When planting soybeans in Kansas, it can be a good insurance policy to inoculate the seed, says Kansas State University agronomy professor Chuck Rice.
"Soybeans are big users of nitrogen, removing about three to four pounds of nitrogen per bushel of seed," says Rice, who is a soil microbiologist with K-State Research and Extension. "Soybeans that are poorly nodulated will have to take up most of the nitrogen they need from the soil. Since nitrogen fertilizer is generally not applied for soybeans, a crop that is poorly nodulated will quickly use up the available nitrogen in the soil and become chlorotic from nitrogen deficiency."
Soybean inoculant contains Bradyrhizobium japonicum bacteria. The Bradyrhizobium bacteria forms nodules on soybean roots, and these nodules fix nitrogen from the atmosphere and supply it to the plants, Rice explains. If soybeans have been grown on the field in previous years, there may be enough Bradyrhizobium bacteria in the soil to nodulate the soybeans adequately. In that case, an inoculant will not benefit the crop.
"But if there is not enough Bradyrhizobium in the soil, the inoculant may increase yields by two bushels per acre or more on fields that have had soybeans in the recent past," he adds. "On fields where soybeans have never been grown, the inoculant can increase yields by 10 bushels per acre or more."
Rice says that soybeans should be inoculated in the following circumstances:
- Where the field has not been planted to soybeans for the past four years or more;
- Where the soil pH is less than 5.5 or greater than 8.5;
- Where soil erosion has occurred since the last time soybeans were grown;
- Where soil organic matter levels are less than one percent; and/or
- Where there has been severe drought or flooding.
If soybean plants are chlorotic and nitrogen deficient despite being inoculated, that probably indicates the inoculant has failed, the agronomist says.
"There may be several causes of poor nodulation and inoculation failure, including: poor quality inoculant; poor storage and handling; or poor seed coverage with inoculant," Rice says.
"Most fungicide seed treatments should not harm the inoculant if applied according to directions, but be sure to check the label of the specific fungicide seed treatment to be used," adds Doug Jardine, K-State Extension plant pathologist.
If the inoculation has failed, producers may need to apply nitrogen to their soybean crop. Producers may need to apply as much as 80 to 100 pounds of nitrogen per acre in that case, says Dave Mengel, K-State soil fertility specialist.