This work presents an assessment of pre-service teachers' argumentative practice, after implementing a novel teaching-learning sequence on soil health including a citizen science programme, which was applied outdoors at the university garden. The sequence was implemented at five Spanish universities with 351 undergraduates studying Early Childhood and Primary Teacher Education. It posed a final assessment task consisting in a real-world situation that involved making decisions on science-related issues: students needed to argue whether it was possible to use a piece of land as a school garden, based on soil data provided in a variety of formats. To assess participants' level of achievement, a rubric was specifically designed by adapting the "Evidence-Explanation Continuum" approach, which was applied to a subsample of 123 answers (35%). Results evidenced that the process of knowledge-building discourse from initial data to final explanations involved a series of transformations of increasing difficulty, since the percentage of students who were able to correctly accomplish them decreased a long the continuum. Including the citizen science programme promoted the development of basic aspects of scientific literacy related to interpreting data and evidence scientifically but, for students to be generally capable of drawing evidence-based conclusions, argumentation practices should be regularly included in science classes.