Starter phosphorus reduces the critical external phosphorus requirements of two tropical pasture legumes

Tropical pasture legumes are expected to improve pasture productivity and forage quality in the extensive grazing systems of northern Australia. However, highly productive C4 grasses often outcompete these legumes, particularly in nutrient-deficient soils. A controlled-environment experiment was conducted to investigate the benefit of starter phosphorus (P) for tropical legume growth across a range of soil P supplies. A low-P soil was amended with basal nutrients and six rates of P fertiliser (0–80 mg P kg^–1 , as KH₂PO₄ in the basal solution). The amended soils were then incubated for five weeks to prepare contrasting ‘native’ P treatments. Following incubation, micro-swards of Centro and Desmanthus were established in the six native P treatments, with or without an application of starter P fertiliser (6 kg P ha^–1 equivalent, as KH₂PO₄ solution). The seed and starter P were applied together in a shallow row. The shoot yield and tissue P concentrations of both legumes increased in response to native P supply. The application of starter fertilizer generally increased shoot yields and reduced the critical external P requirements of both legumes (by 38% for Centro and 74% for Desmanthus). It is likely that the localised application of P fertiliser enabled the legumes to develop more root length so that the soil volume could be explored earlier. This would benefit early legume growth and establishment. These results suggest that starter P applications are likely appropriate for tropical pasture legumes, even in soils that have moderate levels of native P.