RELATIVE IMPORTANCE OF SOIL-PHOSPHORUS TO THE GROWTH OF WIREGRASS (ARISTIDA-ARMATA) COMPARED WITH OTHER GRASSES COMMON TO THE MULGA SHRUBLANDS OF QUEENSLAND

The influence of phosphorus (P) applied to mulga topsoil on the growth of Aristida armata, Cenchrus ciliaris, Digitaria ammophila, and Thyridolepis mitchelliana was examined in 2 glasshouse trials, to test the hypothesis that the invasion of A. armata into mulga shrublands may result from a superior ability to exploit and utilise soil P compared to the other grasses. Growth of all grasses was poor without added P, and all grasses produced large growth responses to added P. In contrast to the other native grasses, A. armata required as much added P as the exotic C. ciliaris to achieve maximum growth. The ability of the grasses to grow without added P was related more to an ability to absorb P than to differences in internal requirements. With no P added, T. mitchelliana had the highest P absorption rate (mug P/g root.day), followed by A. armata, C. ciliaris, and D. ammophila. Adding P reversed this order. Cenchrus ciliaris and D. ammophila had the highest relative growth rates of shoots during early growth stages, especially with P added. Although A. armata had a greater ability than C. ciliaris to access and utilise soil P at low P concentrations. there was no evidence that it was superior to the other native grasses.