Extractable Bases (Ca2+, Mg2+, K+, Na+)

Reagents
Concentrated acetic acid (CH3COOH)
Concentrated ammonium hydroxide (NH4OH)


Preparation of Reagents

CAUTION: ALWAYS ADD ACID TO WATER, NEVER WATER TO ACID!!

1.0N Ammonium acetate (pH 7.0)
Add 1224mL NH4OH (68mL/L) and 1026mL CH3COOH (57mL/L) to an 18 liter bottle containing approximately 10 liters of deionized water and make to volume. This should be done in the fume hood. Allow to cool and adjust to pH 7.0 with appropriate reagent while stirring continuously.

Sample Preparation and Procedure
Weigh, in duplicate, 10g of soil (5g for organics) into 125mL Erlenmeyer flasks. Add 20mL of ammonium acetate to each sample; include blanks. Swirl gently to thoroughly wet, stopper and allow to stand overnight.

Attach the appropriate number of 500mL filter flasks to the vacuum manifold and equip each with a Buchner funnel, moisten with deionized water, and turn on the vacuum. Open stopcock on each line to seal the paper to each funnel. Swirl flask vigorously and pour each sample into the appropriate funnel.

Using vacuum as necessary, draw the ammonium acetate completely through the soil. Leach an additional 80mL through the soil with sequential 20mL aliquots. Some soil may get through the filter paper; refilter these as necessary. Remove funnels, swirl filter flask to thoroughly mix, and pour about 30mL filtrate into numbered plastic bottles. Refrigerate. Discard the remaining sample.

Calculations
meq/100g = [(sample ppm * sample dilution factor) - blank ppm * blank dilution factor)] * elemental constant

Elemental constants:

Calcium 1/200 = 0.00500
Magnesium 1/121.5 = 0.00823
Sodium 1/230 = 0.00435
Potassium 1/390 = 0.00256

Additional Notes
Glassware and funnel should be rinsed with deionized water only, immediately upon completion of extractions.
References
Soil Survey Methods Manual. August 1992.  p.345.  U.S. Department of Agriculture.