Context Tropical pasture species are often established in phosphorus (P)-deficient soils that can limit plant productivity and persistence. Little is known about the root traits and critical P requirements of many tropical grasses and legumes. Characterisation of these traits would allow P-efficient species to be used when soil fertility is poor and agricultural inputs are limited, or for important root traits to be considered if selecting for P efficiency. Aims To assess the shoot yield, root morphology, P acquisition and critical P requirements of a range of commonly grown tropical pasture species. Methods Five tropical grasses and seven tropical legumes were grown in pots containing a light-textured, low-P soil to investigate shoot growth and root traits in response to applied P (0–120 mg P kg−1 soil). Key results The shoot yield of each species increased in response to applied P, yet there were differences in maximum shoot yield (1.7–9.8 g DM pot−1) and critical external P requirements (12.8–38.0 mg P kg−1 soil) among the species. The acquisition of P was associated to varying degrees with the development of root length when plants were grown in soil that enabled near-maximum growth (e.g. R2 = 0.71–0.77 for the grasses and R2 = 0.14–0.43 for the legumes in the 15–30 mg P kg−1 soil treatments). Conclusions Longer roots were associated with higher shoot yields and better P-acquisition efficiency, and the grasses generally had comparable or lower critical P requirements than the legumes. Implications Phosphorus-efficient species should be used when soils are known to be P deficient, or where P fertilisation is not an option, and grasses and legumes could be paired on the basis of their P requirements.
McLachlan et al. (2025) studied this question.