The method is a modification (Colwell 1963) of the original bicarbonate procedure of Olsen et al. (1954). It employs an extracting solution of 0.5 M NaHCO3 adjusted to pH 8.5 with sodium hydroxide (NaOH), a soil/solution ratio of 1:100, and an extraction time of 16 h at 25°C. The manual colorimetric finish is based on the method of Murphy and Riley (1962). There is no prior adjustment of soil pH, or any attempt to remove possible interferences such as arsenate and silicates. Any precipitation during the final determination of soluble P, possibly associated with organic compounds, is ignored. Slight acidification of soil extracts with H2SO4 just before final analysis helps to minimise positive interferences from soluble silicates.
Extracting Solution – 0.5 M Sodium Bicarbonate at pH 8.5
Dissolve 42.0 g sodium bicarbonate (NaHCO3) in deionised water, dilute to almost 1.0 L, adjust pH to 8.5 (usually requires 0.8 g NaOH), and make volume to 1.0 L. Take care not to exceed pH 8.55: refer to Note 1. This Extracting Solution is best prepared on day of use; if storage is necessary, keep under nitrogen or mineral oil or utilise a CO2 trap to prevent entry of this atmospheric contaminant: also refer to Note 2.
2.25 M Sulfuric Acid
1.0 M Sulfuric Acid
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 10.0 mg of P.
Take 100 mL P Primary Standard and dilute accurately to 500mL in a volumetric flask with Extracting Solution (0.5 M NaHCO3 at pH 8.5). This solution should be freshly prepared each time working standards are made.
Phosphorus Working Standards
Add 0, 1.0, 2.5, 5.0, 10.0, 15.0, 20.0, 30.0, 40.0 and 50.0 mL of P Secondary Standard to separate 500 mL volumetric flasks. Dilute to 500 mL with 0.5 M NaHCO3 at pH 8.5 Extracting Solution. These working standards cover the range 0–1 mg P/L and for a 1:100 soil/solution ratio are equivalent to soil concentrations of 0, 2.0, 5.0, 10.0, 20.0, 30.0, 40.0, 60.0, 80.0 and 100.0 mg/P kg.
Weigh 1.00 g of air-dry soil (<2 mm) into a 250 mL extracting bottle and add 100 mL Extracting Solution (0.5 M NaHCO3 at pH 8.5), stopper and mechanically shake end-over-end for 16 h at 25°C. Centrifuge or filter (Whatman No. 42 – tested P free) soil extracts then pipette duplicate 25 mL aliquots into 100 mL volumetric flasks. Add 50 mL deionised water and mix thoroughly. Add 2 mL 1 M H2SO4, mix and, after effervescence has ceased, add a further 5 mL of 1 M H2SO4. Mix well and allow to stand overnight to complete the removal of CO2.
To one set of volumetric flasks add 8 mL Mixed Colour Reagent, then make to 100 mL and mix well. To the duplicate flasks (the reagent blanks) add 8 mL of Reagent A, make to 100 mL and mix well. Concurrently with the samples, take 25 mL of each P Working Standard and treat in a similar manner to the samples.
After 30 min, measure the absorbance at 882 nm of samples, standards, and soil-reagent blanks against deionised water as a reference. The absorbance values remain stable for up to 24 h. If necessary, dilute over-range extracts with NaHCO3 Extracting Solution. Same-day measurement following acidification is preferred to limit the possibility of chemical and/or biological change.
Bicarbonate-extractable P = [Sample value – Reagent blank] mg P/kg
Report bicarbonate-extractable P (mg P/kg) on an air-dry basis.
1. The preferred pH tolerance is 8.48–8.52, ideally measured with a sodium-tolerant glass electrode, with the pH meter calibrated to pH 9.0.
2. If wetting agents/detergents are required for any purpose, including cleaning of apparatus, only those known to be P-free should be used.