Although competent argumentative writing is important for academic success and daily life, it is difficult to teach, especially in middle schools where English is taught as a foreign language (EFL). Previous research has indicated that extensive reading or writing improves the descriptive writing skills of EFL students. However, it is not certain whether the positive effect of extensive reading or writing applies to other domains such as argumentative writing, which is demanding but crucial. In this study, we explored whether the argumentative and descriptive writing skills of middle school EFL students could develop naturally, i.e., via extensive reading or writing, without direct instruction, using descriptive writing as a comparative variable. A total of 297 EFL middle school students in South Korea were divided into three groups: the first group read books ("n" = 119), the second wrote about various topics ("n" = 117), and the third (control) received extended regular instruction ("n" = 61) over two semesters. Pre- and post-writing tests assessed argumentative and descriptive writing. Our results indicated that reading and writing exercises significantly improved argumentative and descriptive writing competence, but regular instruction did not. However, descriptive writing improved more than argumentative writing, particularly in terms of organization, content, and mechanics. Extensive reading or writing may constitute a feasible option for improving general writing skills. However, more context-specific methods to enhance adolescent argumentative writing in EFL schools are required.