** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** PLANT FACILITATION OF RHIZOSPHERE MICROBES PROMOTES PLANT NUTRIENT ACQUISITION FROM SOIL ORGANIC MATTER (SOM), PROVIDING A BASIS FOR SOIL HEALTH AND SUSTAINABLE FERTILITY. MICROBIAL ACTIVITY IS PRIMED BY PLANT EXUDATES AND THIS RHIZOSPHERE PRIMING IS A MAJOR DETERMINANT OF SOM DYNAMICS. HOWEVER, PRIMING MECHANISMS REMAIN POORLY DESCRIBED BECAUSE THEY OPERATE THROUGH COMPLEX MULTIPARTITE INTERACTIONS BETWEEN PLANTS, MICROBES, AND SOILS. WE HYPOTHESIZE THAT PHENOLIC ACID BACTERIA ARE CATALYSTS OF RHIZOSPHERE PRIMING, STIMULATED BY EXUDATES TO METABOLIZE SOM AND ENHANCE PLANT ACCESS TO NUTRIENTS LOCKED IN SOIL. WE PREDICT THAT THIS PRIMING RESPONSE REQUIRES: I) PROLIFERATION OF PHENOLIC ACID BACTERIA, AND II) INDUCTION OF PHENOLIC ACID METABOLISM. WE PREDICT THAT BOTH ROOT EXUDATION AND SOM DYNAMICS CAN PROMOTE PROLIFERATION AND INDUCTION, CAUSING PRIMING TO VARY AS A FUNCTION OF PLANT HOST AND SOIL HEALTH. OBJECTIVE 1 EMPLOYS A FIELD EXPERIMENT INVESTIGATING PLANT CAPACITY TO PROMOTE PROLIFERATION AND INDUCTION OF PHENOLIC ACID BACTERIA. OBJECTIVE 2 DETERMINES THE DISTRIBUTION OF PHENOLIC ACID BACTERIA IN AGRICULTURAL SYSTEMS AND THE DEGREE TO WHICH THESE MICROBES RESPOND TO SOM AND MANAGEMENT. OBJECTIVE 3 IMPLEMENTS A GREENHOUSE EXPERIMENT THAT WILL INCLUDE PLANTS THAT DIFFER IN EXUDATE PRODUCTION, BACTERIAL INOCULA DERIVED WITH OR WITHOUT PHENOLIC ACID ENRICHMENT, AND INOCULA DERIVED FROM SYNTHETIC COMMUNITIES WITH OR WITHOUT INCLUSION OF PHENOLIC ACID BACTERIA. THE PHENOLIC ACID HYPOTHESIS EXPLAINS PLANT-MICROBE INTERACTIONS IN RELATION TO SOIL PROPERTIES AND SOM DYNAMICS. THIS MECHANISM HAS THE POTENTIAL TO EXPLAIN A MAJOR COMPONENT OF BIOLOGICAL SOIL HEALTH THAT UNDERLIES SUSTAINABLE SOIL FERTILITY IN DIVERSE AGRICULTURAL SETTINGS.