In this method, P sorption curves are established by adding graded amounts of PO43– solutions in 0.01 M CaCl2 to air-dry soil and equilibrating for 17 h at 25°C at a soil/solution ratio of 1:10 (different to that of Ozanne and Shaw 1967). The manual colorimetric procedure of Murphy and Riley (1962) is used to determine the concentration of P remaining in solution following equilibration with soil.
0.01 M Calcium Chloride
As for Method 4B1.
Reagent A (Ammonium Molybdate – Sulfuric Acid – Sb Solution)
As for Method 9A2.
Mixed Colour Reagent
As for Method 9A2.
Phosphorus Stock Solution
1 mL contains 500 μg of P.
Dissolve 2.1968 g potassium dihydrogen phosphate (KH2PO4; previously dried at 130°C for 2 h) in 0.01 M CaCl2 and make to volume of 1.0 L. Add 2 drops of chloroform (CHCl3) to suppress biological activity. Store solution in borosilicate glass, preferably cool (≈4°C) and in the dark.
Phosphate Equilibrating Solutions
To obtain final supernatant P concentrations at equilibrium that range from 5–100 μg P/L, several graded solutions are required between 0 and 25 000 μg P/L, corresponding to 0–25 mg P/L. Use Table 9.10 as a guide to their preparation.
Phosphorus Primary Standard
As for Method 9A2.
Phosphorus Secondary Standard
1 L contains 5000 μg of P.
Dilute 50.0 mL P Primary Standard, with constant stirring to 500 mL with 0.01 M CaCl2. This solution should be freshly prepared each time working solutions are made.
Phosphorus Working Standards
Add 0, 0.5, 1.0, 2.0, 4.0, 8.0, 12.0, 16.0, 20.0 and 24.0 mL P Secondary Standard to separate 500 mL volumetric flasks containing some 0.01 M CaCl2. When diluted to volume with 0.01 M CaCl2 these working standards contain 0, 5, 10, 20, 40, 80, 120, 160, 200 and 240 μg P/L. For a 1:10 soil/solution ratio and equal volumes of working standards and soil extracts, the supernatant P values of working standards correspond to 0, 0.05, 0.1 … 2.4 mg P/kg.
Table 9.10. Volumes of P stock solutŠn required for expected P sorptŠn.
Volume of P Stock SolutŠn (mL) |
Initial P concentratŠn of (μg P/L) |
Equivalent amount of P (mg P/kg) |
0.5 |
250 |
2.5 |
1.0 |
500 |
5.0 |
2.0 |
1000 |
10 |
4.0 |
2000 |
20 |
6.0 |
3000 |
30 |
8.0 |
4000 |
40 |
10.0 |
5000 |
50 |
15.0 |
7500 |
75 |
20.0 |
10 000 |
100 |
30.0 |
15 000 |
150 |
40.0 |
20 000 |
200 |
50.0 |
25 000 |
250 |
Wash glassware and filter papers used for filtration and colorimetric analysis before use with dilute (1+4) HCl then rinse and dry.
Weigh 5.0 g air-dry soil (<2 mm) into 100 mL centrifuge tubes or suitable extraction bottles. At least 5 weighings of each soil are required. Select a minimum of five graded P equilibrating solutions to give a desired range of supernatant P concentrations, which will depend on the particular soil type. Separately dispense 50.0 mL of each selected P Extracting Solution to the multiples of each soil. Add 2 drops of chloroform (CHCl3) to each container, stopper, and shake end-over-end for 17 h at 25°C.
Following equilibration, centrifuge or filter (Whatman No. 5) to obtain uncontaminated, particle-free extracts.
Pipette aliquots (usually 25–40 mL) of clear soil extracts and working standards to 50 mL volumetric flasks and add sufficient 0.01 M CaCl2 to give an approximate volume of 40 mL. Dispense 4.0 mL Mixed Colour Reagent into each flask, make to volume with 0.01 M CaCl2, and mix. Maintain a fixed time schedule for these operations including thorough mixing of flask contents. After 30 min, read absorbance at 882 nm in a 50 mm or other suitable cell (colour remains stable for some time beyond 30 min). Prepare a calibration curve (or regression equation) relating absorbance to concentration of P in μg/L and determine the supernatant P concentration (C).
Psorbed(mg P/kg) =
InitialP added(mg P/kg) - Supernatant Pconcentration (mgP/kg)
Repeat for each P solution used.
For each soil, plot P sorbed (mg P/kg) on Y axis against log10C (C expressed as μg P/L), using log-linear graph paper (or plotting software). Extrapolate the line to the point of zero sorption (intercept on X axis) to derive log10 equilibrium P concentration. Derive EPC (units of μg P/L) by taking the antilog of log10 EPC. Also derive the slope of what should be a linear line to obtain the P buffer capacity (PBC; units of mg/kg/log10 μg/L).
Report EPC and PBC on an air-dry basis.