It is now widely recognized that local farmers possess an important body of knowledge concerning soils and their use for agriculture. This article argues that in order for that knowledge to be useful for sustainable development interventions, it is necessary to go beyond the collection of indigenous soil taxonomies and also explore the theories farmers have on soil formation and degradation processes. Based on field research in eastern Burkina Faso, the article demonstrates that farmers' theories of soil go beyond practical rules of thumb and include complex concepts about soil processes and fertility. In this sense they are similar to scientific theories of soil. It is argued that understanding the similarities and differences in soil related concepts, such as that of soil fertility, could do much more to improve communication between farmers, researchers and development workers than only comparing taxonomies. Furthermore, capturing the grammar (theories) rather than the sentences (taxonomies) provides a much clearer insight to how farmers will deal with changing circumstances and new crops than the static way in which local taxonomies are often treated. Finally, local soil theories are a better point of departure in terms of creating the necessary comprehension of farmer practices required for effective collaboration towards sustainable development.