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Feedback loops involving soil microorganisms can regulate plant populations. Here, we hypothesize that microorganisms are most likely to play a role in plant-soil feedback loops when they possess an affinity for a particular plant and the capacity to consistently affect the growth of that plant for good or ill. We characterized microbial communities using whole-community DNA fingerprinting from multiple "home-and-away" experiments involving giant ragweed (Ambrosia trifida L.) and common sunflower (Helianthus annuus L.), and we looked for affinity-effect relationships in these microbial communities. Using canonical ordination and partial least squares regression, we developed indices expressing each microorganism's affinity for ragweed or sunflower and its putative effect on plant biomass, and we used linear regression to analyze the relationship between microbial affinity and effect. Significant linear affinity-effect relationships were found in 75 % of cases. Affinity-effect relationships were stronger for ragweed than for sunflower, and ragweed affinity-effect relationships showed consistent potential for negative feedback loops. The ragweed feedback relationships indicated the potential involvement of multiple microbial taxa, resulting in strong, consistent affinity-effect relationships in spite of large-scale microbial variability between trials. In contrast, sunflower plant-soil feedback may involve just a few key players, making it more sensitive to underlying microbial variation. We propose that affinity-effect relationship can be used to determine key microbial players in plant-soil feedback against a low "signal-to-noise" background of complex microbial datasets.