Composts are commonly used as soil amendments to sustain and improve the functionality of agricultural soil. Compost has abiotic (organic matter [OM], nutrients) and biotic characteristics (microorganisms) and both can influence the soil microbiome. The abiotic and biotic characteristics of compost, in turn, depend on properties of the compost such as maturity. Few studies have investigated the relative effects of abiotic and biotic components of compost on the soil microbial community and crop growth. To bridge this gap, we used a full-factorial design with sterile and live composts that differed in maturity (fresh, intermediate, mature) that were added to sterile and live soil to investigate the separate role of abiotic and biotic characteristics of composts on the resulting soil microbial community and on wheat growth. We found that the changes in the soil microbial community were mainly due to the input of compost with the presence of microorganisms rather than due to the abiotic properties of compost. The majority of the compost-associated microorganisms (more than 70% for bacteria and 90% for fungi) were detected in the soil in the presence of native soil microorganisms. Elimination of native soil microorganisms by sterilization enhanced the prevalence and abundance of compost-associated microorganisms. Adding fresh compost increased wheat biomass production, but the positive effects of compost on plant growth were strongest when sterile composts were used. Hence, our study reports that compost-associated microorganisms are essential to modify soil microbial community but may not benefit crop growth. This highlights the importance of understanding the role of abiotic and biotic properties of composts as common soil amendments on improving the functioning of agricultural soil.