Background and aims Cotton (Gossypium hirsutum L.) is an important cash fiber crop species, but its root traits related to phosphorus (P) acquisition have been poorly understood. Methods Eight cotton varieties that were released between 1950 and 2013 were grown in pots with or without arbuscular mycorrhizal fungi (AMF) at three P supply levels. Eleven root traits were measured and calculated after seven weeks of growth. Results At the lower two P levels mycorrhizal plants acquired more P and produced more biomass than non-mycorrhizal plants. At the highest P level mycorrhizal plants took up more P than non-mycorrhizal plants, but there was no difference in biomass. At the intermediate P level, root diameter was significantly positively correlated with biomass, P concentration and P content of mycorrhizal plants. More recent accessions had smaller root diameters, acquired less P and obtained less biomass, indicating (inadvertent) varietal selection for thinner roots that provided less cortical space for AMF, which increased the need for high P fertilizer. Conclusion Our study provides support for the importance of the outsourcing model of nutrient acquisition in the root economics space framework. Inadvertent varietal selection in the last decades, resulting in thinner roots and a lower benefit from mycorrhizal colonization, has caused a lower productivity of cotton varieties at moderate P supply, indicating the need to rethink cotton breeding efforts in order to achieve agricultural sustainability.