The effect of pH and PBI on the critical phosphorus requirements of two tropical pasture species

Tropical pasture species are often grown in soils with available phosphorus (P) concentrations below their critical P requirements. However, little is known about how key soil traits such as pH and Phosphorus Buffering Index (PBI) influence the critical P requirements of these species. Two controlled-environment experiments were conducted to investigate the effect of different starting pH and PBI on the shoot yield and P acquisition of Digit and Desmanthus. In the first experiment, the two species were grown in low-P soil that was amended to achieve five soil pH treatments (4, 5, 6, 7 and 8). In the second experiment, the two species were grown in a low-P soil mix that contained varying combinations of a low-PBI soil and a high-PBI soil to achieve five soil PBI treatments (65, 145, 225, 305 and 385). Ten soil P treatments (0–120 mg P kg^–1 ) were prepared by adding KH₂PO₄ solution to the soil surface. The shoot yields of Digit and Desmanthus increased in response to the higher application rates of P in both experiments. In the soil pH experiment, both species were most productive in the pH 5–8 treatments. Critical external P requirements were lowest in the pH 7 treatment and increased at lower and higher pH levels. In the soil PBI experiment, critical external P requirements increased significantly with PBI. Nevertheless, critical internal P requirements remained relatively stable. Tissue P tests may therefore be a useful way to determine likely responses of tropical pasture species to P fertiliser application across a range of soil types.