Siripaiboon, C., and P. Chaiwanakupt. “Evaluation fo rDinitrogen fixation and Yield of Soybean Cultivars under Conditions of Zero and Conventional Tillage by 15N Technique”. Thai Agricultural Research Journal, vol. 17, no. 2, Oct. 1999, pp. 116-28, doi:10.14456/thaidoa-agres.1999.12.