EFFECTS OF SUPPLEMENTARY SOIL NUTRIENTS ON HUMMOCK GRASSES

Hummocks of Triodia basedowii E. Pritz, T. pungens R.Br. and Plectrachne schinzii Henr. were treated with ash and with standard and high applications of slow-release fertiliser. Following rain and the ensuing growth and flowering, the soils and the hummocks were sampled for biomass and for nutrient content, and compared to controls. In a similar but separate experiment, seed set in inflorescences was assessed for hummocks treated with standard fertiliser application, for hummocks treated by removal of neighbouring hummocks, and for controls. In the absence of nutrient addition, total topsoil nitrogen concentration was higher under hummocks than outside, whereas total topsoil phosphorus concentration was lower under hummocks than outside. The absolute amount of phosphorus in the hummock itself was insufficient to account for the depletion of phosphorus in topsoil under the hummock. Available soil phosphorus was about 1-4% of total phosphorus, and was generally highest towards the edge of hummocks. Additions of ash did not significantly affect soil nutrients, while addition of fertiliser produced substantial increases in total nitrogen and in available and total phosphorus. The increases did not extend to the area surrounding the hummock. The increases in soil nutrients did not result in significant increases in new green growth, or in inflorescence biomass or number. However, nutrients appear to have been taken up into the plants, including into old tissue, because there were increases in nutrient concentration in plant tissues, which sometimes were statistically significant and sometimes not. In the second experiment, neither increased soil nutrients nor removal of neighbours increased the number of inflorescences produced, or the proportion of florets that gave rise to filled grain.