In this chapter we introduce soil biogeochemical processes that have broad influence on Critical Zone function. Soil comprises all unconsolidated material above competent bedrock that is open to - and structured by - fluxes of energy and matter. These fluxes drive biotic and abiotic reactions that result from the open system disequilibria. While (bio)geochemical reactions occur at the molecular scale, they have implications for ecosystem function and human sustainability. Many of the characteristic changes of the Anthropocene - including climatic and land use changes - have direct influence on soil biogeochemistry with cascading effects on the capacity of the Critical Zone to sustain human life. Here we highlight several of these "scalable" soil biogeochemical reactions, give examples on how they are impacted by external forcings, and show how they "reach" beyond the extent of soils to impact atmosphere, surface- and groundwater composition.