540 EXTRACTABLE PHOSPHORUS IN BOTANICAL MATERIALS
PO4-P
Total extractable phosphate by 2% acetic acid extraction. Quantitative determination by ascorbic acid reduction of phosphomolybdate complex and quantitative measurement by flow injection analysis.
Summary: This method quantitatively determines the amount of phosphorus (PO4-P) in botanical materials by extraction with a solution of 2% acetic acid. Phosphorus concentration in the extract is determined spectrophotometrically by reacting with ammonium molybdate and antimony potassium tartrate under acidic conditions to form a complex. This complex is reduced with ascorbic acid to form a blue complex which absorbs light at 880 nm. The absorbance is proportional to the concentration of phosphorus in the sample. The method has shown to be correlated to plant deficiency and response to phosphorus fertilization. Samples are analyzed using an automated Flow Injection Analyzer (Lachat). The method has a routine detection limit of 50 ppm but samples that fall below 200 ppm are re-extracted yielding a detection limit of 10 ppm.
Sample amount required: 50 mg is required but 2-3 g is requested to allow for quality control and reporting on a 100% dry basis.
Prokopy, W. R. 1995. Phosphorus in acetic acid extracts. QuikChem Method 12-115-01-1-C. Lachat Instruments, Milwaukee, WI.