This thesis examines the conditions presiding over conceptual changes in soil sciences in Québec since 1960, and the legitimation of research in agrobiology. At the beginning of the seventies, researchers in soil sciences opted for a physico-chemical interpretation of soil fertility phenomena and for a scientific practice that involved analytical and experimental tools centered around the ionic movements of nutrients in soil solution and the attainment of high yields. Following soil degradation problems, researchers’ turned toward composting practices fostered by agrobiologists and foresters. After recognizing the role of organic matter in agricultural productivity, soil scientists adopted a biological interpretation of soil fertility and studied the role of microorganisms in the evolution of organic matter and in the provision of nutrients. The knowledge produced in soil biology benefited from the existence of a network of agrobiologists who, while insuring the diffusion of that knowledge, secured the agrobiological identity of composting practices for soil conservation. The commensurability of experimental practices and theoretical entities in soil fertility and agrobiology, as well as the support of agrobiologists for applying the knowledge produced in soil biology legitimated research in agrobiology.