Dilute (0.005–0.01 M) CaCl2-extractable P is highly correlated with soil solution P concentration (Moody et al. 1988), and therefore favours the intensity component of labile P.
Calcium chloride-extractable P (0.01 M) was the best predictor of grain yield response by soybeans to P on a wide range of soil types in south-eastern and central Queensland (Moody et al. 1983). It can also predict the P fertiliser requirements of subterranean clover (Dear et al. 1992). The extractant at 0.005 M is also useful for identifying soils with an elevated risk of P loss to the water environment: see Method 9K for more details.
The method described (Moody et al. 1988) involves 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). Because of the small amounts of P extracted by this method, results are commonly expressed as μg P/kg, rather than mg P/kg.
Extracting Solution – 0.005 M Calcium Chloride
Dissolve 0.74 g calcium chloride (CaCl2.2H2O) and make to 1.0 L with deionised water.
Reagent A (Ammonium Molybdate – Sulfuric Acid – Sb Solution)
As for Method 9A2.
Mixed Colour Reagent
As for Method 9A2.
Phosphorus Primary Standard
As for Method 9A2.
Phosphorus Secondary Standard
1 L contains 5.0 mg of P.
Dilute 50.0 mL P Primary Standard, with constant stirring, to 500 mL with 0.005 M CaCl2. This solution should be freshly prepared each time working standards are made.
Phosphorus Working Standards
Add 0, 0.2, 0.5, 1.0, 2.0, 4.0, 8.0 and 10.0 mL P Secondary Standard to separate 500 mL volumetric flasks and make to volume with 0.005 M CaCl2 solution. These working standards contain P concentrations of 0, 0.002, 0.005, 0.01, 0.02, 0.04, 0.08 and 0.10 mg P/L.
Wash glassware and filter papers used for filtration and colorimetric analysis before use with dilute (1+4) HCl then rinse and dry. Weigh 10.0 g air-dry soil (<2 mm) into 100 mL centrifuge tubes or suitable extraction bottles. Dispense 50.0 mL of 0.005 M CaCl2 Extracting Solution into each tube. Add 2 drops of chloroform (CHCl3) to each container, stopper, and shake end-over-end for 18 h at 25°C.
Centrifuge or filter, and pipette identical aliquots (usually 25–40 mL) of clear soil extracts and working standards to 50 mL volumetric flasks, and add sufficient 0.005 M CaCl2 to give an approximate volume of 40 mL. Dispense 4.0 mL Mixed Colour Reagent into each flask, make to volume with 0.005 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 of absorbance against concentration of standards. Interpolate the P concentrations of the sample solutions 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.