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Soil organic matter (SOM) is a key indicator of soil fertility, and building SOM is assumed to decrease reliance on external inputs and ensure stable crop production. Recent syntheses of field data support this assumption with positive SOM–productivity relationships that asymptote at ~4% SOM. Teasing out the directionality of this relationship—the extent to which SOM increases crop growth vs. greater growth leading to higher SOM concentrations—requires controlled experimentation. To disentangle this causative pathway, we conducted a greenhouse experiment whereby we manipulated SOM concentrations from 1% to 9% and evaluated whether the SOM–productivity relationship differed for spring wheat (Triticum aestivum, L.) under nitrogen fertilization crossed with irrigation due to the expectation that SOM buffers the effects of reduced fertilization and/or irrigation. We found that higher concentrations of SOM led …