Purpose: The aim of this article is to present and apply a method to investigate farmers' socio-psychological determinants of conservation practice adoption, as an aid in extension, policy and conservation practice design. Design/methodology/approach: We use a sequential mixed method, starting with qualitative semi-structured interviews (n = 24), that serve as the basis for a quantitative survey (n = 138). The survey uses the expectancy value method to unravel the foundations of farmers' socio-psychological determinants. Findings: Some of the main findings are that non-adopters exhibit rational behaviour, given their own point of view of potential consequences of conservation practice adoption. Further, the majority of farmers are mainly under social influence by parties that are generally not involved in conservation practice promotion. Practical implications: Extension approaches should be targeted towards a broader range of issues and should involve local governments and rural dwellers. The results also suggest the value of co-learning and co-development in a combined farmer-researcher approach to design conservation practices such that the potential negative impact is reduced. Originality/value: The study has empirically proven that non-adopters in fact behave rationally, given their expectation about the broad range of issues they consider when thinking about conservation practice adoption. Its value is the introduction to agriculture of an approach that has proven its usefulness in general environmental psychology. (Contains 4 figures and 6 tables.)