KBS036: MCSE: Inorganic and Organic Soil Phosphorous Fractions

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Abstract

Sampling Frequency: every 5 years

Composite soil samples were dried and sieved through a 250-um (60mesh) sieve . Subsamples of 1.0 g were extracted sequentially for the following phosphorus (P) fractions: Resin inorganic P (Pi), 0.5 M NaHCO 3 Pi and organic P (Po), NaOHPi and Po, NaOH plus sonication Pi and Po, HCl Pi, and residual P . The P fractionation method was modified from Hedley et al . 1982 . Phosphorus was determined by the ascorbicacid- molybdate method (Olsen and Sommers, 1982) .

Protocol

Overview

Phosphorus Sequential Fractionation Method

Modified Procedure from Hedley et al. (1982)

  1. Weigh 1 g (60 mesh- 250 m m) of soil in a 50 ml centrifuge tube

  2. Add 30 ml deionized water plus the anion exchange resin in the bicarbonate form (procedure to prepare resin
    bags attached).

  3. Shake for 16 h, remove the resin bag and wash any soil that might adhere to the bag back into the tube,
    centrifuge the tube at 10 000 RPM for 10 min and discard the supernatant.

  4. Extract the P from the resin bag, by shaking the resin bag with 20 ml of 0.5 N HCl for 24 hours. Save in
    the cooler for determination of inorganic P in the resin extract.

  5. Add 20 ml of 0.5 M NaHCO
    3
    to the soil in the centrifuge tube, shake for 16 hours. Centrifuge for 10 minutes at 7000 rpm. Save
    the extract in the freezer for determination of inorganic and total P in the NaHCO
    3
    extract.

  6. Add 30 ml of 0.1 N NaOH, shake for 16 hours, centrifuge for 10 minutes at 7000 rpm. Save the extract in
    the freezer for the determination of inorganic and total P in the NaOH 1 extract.

  7. Repeat step 6. Save the extract in the freezer for the determination of inorganic and total P in the NaOH
    2 extract.

  8. Add 20 ml of 0.1 N NaOH. Put in ice bath. Sonicate the samples for 2 min. at 75 W. Shake for 16 hours,
    Centrifuge for 10 minutes at 7000 rpm. Save in the freezer for the determination of inorganic P and total P
    in the SNaOH extracts.

  9. Add 30 ml of 1 N HCl, Shake for 16 hours, Centrifuge for 10 minutes at 7000 rpm. Save the extracts in the
    cooler for determination of inorganic P in the HCl extract.

  10. Digest the remaining soil with H
    2
    SO
    4
    and H
    2
    O
    2
    . Save the digests in the cooler for determination of residual P.

Notes: Always prepare fresh NaOH solution for extraction and neutralization of samples. A lot of P
contamination comes from old NaOH solution.

Preparation of resin bags

  1. Weigh 0.65 g of wet Dowex anion exchange resin (42 % moisture) or 0.4 g dry resin into nylon mesh bags
    (diameter 0.53 um) and seal the bags.

  2. Shake the resin bag with 30 – 50 ml NaHCO

  3. Repeat the above step.

  4. the bags with deionized water. The bags are ready to use.

  5. After use, rinse the bags very well from the acid solution, otherwise the nylon mesh will deteriorate.
    Store the bags in deionized water.

  6. To use the bags again, regenerate by repeating steps 2 to 4.

Total P in the extracts

EPA methods for analysis of water 1978

Reagents

  1. Sulfuric acid solution, 11 N : Slowly add 310 ml concentrated sulfuric acid to 600 ml distilled water.
    When cool, dilute to 1.0 liter.

  2. Ammonium Persulfate

Procedure

  1. Pipette 5 ml sample into a 125 ml Erlenmeyer flask.

  2. Add 40-45 ml distilled water.

  3. Add 1 ml H
    2
    SO
    4
    (11N) { for NaOH extracts, add 1.5 ml H
    2
    SO
    4
    ).

  4. Add 0.4 g Ammonium Persulfate.

  5. Boil gently on a hot plate for about 30-40 minutes or until a final volume of about 10 ml is reached. Do
    not allow to get to dryness. Cool. Make up volume to 50 ml.

  6. Cover with parafilm. Handshake well. Take a 20-ml aliquot into a plastic vial.

  7. Adjust the pH with 1 N NaOH. (Procedure follows).

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