Intrinsic capacity for nutrient foraging predicts critical external phosphorus requirement of 12 pasture legumes

The mainstream pasture legume species such as Trifolium subterraneum, T. repens and annual Medicago spp. used in the temperate pasture systems of southern Australia have high critical external requirements for phosphorus (P) (i.e. P required to achieve 90% of maximum yield). This work aimed to identify alternative pasture legume species that could be used in systems with lower P input. Shoot and root biomass of 12 species of pasture legume was measured in response to seven rates of P applied to the top 48mm of soil in a pot experiment. Most species had maximum yields similar to T. subterraneum, but some required only one-third of the applied P to achieve this. The critical external P requirement of the species, ranked from lowest to highest, was as follows: Ornithopus compressus≤O. sativus<Biserrula pelecinus<T. michelianum≤T. vesiculosum≤T. glanduliferum<T. hirtum≤Medicago truncatula≤T. purpureum≤T. incarnatum<T. spumosum≤T. subterraneum. An ability to maximise soil exploration through a combination of high root-length density, high specific root length and long root hairs (i.e. a large specific root-hair-cylinder volume) was associated with a low critical external P requirement. The results indicate that Ornithopus spp. could be used to achieve productive, low P-input pasture systems.