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Soil C status is acritical component of greenhouse gas mitigation efforts, and supports food security through affecting crop growth and management. In much of the world, laboratory-based measures of soil C are expensive and logistically challenging, whereas map-based predictions generated at the continental scale (e.g., African Soil Information Service [AfSIS]; www.soilgrids.org) may be unreliable at the management-relevant, policy-relevant, field, and regional scales. We test whether an USD 350, open source, field portable reflectometer can provide site-specific estimates of soil C status, and also predict whether a crop will respond to fertilizer across 1,155 sites in central and southern Malawi based on an established threshold of 9.4 g C kg −1 soil. When compared with soil C measured by combustion, the scanner calibrated with covariates of field-estimable texture class and slope class provides unbiased (0.42 ± 0.44 g C kg −1 soil; p = .06), precise (R 2 = .57), and actionable (area under the receiver operating characteristic curve [AUC] = 0.88) data at the field scale, including at unmeasured locations (relative prediction error 19-23%) and at the village scale. The reflectometer outperformed predictions from the continental-scale AfSIS database, which were neither precise (R 2 = .044) nor actionable (AUC = 0.63) at the field scale, and underestimated soil C in the region by 2.5 ± 0.5 g C kg −1 soil (p < .001). The reflectometer is an accessible tool to monitor soil C in sub-Saharan Africa to improve field-level management, and guide regional policies that support food security while combating climate change.

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