EFFECT OF NUMBERS OF BRADYRHIZOBIUM-JAPONICUM APPLIED IN COMMERCIAL INOCULANTS ON SOYBEAN SEED YIELD IN ONTARIO

In the absence of Bradyrhizobium japonicum populations in the soil, yields of field-grown soybean (Glycine max (L.) Merrill) usually respond to inoculation with B. japonicum. The objective of this research was to determine the relationship between numbers of B. japonicum per seed in inoculants and soybean nodulation and yield. A total of six field experiments were conducted in 1989 and 1990 on new soybean soils. In dilution trials, Grip inoculant was applied to provide approximately 10(6), 10(5), 10(4), and 10(3) B. japonicum per seed at two locations in 1989. Nodule number and mass, as well as seed yield, increased curvilinearly upward with increasing log10 most probable numbers (MPNs) of B. japonicum. The yield response curve was best fit by a cubic equation, which accounted for 97% of the variation in yield. Seed yields increased 19% (1.83 to 2.13 Mg/ha) from 10(5) to 10(6) B. japonicum per seed. In field experiments involving 8 commercial inoculants in 1989 and 10 in 1990, and conducted at two locations in each year, responses to increasing log MPNs in the inoculants also were concave upwards and cubic. In the two years, 78 and 46% of the yield variation was accounted for by log MPN per seed. Increasing MPN per seed from 10(5) to 10(6) improved yields in first-time fields by an average of 24%, indicating the present minimum standard of 10(5) B. japonicum per seed should be increased.