Effect of Cropping History on Soybean Growth and Nodulation and Soil Rhizobia

Published results of Rhizobium research studies are often conflicting and inconclusive as to whether soybean [Glycine max (L.) Merr.] inoculation is beneficial in soils previously cropped to soybeans. Greenhouse studies were conducted to clarify the effect of previous soybean cropping histories (soybeans grown 0 to 11 years previously or never) on soybean nodulation and growth responses and Rhizobium japonicum serogroups and population in southern Illinois soils. Sterile soybean seeds were inoculated with the supernatant from soil suspensions representing different soybean cropping frequencies and variable years since the last soybean crop. Seeds were planted in pots of sterilized sand in the greenhouse and soybean growth, nodulation, and nodule mass or dry weight were evaluated. Dominant nodule serogroup determinations were made on selected treatments. Relative N₂ fixation was estimated by means of an acetylene-reduction assay (ethylene production) and MPN determinations were made on cropping history samples in one experiment. Correlations were not significant between soybean cropping frequency and nodule number, nodule mass, or ethylene production with but one exception. In most instances, the soybean cropping history of the soils used to inoculate seed did not influence soybean top or root growth if soybeans had ever been grown on a field. No aspect of soybean cropping history significantly influenced the predominant serogroups present in soybean nodules. These results indicate that under the conditions of these experiments sufficient populations of Rhizobium japonicum persist in southern Illinois soils for at least 10 or 11 years, resulting in good soybean nodulation and growth. Based on these studies, the recommendation of inoculating soybean seed before planting for “inexpensive insurance” regardless of soybean cropping history is questionable.