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In the infertile sandy soils of northeast Thailand, field experiments were conducted to evaluate the effects on rates of biological nitrogen fixation (BNF) of inoculating soybeans with selected strains of Bradyrhizobium japonicum and adding herbicides. Inoculating soybean seeds with effective B. japonicum strains increased the acetylene reduction by nodulated root systems, and increased the nodule numbers and dry weights associated with soybean roots. The amounts of Nz fixed, as measured by N-15 isotope dilution methods, were also increased by adding the B. japonicum. Inoculation of soybeans with a nif -strain of B. japonicum resulted in increased nodule weights and numbers, but did not increase the acetylene reduction rates or the amounts of Nz fixed. There was a significant straincultivar interaction. Application of metolachlor, alachlor, fluazifop butyl or sethoxydim did not reduce the levels of Nz fixed by B. japonicum in association with soybeans, but paraquat did significantly reduce the Nz fixed by the inoculated plants.

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Field experiments were conducted in northern Thailand to evaluate the effect of five herbicides and three watering schedules on soybean yields and Nz fixation. The herbicides sethoxydim, alachlor, fluazifop butyl and metolachlor when added at recommended rates for weed control did not result in detrimental effects on seed yields or benefits of N2 fixation. Paraquat additions significantly reduced the amount of N2 fixed as measured by ~SN dilution methods. Inoculation of the soil with a mixed strain inoculum of Bradyrhizobium japonicum increased seed yields and measurements of Nz fixation. The application of water to soybeans only when water stress symptoms such as wilting were observed resulted in drastic reductions in both yield and N 2 fixation of four cultivars of soybeans. Reducing the water application from weekly to biweekly did not seriously reduce yields or N2 fixation benefits.

The effects of Bradyrhizobium japonicum inoculation and pre‐plant additions of N fertilizer on soybean (Glycine max L. Merrill) yields and levels of N2 fixation were studied under field conditions at two sites in Thailand. Bacterial inoculants were composed of B. japonicum strains selected for high N2 fixation levels in Thai soils. Nitrogen fertilizer addition rates used were from 0 to 250 kg N/ha in 50 kg N/ha increments. At the Chiang Mai site in northern Thailand, bacterial inoculation increased nodule weights on plants receiving 100 kg N/ha or less. Increases in nodule parameters due to inoculation were evident at 45 d after planting (DAP) but disappeared by 60 DAP. Addition of N fertilizers decreased the incidence of nodulation and sap ureide contents and decreased the contribution of N2 fixation to the N content of plants at maturity as measured by N‐15 isotope dilution methods. At the Kampang Saen site in central Thailand, bacterial inoculation had significant positive effects on nodule numbers and weights, ARA, sap ureide contents and levels of N2 fixed as measured by N‐15 isotope dilution methods. Addition of N fertilizers at this site also reduced the effectiveness of N2‐fixing symbioses. It was concluded that small additions of N fertilizer added before planting did not significantly decrease N2 fixation levels, but did have a significant positive effect on plant growth. Larger N additions would reduce N2 fixation levels in excess of the benefits of adding more N in chemical form.