This method is very similar to Method 9I2a, apart from its analytical finish. This is undertaken by the colorimetric vanadate finish, which is suited to higher solution P concentrations than is the Murphy and Riley colorimetric finish.
0.01 M CaCl2 solution, the Phosphate Equilibrating Solution, and the P Primary Standard (1L contains 100 mg P) are identical to those for Method 9I2a.
Vanadium molybdate solution: Dissolve 40.00 g ammonium molybdate and 2.00 g ammonium (meta) vanadate in 1.5 L warm distilled (preferred) or deionised water (temperature should not exceed 50°C). Add 280 mL nitric acid (sg. 1.20, see below) and make to 2.0 L with water. See Note 1 of Method 9B2 on the reason for preferring distilled water.
Dilute 780 mL concentrated nitric acid (sg. 1.42) to 2.0 L with distilled or deionised water.
Add appropriate aliquots of the P Primary Standard to 0.01 M CaCl2 to achieve final working standards ranging from 0 to 100 mg P/L in 0.01 M CaCl2.
Weigh 7.0 g air-dry soil (<2 mm) into 100 mL bottle or centrifuge tube and add 70 mL of P Equilibrating Solution and 3 drops of chloroform (CHCl3) and proceed with soil equilibration as for Method 9I2a.
For the vanadomolybdate analytical finish, pipette 10 mL aliquots of cleared soil extracts (as per Method 9I2a) and Working Standards (0 to 100 mg P/L) into 100 mL volumetric flasks and add sufficient 0.01 M CaCl2 to give an approximate volume of 60 mL. Next add 25 mL of vanadomolybdate solution, make to volume with 0.01 M CaCl2, and mix. After 60 min, read absorbance at 400 nm in a 10 mm or other suitable cell (colour remains stable for some time).
Proceed as for Method 9I2a (and Note 1 for that method).
Report PBI+ColP on an air-dry (40°C) basis.