Effect of Inoculum Rate on Competitive Nodulation of Glycine max L. Merrill. II. Field Studies1

Abstract 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.