Field evaluation of a leaf test for assessment of the phosphorus status of subterranean clover and for prediction of its response to phosphorus

Abstract

A series of field experiments (5 phosphorus levels x 6 replicates) on the southern Tablelands of New South Wales was used, firstly, to confirm the suitability of a modified test for inorganic phosphorus (Pi) concentrations in fresh clover (Trifolium subterraneum) leaves as an index of the current phosphorus status of subterranean clover-based pastures; and, secondly, to evaluate the usefulness of Pi as a predictor of responses to phosphorus applications. A close relation (R2 = 0.910) was found between Pi in healthy green leaf tissue sampled in winter and field responses to phosphorus measured at the same time. The fitted curve had a critical value of 154 ppm Pi at 90% of the fitted asymptote for relative yield. Close relations were also found between total herbage yield measured over the season and Pi in leaf tissue sampled in late autumn and early winter (R2 = 0.896) and in leaves sampled in late winter and early spring (R2= 0.877). Critical values were 160 and 153 ppm Pi respectively. The relation was less close for the third sampling (seed set and flowering, R2= 0.809) and the critical value had declined to 118 ppm Pi. It is concluded that Pi determined in clover leaf samples provides a simple measure of the current phosphorus status of subterranean clover-based pasture, and of its likely response to phosphorus. A critical value of 150 ppm Pi is confirmed for assessing the current phosphorus status, and is also proposed for predictive purposes. The method described for the estimation of Pi is a further simplification of the method presented previously. It is also more rapid and achieves a saving of 80-90% in chemicals. The correlation coefficient for the straight line relationship between the two methods was 0.956 (68 observations).