An important question in current soybean (Glycine max (L.) Merrill) production is whether inoculation is beneficial when seed is planted in soils where effectively nodulated plants have been produced previously. These studies were conducted to measure the effective of inoculating soybeans with commercial cultures of R. japonicum on seed yield and protein percentage when soybeans were grown on soils which had grown nodulated soybeans previously. Paired samples of different lots of soybean seed taken before and after commercial inoculation with R. japonicum were planted at two Minnesota locations in 1967. More extensive studies were conductedi in 1968 at five locations in cooperation with various inoculant companies. Seed from a common lot was sent to the participating companies to be inoculated. Two treated samples and an uninoculated check were returned from each company. Two samples were treated immediately prior to planting with humus samples from each inoculant company. Three humus cultures of single strains of R. japonicum, selected from the more effective strains in an inoculation study on rhizobia free soils in Minnesota, also were used as inoculants. Soybean seed yields and seed protein percentage were not significantly increased by inoculating soybean seeds with Rhizobium japonicum at planting time. The uninoculated checks were adequately nodulated by rhizobia which were in the soil from previous soybean crops. Serotyping of single strain inoculants indicated a substantial range of recovery (0–17%) depending on the strain and location.

Soybeans (Glycine max (L.) Merr.) obtain nitrogen from soil and from symbiotic fixation when nodulated with effective strains of rhizobia. These two sources of N may need to be supplemented with fertilizer N for maximum seed yields. The objectives of the present study were to determine the effects of N and sulfur fertilization on soybean seed yield and other seed properties (protein, oil, amino acid, fatty acid, and S composition). Chippewa 64 and a nodulating‐nonnodulating (nodnonnod) pair of isolines of Chippewa background were examined at three field locations over 2 years for yield response to different forms of fertilizer N (224 kg/ha) and to fertilizer S (224 kg/ha). Three readily available fertilizers (ammonium nitrate, urea, and urea + sulfur) and two slow release fertilizer (S‐coated urea and ureaformaldehyde) were mixed in the top 20 cm of soil before planting. The soils were Nicollet silty clay loam (Aquic Hapludoll), McIntosh silt loam (Aeric Calciaquoll) and Webster clay loam (Typic Haplaquoll). Response to N fertilizer was greater with the nonnod isoline than with the other two lines. Nitrogen fertilization increased seed yield, weight per seed, seed protein percentage, and kg protein/ha. Plant height and lodging were either increased or unaffected, depending upon locations and/or year. Seed oil percentage decreased following N fertilization. However, total oil production usually increased due to larger yields. All sources of fertilizer decreased N2 fixation, plant nodule weight, nodule number, and weight per nodule. Amino acids increased uniformly with protein increases but fatty acid composition was unchanged. Sulfur fertilizer increased seed yield in one case and decreased seed yield in two cases depending on location and year. Total S percentage of the seed and S‐containing amino acids of the seed were not increased with S additions. Increases in seed yield and/or seed protein percentage in the nodulating lines suggest N2 fixation failed to supply amounts of N essential for maximum seed yield and/or protein percentage. With the recent concern for energy, N2 fixation must be increased to provide the N needed for maximum seed yields of soybeans rather than relying on fertilizer N.

Soybean [Glycine max (L.) Merr.] seed yields and seed protein percentages have been increased significantly with inoculation where soybeans were not grown previously. However, when the soil contains Rhizobium japonicum from previous inoculation, significant differences in seed yield are usually not obtained from inoculation. Generally, about 5 to 10% of the nodules are produced by the inoculant applied to the seed with the other 90 to 95% of the nodules produced by R. japonicum already present in the soil. Field studies were used to identify soybean genotype‐R. japonicum strain combinations which might enhance soybean yields through more effective biological N2 fixation. Soil types in the experiments were Waukegan silt loam (Typic Hapludoll), Nicollet silty clay loam (Aquic Hapludoll) and Webster silty clay loam (Typic Haplaquoll). The first approach involved screening soybean genotypes for their ability to recover introduced R. japonicum strains from a mixed indigenous soil population. The second approach involved evaluating over 1,600 soybean lines for their degree of nodulation (relative to commonly‐grown cultivars) with the indigenous strains of R. japonicum. Twenty‐four soybean lines were selected from the 1,600 for further study in yield trials. Some lines were better nodulated than the commonly‐grown cultivars while other lines were very poorly nodulated. High recoveries of inoculant strains usually produced positive yield responses. Two of the tested soybean lines (PI 358.319 and PI 70.017) recovered an introduced strain (R. japonicum USDA 110) at enhanced levels (as high as 55% of the plant nodules were from the inoculated strain) and responded with increased seed yield at both test locations. The seed yields of PI 358.319 and PI 70.017 were increased by as much as 437 and 459 kg/ha, respectively, while the seed yields of commonlygrown cultivars were not increased. Recovery of inoculant strains was found to be independent of the soybean genotype's ability to nodulate with the indigenous R. japonicum strains. Generalizations on the recovery and effectiveness of introduced R. japonicum strains could not be made based on the soybean line's ability to nodulate with the indigenous R. japonicum strains. Each soybean genotype must be examined individually for nodulation with indigeous R. japonicum, for recovery of introduced strains, and for the general effectiveness with introduced strains to increase seed yields.

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Serogroups of Rhizobium japonicum present in the nodules of soybeans and their relationship to soil properties were studied on 613 nodule samples obtained at 75 locations during 1965 and 1966. The homogenized nodule suspensions were typed serologically by a quick agglutination test using somatic antisera of cultures of R. japonicure representing seven serogroups. Only 2.8% and 1.1% of the nodules tested failed to react with one of the seven antisera in 1965 and 1966, respectively. R. japonicum serogroup 123 was the dominant serogroup found in soybean nodules on soils below pH 7.5, while serogroup 135 dominated when the soil pH was greater than 7.8. Individual serogroups were related to at least 2 and as many as 6 soil properties. In 1966, 81% of the variation in the occurrence of serogroup 135 was attributed to soil pH. Relating the occurrence of serogroup 135 to all soil properties accounted for 84% of the variation or only 3% more than pH alone. Less of the variation in other serogroups was attributed to soil properties.