Basic components of this method (Moody et al. 1988) involve extraction for 18 h by 0.005 M CaCl2 at a 1:5 soil/solution ratio, with a colorimetric finish based on the method of Murphy and Riley (1962).
The automated, segmented-flow colorimetric finish described that uses AutoAnalyzer I/II technology is based on Warrell and Moody (1984). Microbore or FIA are appropriate alternatives, ideally without any heating above 37°C to speed colour development. Irrespective of the technology employed, the analyst must be guided by the operational instructions and directions given by the manufacturer. This could require changes to the specified reagents and to the flow diagram provided for this method. Whichever technology is used, the results obtained should be close to those obtained by Method 9F1. See preamble to Method 9F1 for further information on this soil P test.
Extracting Solution – 0.005 M Calcium Chloride
As for Method 9F1.
Ammonium Molybdate – Sulfuric Acid Solution
As for Method 9B2.
Dissolve 2.0 g l-ascobic acid (C6H8O6) in distilled water and dilute to 100 mL. Prepare freshly daily.
Potassium Antimony Tartrate Solution
As for Method 9B2.
Working Colour Reagent/Colour Reagent
Mix 100 mL Ascorbic Acid Solution with 140 mL Ammonium Molybdate Solution and then add 20 mL of Potassium Antimony Tartrate Solution. Dilute 1 mL of Levor IV wetting agent or a suitable substitute (e.g. Skalar 13902) in about 50 mL water and add to the mixture. Finally, add 25 mL ethanol and make volume to 500 mL with distilled water. Prepare fresh daily.
Acid-Phosphate Diluent
Pipette 8 μg P as KH2PO4 (e.g. 8.0 mL of P Secondary Standard from Method 9A2; 1 L contains 1.0 mg P) into a 200 mL volumetric flask. Add 72 mL 0.75 M H2SO4 and make to volume with deionised water.
Phosphorus Primary Standard
As for Method 9A2.
Phosphorus Secondary Standard
As for Method 9F1.
Phosphorus Working Standards
As for Method 9F1.
Prepare particulate-free soil extracts as described in Method 9F1 and transfer to clean dry tubes for automated colorimetric analysis.
Ensure individual components of the continuous flow analyser system are connected as detailed on the flow sheet (Figure 9.10) and are in accord with manufacturer’s instructions. ‘Condition’ the manifold before use and check instrument settings, then run calibration standards on commencement, after at least every 30 soil extracts and on completion. Prepare a calibration curve from the P Working Standards and interpolate concentrations of samples from the graph (or from a regression equation).
Convert P concentrations in sample solutions from mg P/L to μg P/kg of soil by multiplying by 5000 (the soil/solution ratio is 1:5 for this extraction).
Report CaCl2-extractable P (μg P/kg) on an air-dry basis.
Figure 9.10. A continuous, segmented flow sheet (AutoAnalyzer I/II technology) for calcium chloride-extractable P.