Plant-soil feedback (PSF) has been shown to be a key factor driving plant performance and species diversity. However, it is unclear how soil conditioned by diverse species mixtures modulates the plant productivity-species diversity relationship through PSF. Here, we tested the effects of a gradient of soil conditioning diversity and concurrent plant species diversity on plant biomass production in a full factorial experiment. Soils conditioned by multiple species produced the well-known positive diversity-productivity relationship. In contrast, when soils were conditioned by a single species, there was no diversity-productivity relationship. However, while overall productivity increased with soil conditioning diversity, that effect rapidly plateaued at higher levels of conditioning diversity. Our results highlight that a soil-borne legacy of species diversity is required for biodiversity-productivity relations to arise, suggesting that PSFs overrule concurrent complementarity effects. Our study highlights the importance of conditioning soil diversity for optimal biomass productivity, particularly at lower levels of conditioning diversity.