based nitrogen level is very high, “it'll inhibit nodules and the (bacteria) will shut down and won't produce nitrogen until the plant uses up the supply in the soil. That's one of the big reasons that, in Arkansas, we don't recommend a nitrogen starter fertilizer for soybeans. It takes about two weeks, once the soil-borne nitrogen is depleted, for the fixation process to get going. So a starter fertilizer pushes any efficiency gains into the future.”

In the past, there were two different carriers or inoculant products: peat and water. Peat-based products are for in-box or in-furrow applications. Water-based products are typically used as a seed coat. Now, there are some newer inoculants. Sterile carriers are being used, “which don't introduce other bacteria or contaminants into the inoculants.”

Also, different packaging techniques are being employed. “Those not only increase the number of bacterial cells per volume but also enhance the life expectancy of the inoculants. So, not only are we getting more inoculant per volume but more of it is living.

“Companies are also pushing growth promoters. A common one is LCO. These promoters are nutrients or chemical enhancers that stimulate root growth or enhance the communication between bacteria and roots to make the nodules form a bit quicker than normal.”

Ross has collected inoculant-related data over the last several years. In 2008, “we had replicated trials in Arkansas. The first was in Poinsett County behind rice with an untreated check, Cell-Tech and Optimize seed treatments. There wasn't a statistical difference in yield” with all three hovering around 60 bushels per acre.

Another test, in Phillips County, involved soybeans behind cotton. “There was a big concern in 2008, with so many cotton acres going to soybeans. I know of several instances where cotton farmers began planting soybeans and, after they finished, realized they hadn't put an inoculant out. I got several calls: ‘What can I do?’ I told them the only options, at that point, were to wait and see if they got nodulation, or to go ahead put on extra nitrogen. They didn't like hearing that, especially with the price of urea. Fortunately, they did get nodulation a bit later in the season, although it wasn't as robust as if they'd inoculated.”

The Phillips County location, which had been in cotton at least 50 years, “didn't get a significant increase over the check with Cell-Tech — 60 bushels untreated versus 66 bushels. We had some problems with some of the replicated portions of the fields, so the Optimize would likely have had an increased yield over the untreated check — but they both ended up at 60 bushels.”

In tests conducted by Iowa State University looking at several inoculants, “there were no significant differences between the inoculant compounds and untreated checks. Of course, it should be kept in mind, that in these particular tests, the fields have been in continuous soybeans for 50 or 60 years so there's been a population of Bradyrhizobium japonicum (working) for a while.”

Other data from Mississippi State University show the relationship between inoculant compounds and the number of nodules per plant. The critical range is between seven and 14 nodules for maximum nitrogen fixation. The studies show the untreated check nodule numbers “were quite a bit lower, while the treated plants produced seven to 14 nodules.

“It should be noted that, unlike in the past, companies have recently stopped recommending the application of molybdenum with the inoculants. That's just because the salt in the molybdenum seems to cause the desiccation of the bacteria — kind of like pouring salt on a slug.”

Ross pointed to these current recommendations for the use of inoculants:

  • Fields that have never been planted to soybeans definitely need a dose of Bradyrhizobium japonicum.

    “From data I've seen, it seems most companies are still using the same bacterial strains. In the future that may change — I'm sure many are trying to find better forms and strains, looking at different extenders and compounds that enhance the bacteria. We'll continue to looking at this in Arkansas, for sure.”

  • A field that hasn't been in soybeans in the past three to five years needs an inoculant application.

  • If soil pH is below 6.0, Ross recommends an inoculant.

  • Consider an inoculant based on soil type/sand content.