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Soil Double Acid (Melich #1) Extraction for Metals: (K, Ca, Mg,) and available P

Last revised July 22, 1997
-------------------------

Page contents (click to skip down):

* [Equipment]
* [Reagents]
* [Extractant solution]
* [Procedure]
* [Post-analysis calculations]

This extraction procedure was developed for use with acid, sandy soils found in the southeastern U.S., having less that 5% organic matter

Equipment

1) 125 ml screw-cap Nalgene extraction bottles

2) mechanical shaker

3) Whatman #1 filter paper (15 cm dia.)

4) glass funnels (8 cm.) and funnel racks

5) scintillation vials with plastic (not metal-lined) caps

6) 2-liter volumetric flask

7) automatic pipetter, variable volume (1-10 ml)

Reagents

1) concentrated HCl

2) concentrated H2SO4

3) fresh distilled deionized water

4) Quality-control samples: NBS 2709, 2710, 2711 standard soils

Double Acid Extraction Solution

Place ~1 L deionized H2O in the 2 L volumetric flask on stir plate. Add 8 ml concentrated HCl and 1.4 ML concentrated H2SO4 and mix. Remove stir bar and bring to full volume with d-i H2O. Cap with doubled parafilm and invert several times to mix. Let stand ~30 min. and re-check volume. The resulting extract solution is 0.05N HCl and 0.025N H2SO4.

Procedure

1) Label extraction bottles and scintillation vials to correspond to original sample containers.

2) Scoop 5 gm (alternatively, 4 cubic cc. volume) of sample material into an extraction bottle

3) Add 20 ml extraction reagent

4) Place all extraction bottles on rack of mechanical shaker and shake at 250 cycles per minute for 5 minutes.

5) Pour the extract solution in each bottle through a funnel lined with filter paper; capture the filtrate in labelled scintillation vials. Cap and refrigerate.

6) Analyze filtrate for metals by atomic absorption spectrophotometry; for extracted orthophosphate by Alpkem (colorimetric, automated Murphy-Riley)

Post-analysis calculations:

1) Expected extract content of NBS 2711 standards (for choosing suitable calibrant ranges):
 NBS certified content:
   
     Ca  2.88%  (+/- 0.08% ) by weight
     K   2.45%  (+/- 0.08% )  "    "
     Mg  1.05%  (+/- 0.03% )  "    "
     P   0.086% (+/- 0.007%)  "    "

 5.0 g sample contains:

     Ca  0.144 g  =  144 mg in 20 ml extract  =  7200 mg/liter
     K   0.123 g  =  123 mg  "  "  "    "     =  6150    "
     Mg  0.053 g  =   53 mg  "  "  "    "     =  2650    "
     P   0.004 g  =    4 mg  "  "  "    "     =   200    "
      

2) Back-calculation of dry soil content from determined extract values;
Soil content =

(determined analyte mg.)     (volume of extract sol'n)
------------------------  x  ------------------------- 
 1 liter extract sol'n       (weight of soil extracted)


(7200 mg. Ca)     (20 ml sol'n)     28.8 mg Ca
-------------  x  -------------  =  -----------
  1000 ml          (5.0 g soil)      1 g soil


                                    .0288 mg Ca
                                 =  -----------
                                     1 mg soil

                                 =  2.88 % Ca by weight
      

If any dilution of the extract solution is necessary to obtain samples within the analytical range of the instruments, this dilution must also be accounted for. The dilution factor df is

             final volume after dilution
   df  =  --------------------------------
           initial volume before dilution
      

and the colorimetrically determined content of the diluted sample must be multiplied by the inverse of the df. (The Alpkem driver software does this automatically when using a dilution loop to apply the same dilution to an entire sample run.)

References:

1. Jones, J. Benton Jr. 1990.
Universal soil extractants: their composition and use. Commun. in Soil Sci. and Plant Anal. 21(13-16), 1091-1101.

2. Mehlich, A. 1953.
Determination of P, K, Na, Ca, Mg and NH4. Soil Test Division Mimeo, North Carolina Department of Agriculture, Raleigh, NC USA.

3. Murphy, J. and J. Riley. 1962.
A modified single solution for the determination of phosphate in natural waters. Anal. Chim. Acta 27, 31ff.

4. Nelson, W. L., A. Mehlich, and E. Winters. 1953.
The development, evaluation and use of soil tests for phosphorus availability. pp. 153-188. In W. H. Pierre and A. G. Norman, eds., Soil and Fertilizer Phosphorus, v.II, Agronomy, A Series of Monographs, Academic Press, New York.

5. U. S. Environmental Protection Agency. 1983.
Calcium: Method 215.1 (atomic absorption, direct aspiration). pp. 215.1-215.2. In Methods for Chemical Analysis of Water and Wastes, EPA-600/ 4-79-020. U.S.E.P.A., Cincinnati, Ohio, USA.

6. U. S. Environmental Protection Agency. 1983.
Magnesium: Method 242.1 (atomic absorption, direct aspiration). pp. 242.1-242.2. In Methods for Chemical Analysis of Water and Wastes, EPA-600/ 4-79-020. U.S.E.P.A., Cincinnati, Ohio, USA.

7. U. S. Environmental Protection Agency. 1983.
Phosphorus, all forms: Method 365.1 (colorimetric, automated, ascorbic acid). pp. 365.1-365.7. In Methods for Chemical Analysis of Water and Wastes, EPA-600/4-79-020. U.S.E.P.A., Cincinnati, Ohio, USA.

8. U. S. Environmental Protection Agency. 1983.
Potassium: Method 258.1 (atomic absorption, direct aspiration). pp. 258.1-258.2. In Methods for Chemical Analysis of Water and Wastes, EPA-600/ 4-79-020. U.S.E.P.A., Cincinnati, Ohio, USA.


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