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

Abstract

<p>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.</p>