Soil organic matter underlies crop nutritional quality and productivity in smallholder agriculture

•Soil organic matter is significantly related to crop nutrient composition.•Soil organic matter is impacted by land use type and farm management.•Increasing soil organic matter by 1% can meet the nutrient needs of an extra 0.5 persons per hectare. Global crop yield gains have not be associated with increases in the many macro- and micro-nutrients needed for a balanced human diet. There is thus growing interest in improving agricultural practices to increase nutrient availability to people. Because nutrients in crops come from soil, soil management—such as building soil organic matter—could be a tool in managing agriculture to produce more nutritious food. To understand the relationship between soil organic matter and nutritional quality, we measured soil organic matter fractions, crop yield, and wheat nutrient composition on smallholder farms along a land-use and land-cover gradient in Ethiopia. We found that wheat yields and protein content were related to organic matter nitrogen, and zinc content was related to organic matter carbon. Increasing organic matter carbon by 1% was associated with an increase in zinc equivalent to the needs of 0.2 additional people per hectare; increasing organic matter nitrogen by 1% was associated with an increase in protein equivalent to the daily needs of 0.1 additional people per hectare. Soil organic matter—and its associated fractions—was greatest in soils closest to a state forest and in home gardens (as opposed to in wheat fields). Wheat fields closer to the forest had elevated soil organic matter fractions relative to wheat soils closest to the market town. Our results indicate that realistic gains in soil organic matter could make human-health-relevant increases in wheat nutrient content. Soil organic matter management can therefore be an additional tool for feeding the world well.