Effect of topdressed phosphorus fertilizer on established white clover based pastures in south-east Queensland. 2. Macronutrient status and prediction of yield responses using plant chemical tests

Details are given on the effect of topdressed phosphorus, at rates up to 60 kg P ha-1, on macronutrient concentrations and phosphorus uptakes of components of white clover based pastures from phosphorus responsive and non-responsive sites. These data were obtained from 17 short-term field experiments conducted over a five year period in south-east Queensland. Plant indices for predicting yield responses to phosphorus by these pastures were derived from nutrient concentrations in tops and in 'diagnostic' samples of white clover collected during spring to early summer. Mathematical approaches used had little effect on critical values obtained. For white clover tops, which comprised fresh leaves, petioles and flowers, a critical phosphorus concentration of from 0.28 to 0.30% was established. Alternatively, an N : P ratio of 15 effectively separated responsive from non-responsive sites. The critical phosphorus concentration based on 'diagnostic' samples, which differed from tops in that flowers were excluded, was 0.30%. There was no practical difference between diagnostic indices based on white clover or total pasture production.