This article uses the theoretical and methodological tools of cultural historical activity theory and critical realism to examine three case studies of the introduction and expansion of sustainable agricultural practices in southern Africa. The article addresses relevant issues in the field of agricultural extension, which lacks a theoretical "bridge" between top-down knowledge transfer and bottom-up participatory approaches to learning. Further, the article considers the learning environments necessary for sustainable agriculture. Such environments provided research participants with encounters with "postnormal" scientific practices that recognise and engage plural ways of knowing. Our research explored why farmers learn and practise sustainable agriculture, how they learn and practise it, the contradictions they are facing, and how these contradictions can be overcome in a context of change-oriented learning. (Contains 4 figures, 5 tables and 4 footnotes.)