<p><i>Aims</i> The experiment was conducted to understand how root morphological traits contributed to the contrasting P-acquisition efficiencies of two <i>Trifolium subterraneum</i> cultivars.</p><p><i>Methods</i> Phosphorus acquisition, root length proliferation and root acclimation to P deficiency were investigated by varying the P supply (20-300 mg P kg⁻¹) and planting density (1-12 plants pot⁻¹) of clover micro-swards.</p><p><i>Results</i> The shoot yield of both cultivars increased in response to increased P supply and increased planting density. Higher planting densities increased shoot yield by reducing the time to leaf canopy closure. In P-deficient soil, the ‘less P-efficient’ cultivar acquired less P and yielded relatively poorly compared with the ‘more P-efficient’ cultivar when planting density was low (<6 plants pot⁻¹). However, the P acquisition and yields of the cultivars were equivalent when planting density was high (~6-12 plants pot⁻¹). Both cultivars proliferated nutrient-foraging roots in response to P limitation and space (i.e. lower planting densities). However, the proliferation responses of the cultivars differed, particularly in response to space.</p><p><i>Conclusions</i> Differential proliferation responses, combined with differences in specific root length, explained how the cultivars differed in P-acquisition efficiency. The results indicate that inappropriate cultural conditions (e.g. planting densities) may confound attempts to define P-efficient root traits and to identify genotypes with improved P-acquisition efficiency.</p>