This protocol is a basic and can be used for different purposes. In our lab we mostly use this type of extraction for phosphate and sulfate assay. Plants can take up only inorganic forms of phosphorus
– phosphates (P i ). P i assay shows metabolically active phosphorus in a plant. Sulfur is available to plants primarily in the form of anionic sulfate (SO 4 2− ) present in soil. SO 4 2− is actively transported into roots and then distributed, mostly unmetabolized, throughout the plant. SO 4 2− assay reflects amount of sulfur taken up by plant.
The determination of P i is based on the colorimetric method in which a blue colour is formed by the reaction of ortho-phosphate molybdate ion and antimony ion followed by the reduction with ascorbic acid at an acidic pH. The phosphomolybdenum complex is read at 620 nm. SO 4 2− ion is precipitated in a strongly acid medium with barium chloride. The resulting turbidity is measured photometrically at 420 nm. In both assays photometric results are compared with appropriate calibration standard solutions.