To provide a scientific basis for predicting successful competitive nodulation of soybeans (Glycine max L. Merr.) by Rhizobium japonicum, the quantitative relationship between soil and inoculum rhizobia, in nodule formation, was determined in the field. Soybean seeds were planted at a rate of 1 seed/2.5 cm of row and were inoculated with up to 3.3 ✕ 1038 R. japonicum cells/2.5 cm of row. Field plots were grown on six soils in Iowa having a range of soil rhizobia populations of 1.2 ✕ 101 to 2.3 ✕ 105/g of soil. Numbers of nodules on tap and lateral roots of soybeans grown on soils containing fewer than 12 rhizobia/g were increased when inoculation rates exceeded 3.3 ✕ 104 cells/seed. Neither numbers of nodules nor total nodule mass was increased by inoculation of soybeans grown on soils containing more than 1 ✕ 103 rhizobia/g. When an inoculation rate of 3.3 ✕ 106 rhizobia/seed was used on soybeans grown in soils with fewer than 12 rhizobia/g and in soils with approximately 1 ✕ 103 rhizobia/g, the inoculum strain produced, respectively, 65% and 35% of the nodules. From our results it may be predicted that if the inoculum rhizobia are to form 50% or more of the nodules an inoculum rate of at least 1,000 times the soil population (per g soil) must be used. Commercial inoculants in the United States probably are not supplying adequate numbers of rhizobia for successful competitive nodulation of soybeans grown on land previously cropped to soybeans.

Employing a rapid agglutination test, serogroups of Rhizobium japonicum were distinguished by using homogenized nodule suspensions as antigens. Most of the nodules obtained from soybeans on Iowa soils were in one of four serogroups: 123, 135, 31, and 3. Serogroup 123 was dominant, averaging 52% in all soils except a few highly alkaline soils dominated by 135. Serogroups 3 and 31 were found in small percentages in many soils. Serogroups 71a, 110, and 117 were rarely found. Generally, variability between soil sites on the same soil type was less than variability between soil types. Serogroup 123 occupied from 79 to 100% of the nodules in 4 soybean fields on Webster, and was present in about 40% of the nodules from Clarion, Winterset, Shelby, and Lagonda soils. In a soil toposequence grading from Clarion (pH 5.9) to Harpster (pH 8.3), 80% of the nodules from Clarion were serogroup 123, but 92% of the nodules from Harpster contained serogroup 135. The predominance of serogroup 123 or 135 seemed related to soil pH. In sterile soil, serotype 123 grew well at pH values from 6.1 to 8.2; in liquid culture, it had a lower pH limit of about 4.0. In sterile soil, growth of serotype 135 appeared best around pH 8.2; in liquid culture, it had a lower pH limit of 5.5. Cropping with soybeans appeared to increase the percentage of 123, but soil type had a greater influence on the distribution of serogroups than did previous cropping history. The factors which affect the dominance of a serogroup in soybean nodules need further evaluation.

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