<p>In the last decades, extensive research has been conducted on argumentation and classroom discourse, but few studies have developed combined analyses on interaction patterns and co-construction of arguments. In this paper, we analyse the discussion of a group of ninth-grade students (ages 14–15) concerning soil science, with the aim of investigating the co-construction of arguments and the interactions in a naturalistic context. In order to collect data, pupils’ discussions during a session were recorded in audio and video files. The session was framed in a teaching unit in which the students were presented with a land allocation problem, and they had to solve it using information about soil properties. A network to analyse interactions and Toulmin’s scheme to analyse argumentation were used. The results suggest that the roles adopted by the pupils and the teacher were different from those of a traditional classroom, and that the tasks succeeded in encouraging the pupils to reason and discuss. On the other hand, when analysing the co-construction of the arguments, we observed that the quality of discourse was not the same in all the analysed episodes. In the first one, an argument with a greater articulation of ideas is constructed, whereas in the last one, we found different ideas that were merely juxtaposed, constructing parallel arguments. We conclude that the quality of the classroom dialogue is high when the participants manage to articulate ideas that interact with each other, referring to the remaining ideas during the process of constructing meanings.</p>