Predicting soil organic carbon (SOC) stocks in forests is crucial to determining the C balance, yet drivers of SOC stocks remain uncertain at large scales. Across a broad environmental gradient in Switzerland, we compared measured SOC stocks with those modeled by Yasso, which is commonly used for greenhouse gas budgets. We show that soil mineral properties and climate are the main controls of SOC stocks, indicating that better accounting of these processes will advance the accuracy of SOC stock predictions.