Soil and Water Conservation Laboratory
Standard Operating Procedure
Phosphorus Sorption Index
Reference Methods:
Bache, B.W. and E.G. Williams. 1971. A Phosphate Sorption Index for Soils. Journal
of Soil Science 22 (3):289-301.
Robertson, G.P., et.al. 1999. P Sorption in Standard Soil Methods for Long-Term Ecological Research.
Oxford University Press, Inc., New York, p. 124-128.
Purpose
To measure the phosphorus sorption capacity of soils and create an index that allows for meaningful
comparisons between soils of different types.
Materials and Equipment
Reagents
 Potassium phosphate, monobasic (KH2PO4)
 Calcium chloride, dihydrate (CaCl2∙2H2O)
 Deionized water (dI H2O)
Equipment
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Analytical balance, accuracy 0.0001g
Refrigerator (to keep soil samples cool)
Drying oven, fixed temperature of 105ºC ± 5º
Dessicator with drying agent
Tabletop shaker, capable of shaking samples at 120 rpm for 24 hours
Repipettor calibrated to 25.00 mL
Clean acid bottle (larger than 2L) to fit repipettor
Volumetric flasks; 50 mL, 100 mL, 1000 mL, 2000 mL
Magnetic stirrer & stir bars
50 mL screw-cap polypropylene centrifuge tubes
Weighing device to hold centrifuge tube, i.e. Styrofoam square
Container to securely hold centrifuge tubes on shaker, i.e. Styrofoam ‘egg’ crate
Plastic funnels that fit inside centrifuge tubes
Whatman No. 42 filter paper
Weigh boats
Disposable plastic pipets
Plastic Spoons & Forks (for measuring soils)
Metal spatulas (for measuring reagents)
Dionex microtubes with 0.45μm filter caps
IC loading cartridges
Stock solution of 130 mg PO4-P/L in 0.01M CaCl2
Stock solution of 0.01M CaCl2
Nitrile powder-free gloves & appropriate lab attire
Stock Solution Preparation
1. 0.01M CaCl2 stock solution
a. Using a metal spatula and analytical balance, weigh 1.4701g CaCl2∙2H2O into a
weigh boat.
b. Transfer the calcium chloride into a 1000 mL volumetric flask.
c. Add a stir bar to flask and fill about ¾ with dI water.
d. Stir solution using magnetic stirrer.
e. Remove stir bar and dilute to volume with dI water.
f. Store solution at room temperature.
2. Phosphate stock solution: 130 mg PO4-P in 0.01M CaCl2
a. Dry KH2PO4 in oven at 105°C for 1 hour.
b. Allow KH2PO4 to cool in dessicator for at least ½ hour.
c. Weigh 1.1424g of KH2PO4 into a weigh boat.
d. Transfer KH2PO4 to a 2000 mL volumetric flask.
e. Weigh out 2.9402g of CaCl2∙2H2O and add to the 2000 mL volumetric flask.
f. Add a stir bar to the volumetric flask and fill about ¾ with dI water.
g. Stir solution on magnetic stirrer.
h. Remove stir bar and dilute to volume with dI water.
i. Transfer solution to glass jug and store in refrigerator.
3. Calibration Curve solutions
a. Fill a 50 mL volumetric flask with 130 mg PO4-P in 0.01M CaCl2.
b. Transfer to a 100 mL volumetric flask and dilute to volume with 0.01M CaCl2.
This is the 65 mg PO4-P solution.
c. Fill a clean 50 mL volumetric flask with the 65 mg PO4-P solution. Pour the
remaining 65 mg PO4-P solution into a 50 mL centrifuge tube and cap.
d. Transfer the 50 mL of 65 mg PO4-P into a clean 100 mL volumetric flask and
dilute to volume with 0.01M CaCl2. This is the 32.5 mg PO4-P solution.
e. Repeat steps c-d to create the remaining solutions for the calibration curve:
16.25 mg PO4-P and 8.125 mg PO4-P.
Procedure
1. Determine target wet weight of soil.
2. Put uncapped 50 mL centrifuge tube in weighing device (i.e. Styrofoam square), weigh the
total and zero.
3. Using plastic spoons, add the target amount of soil to the centrifuge tube, and weigh to the
nearest 0.0001g. Record weight, cap tube, and store in refrigerator.
4. Repeat steps 1-3 for all soil samples in batch.
5. Prepare stock solutions and solutions for phosphate calibration curve.
6. Using the repipettor, add 25.00 mL of 130 mg P/L in 0.01M CaCl2 stock solution to each
centrifuge tube.
7. Cap tubes, securely place in Styrofoam holder, and shake on tabletop shaker for 24 hours at
120 rpm.
8. Prior to end of shaking time, prepare filters, one filter per sample (average prep time is 3.5
hours):
1. Fold Whatman No. 42 filter paper into cone shape and place in funnel.
2. Rinse all filters with dI water.
3. When water has passed through, repeat the dI water rinse.
4. Do final rinse using 0.01M CaCl2 solution.
5. Place prepared filters and funnels onto new centrifuge tubes.
9. After 24 hours, remove samples from shaker and give hard manual shake for about 10
seconds.
10. Pour samples into prepared filters to remove any large soil and non-soil particles. Also pour
two blanks, using the 0.01M CaCl2 stock solution as the sample.
11. When target solution volume has filtered into tube, remove funnel, cap tube, and place back
into refrigerator for further settling.
12. When samples have settled, pour solution into Dionex microtubes, cap samples with 0.45 μm
filter and place into IC loading cartridges. Follow same procedure with stock solutions.
13. Phosphate concentration remaining in solution determined by ion chromatographic analysis
(Dionex IC) at WRRC at UH Mānoa.
14. Compute phosphorus sorption for each soil to build index.
Notes
All glassware and reusable plastic equipment was acid-washed.
Calculations
1. Determination of Target Wet Weight of Soil
Target Wet Weight = 2 * Wet:Dry Ratio (if Ratio < 2.5), otherwise = 5
Ex. Koheo-1W
Target Wet Weight
= Wet Mass/Dry Mass * 2
=
2. Actual Dry Weight
Actual Dry Weight = Actual Wet Weight / (Wet:Dry Ratio)
Ex. Koheo-1W
Actual Dry Weight
=
3. Concentration of Stock Solutions
[PO4-P] (mg/L) = weight KH2PO4 * FW (P:KH2PO4) / amount of solution
Ex. Batch 1
1.1429g KH2PO4
(30.97376g P/136.08554 g KH2PO4)/2L = 130.064922 mg
PO4-P
Concentration of CaCl2 = weight of CaCl2/FW/L of solution
Ex. Batch 1
2.9397 g CaCl2·2H2O (1 mole/147.01456g CaCl2·2H2O)/2L = 0.00999799M CaCl2
4. Phosphorus Sorption Index (PSI)
PSI = X / log C
C = [PO4]remaining in solution
X = mg Psorbed / 100 g soil = ([PO4]initial – C)(0.025 L)(100 / actual dry weight of sample)
Ex. Koheo-1W
PSI = [(130.06 – 48.04)(0.025)(100/1.9978)] / log 48.04 = 60.99