This study examined the impact of instructional support of fourth-grade students' basic psychological needs (autonomy, competence, relatedness) on their motivation to participate in asynchronous online discussions and their argumentation skills. An instructional intervention called Need Supportive Blended Learning (NSBL) was developed and tested in the context of a blended social studies curriculum using a quasi-experimental research design. Ninety-five fourth-grade students and four teachers participated in the study. Four classrooms were randomly assigned to two conditions: NSBL and active control. Teachers in both conditions were trained to support students' argumentation skills, but only the NSBL teachers were trained to support students' psychological needs. The findings indicate that students in the NSBL condition were significantly higher on need satisfaction, motivation to participate in online discussions, and the ability to integrate arguments with counterarguments, compared with students in the active control group. Results of a path analysis imply that motivational factors are critical antecedents of learning argumentation skills through online discussions and must be considered in the context of blended classrooms. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]