Phosphorus compounds measured in a rapid and simple leaf test for the assessment of the phosphorus status of subterranean clover

Abstract

Experiments were done to determine what proportion of the phosphate extracted from fresh leaf tissue by five drops of 10 N H2SO4 represents inorganic tissue phosphate, and to what extent hydrolysis of organic phosphates during and after the extraction, and during the development of the blue phosphomolybdate complex, could contribute to the values obtained. The extraction is the basis of a simple and rapid test for the assessment of the phosphorus status of subterranean clover (Bouma and Dowling 1982). Extraction of leaf tissue of subterranean clover and sunflower with 0.2 M HClO4 at O¦C, which was shown to extract inorganic leaf phosphorus without causing significant hydrolysis of organic phosphates, gave values not significantly different from those in H2SO4 extracts. The rate of hydrolysis of endogenous organic phosphates in tissue, extracted and left at room temperature for periods of up to 40 min. after adding H2SO4, did not differ significantly from zero. Errors due to hydrolysis during the 30 min. previously recommended for colour development are reduced to negligible proportions by reducing the time for colour development to 10 min. and by adding citric acid at this point. Anion-exchange chromatography of 10 N H2SO4 and 0.2 M HClO4 extracts confirmed the similarity of their composition and provided estimates of the various phosphate compounds present. The extraction of fresh leaf tissue with 10 N H2SO4 provides a satisfactory estimate of the endogenous inorganic phosphorus content.