Abstract The growing popularity of the idea of carbon budgets has contributed to a reshaping of climate policy goals in terms of carbon neutrality (net zero emissions) instead of strict emissions reductions. As a consequence, new legitimacy has been given to debatable efforts to use negative emissions for offsetting purposes. In this respect, sequestration by agricultural soils is now presented as a promising way to offset fossil carbon emissions. Adopting a historical perspective, this article studies the way soil sciences and economics amplified the political promise of agricultural sequestration, despite enduring concerns about its non-permanence and reversibility. First, it shows how, from the 1980s, the soil sciences conveyed a mechanical representation of soil as a carbon sink, and on this basis worked to assess its sequestration potential. Second, it shows how agricultural economics helped to translate this physical potential into economic opportunity, extolling its low cost relative to decarbonisation options. Both tendencies contributed to the institutionalisation of a new equivalence between the reduction of CO2 emissions and organic carbon sequestration through the enlargement of the international system of carbon accounting, and both encouraged the development of soil-based offsets, at the cost of environmental integrity.