Field evaluation of a test for phosphorus deficiency in pastures based on dry matter responses induced in detached subterranean clover leaves

Four field experiments in widely differing locations in New South Wales were used to evaluate a new approach to the assessment of the phosphorus status of subterranean clover (Trifolium subterraneum L.) based pastures. Leaves sampled from plots which had received different phosphate levels were placed in either water or 6 mM NaH2PO4.2H2O for 7–8 hours. The latter were then transferred to water and both groups placed under fluorescent lights for 7 days (19 hours light, 3000 ft.c, 5 hours dark). There were no differences in dry matter increases between treated and untreated leaves from plots not deficient in phosphorus. Untreated leaves from phosphorus deficient plots showed smaller increases in dry matter than phosphate treated leaves, indicating that it may be possible to distinguish in this way between deficient and non‐deficient pasture plots. Differences between treated and untreated leaves increased with the severity of the phosphorus deficiency of the pasture plots from which the leaves were sampled. This was largely due to a decrease in dry matter produced by the untreated leaves, the increase by phosphate treated leaves being relatively independent of their initial phosphorus status. The correlation coefficient (r) over all four experiments between pasture response and induced leaf dry matter differences, the latter measured as “nutrient stress”;, was ‐0.74.