Pre-service teachers' mental models of the nature of soil were investigated in a sample of 181 students from four different Spanish universities, using three different methodological approaches: a phenomenographic analysis of definitions, a categorisation of labelled-drawings and the analysis of answers to a questionnaire consisting of both open- and closed-ended questions. Based on the phenomenographic analysis, four explanatory categories were defined: paedological (soil as a highly complex system); anthropocentric (soil from a utilitarian point of view); structural (soil as a layer of Earth) and naive view (soil as a surface of unknown composition and function). The most represented category in the studied sample was the structural one. Based on the questionnaire and the drawing analysis, students have some notions about soil composition, but their understating of its origin and degradation processes is scarce. No significant correlation was found between the analyses conducted using the three different instruments, thus indicating the need to use different approaches to better understand students' conceptions and their 'intermediate' epistemic models of soil. Finally, some implications for soil education are discussed.