Effect of topdressed phosphorus fertilizer on established white clover based pastures in south-east Queensland. 1. Prediction of yield responses using soil tests

Seventeen short-term field experiments were conducted over a five year period in south-east Queensland in which rates of up to 60 kg P ha-1 as monocalcium phosphate were topdressed onto established, previously grazed, grass-white clover (Trifolium repens) pastures, Increases (P < 0.05) in yields of white clover were obtained at seven sites, but concurrent increases in grass production occurred at only four sites. Higher total pasture production resulted at six of these sites. One quantitative (total) and two empirical (0.005 M H2SO4 and 0.5 M Na HCO3) estimates of phosphorus status in 0-10 cm soil samples, collected prior to topdressing treatments, were separately correlated with relative yield responses of white clover, grass and total pasture components. Although soil phosphorus levels by all methods were statistically intercorrelated (P < 0.01), acid-extractable and total phosphorus tests were generally unsuitable for predictive purposes, having low coefficients of determination for regressions and Cate-Nelson separations of responsive from non-responsive sites. Bicarbonate-extractable phosphorus proved the most suitable soil test. It accounted for 60 and 44% of the variance in relative yields of white clover and total pasture, respectively, but was poorly correlated with relative yields of grass. The suggested critical level of soil phosphorus (bicarbonate extraction) for white clover is 28 ppm P. For total pasture, responses are likely below 22, unlikely above 28 and uncertain between 22 and 28 ppm P, respectively. Percentage variance in relative yields already explained by both empirical tests was not significantly increased by inclusion of terms for pH and exchangeable calcium into the X variable.