The Paris Agreement reached at the COP 21 signals the new centrality of carbon sinks, including soils, as a key means of enabling a net zero carbon global balance through the development of ‘negative emission technologies’. This article focuses on the 4 per 1000 Initiative, which the French government launched to foster soil carbon sequestration with an international scope. We present and discuss the multidisciplinary design of our research, the methodological tensions that it entailed and the assets that it eventually represented for a better consideration of soil agency and vulnerability. Drawing on an in-depth qualitative investigation, we discuss soil carbon sequestration promises in light of the broader politics of soil knowledge. We argue that the 4 per 1000 promise may have failed to give adequate attention to soil’s complex agency (e.g. its capacity to both release and sequester carbon and other greenhouse gases) and soil’s vulnerability (e.g. to urbanisation and intensive agriculture, which have long been causing soil degradation, including carbon loss). Committing to a richer conception of soil agency and soil fragility is crucial to enact the social and environmental conditions for fostering efficient and sustainable technological and political change in relation to climate change.