<p>Plant-soil microbe interactions are major drivers of ecosystem productivity in many systems, yet our understanding of how aboveground diversity can moderate these interactions is still developing. Additionally, the role that soil microbes play in influencing floral phenology and production is not yet clear. To understand how aboveground plant diversity can moderate the impact that soil microbes have on plant phenology and productivity, we conducted a factorial growth chamber experiment, examining effects of soil microbes associated with both high and low diversity grasslands on growth and flowering of <i>Chamaecrista fasciculata</i>, an annual legume species native to Eastern US grasslands. We found that plants grown with only the soil bacterial communities originating from low diversity grasslands had reduced plant productivity and phenology – reducing biomass, floral production and duration, causing earlier floral initiation, greater skew in flowering distributions, and decreased co-flowering times of conspecifics compared to those grown in sterile soils and in soils with fungal communities. Soil microbial influence from high diversity grasslands was much less profound. These results indicate that soil microbes can have significant impacts on plant productivity and phenology, possibly due to a build-up of pathogenic taxa in low-diversity plant communities. Additionally, we found evidence that soil bacteria can vary more widely in their effects on plants than soil fungi, as has been observed in other systems. Together, these results build on our understanding of how plant community composition can influence soil microbial communities, and then in return, how these soil communities can influence individual plant performance.</p>