Is there enough Rhizobium in the soil to make a good soybean crop in many southern growing areas? No, says Brewer Blessitt, a Mississippi Delta soybean pathology/agronomy researcher who has field-trial results that back up calls for inoculating beans in “lighter, sandier ground with little history of soybean production.”

Blessitt, research associate at the Delta Research & Extension Center, Stoneville, MS, adds that even though a shot of starter fertilizer can also boost bean growth early on, it may disrupt nodulation in the long run and even hurt yields in some instances.

The in-question inoculation program is aimed at land that for decades has hosted cotton production. The pH levels may not be high enough and the Rhizobium level is too low to produce the best bean yields.

Blessitt's field and lab studies indicate that the use of inoculants on such fields will enable plants to green up quicker and generate better growth. And newer inoculant products showed a substantial increase in yields.

The inoculants researched included Launcher Pro (with PBX and ProSurge) from Precision Laboratories, Cell-Tech (with dual strains) from EMD Crop BioScience and Optimize (with an LCO promoter technology) from EMD. They were compared to Vault, a peat-based inoculant (with TA-11 Nod+) from Becker Underwood, Inc.

Each inoculant was applied in a silt-loam soil common to the Delta region. The soil pH was 6.5 and there was no history of soybean production. For each inoculant, seed was treated with Apron Maxx RTA seed treatment. An addition, molybdenum, was included on half the strip-trial plots for each inoculant.

“We expected to see higher yields from the more efficient nodule-forming inoculants and we did,” says Blessitt. “There were some noticeable differences in how the newer inoculants performed. There was as much as a 10-bu./acre difference in yields in some cases.”

IN THE STUDIES, Launcher Pro had a yield of 75 bu./acre. That compared to 68 bu. for Optimize, 67 bu. for Cell-Tech and about 64 bu. for Vault.

“Pods per plant were similar in number for the plants receiving the various treatments,” says Blessitt. “But when it got down to seed size, the Launcher Pro had the larger seed.”

At eight to nine weeks growth, plants were pulled up to observe nodulation. “Old literature suggests seven to 14 nodules typically produce optimum yields,” says Blessitt. “Launcher Pro had 13 nodules, and Optimize- and Cell-Tech-treated plants had 11. Vault plants were a little below that.”

Since soils were so low in nutrients, a starter fertilizer of about 15 lbs. of nitrogen was applied on some plots; plants kicked off well. However, areas receiving the starter fertilizer saw less nodulation in the end.

In plants that received the molybdenum application, yields were impacted very little. “If there had been some stress on the plants, we feel the moly-treated areas would have seen lower yields,” says Blessitt.

Older projections showed the need for seven to 14 nodules/root system to produce optimum yields. However, Blessitt says his studies indicate that more nodules may be needed for weaker soils, such as those formerly in cotton production.

“When the points were plotted out, a weak relationship showed yields becoming more stable and steadily increasing upwards of 50+ nodules/plant,” he says. “This seems to point to the fact that with our modern yield potentials, that nodule threshold may need to be re-evaluated.”

Blessit is encouraged by the research, but wants to see more studies in 2009 to get a clearer view of how the newer inoculants will perform.

“We expect to see close to 4 million acres of soybeans in Mississippi in 2009,” he says. “Many of those acres will be on light soil. Growers should consider using some of these inoculants in their soybean programs to help improve yields.”

Growers and their crop consultants should determine which inoculants might best be suited for their soil and overall situation, he says.