Extractable Potassium, Sodium, Calcium, and Magnesium- Ammonium Acetate
Method
A. Reagents
1. Ammonium acetate, 1N - Add 68 ml of ammonium hydroxide (C.P. reagent 2830% NH3) to about 800 ml of distilled water. Then add 57 ml of glacial acetic acid
(99.8%) and dilute to 1 liter. Adjust the solution to pH 7.0 by adding either
ammonium hydroxide or glacial acetic acid.
2. Standard solutions
a. Standard stock solutions
 Calcium (500 ppm Ca) - Dissolve 1.249 g of CaCO3 in 1:1 HC1 and evaporate
to dryness on a hot plate. Dissolve the residue and make to 1 liter with distilled
water.
 Magnesium (500 ppm Mg) - Dissolve 0.50 g pure Mg ribbon in 1:1 HC1 and
evaporate to dryness on a hot plate. Dissolve the residue and then dilute to I
liter with distilled water.
 Potassium (500 ppm K) - Prepare a standard solution of K by dissolving 0.9535
g oven dried KC1 in a small volume of distilled water and diluting to 1 liter
with distilled water.
 Sodium (500 ppm Na) - Prepare a standard solution of Na by dissolving 1.271
g NaC1 in a small volume of distilled water and diluting to 1 liter with distilled
water.
b. Standard Work Solutions
 Calcium (Ca), Magnesium (Mg) and Sodium (Na) Standard solutions:
Pipette the following aliquots of 500 ppm stock solutions into 100 ml
volumetric flasks. Bring to 100 ml volume with ammonium acetate. Mix
thoroughly and store in plastic bottles.
Flask or
Ca
Mg
Na
Standard
Aliquot
ppm of
Aliquot
ppm of
Aliquot
ppm of
ml
solution
ml
solution
ml
solution
1
5
25
1
5
1
5
2
15
75
3
15
2
10
3
25
125
5
25
4
20
4
35
175
8
40
5
25
 (ii) Potassium (K) standard solution: Pipette 1, 2, 3, 4, and 5 ml aliquots of
500 ppm stock solution into 100 ml volumetric flasks. Bring to volume with
ammonium acetate. Mix thoroughly and store in plastic bottles. The final
concentration of the standards ore 5, 10, 15, 20, and 30 ppm K.
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B. Procedure
1. Weigh 2 g of soil into a 70-m1 extracting vessel, add 40 ml of the ammonium
acetate extractant, and place the extracting vessel containing the sample on the
shaker for 30 minutes.
2. Filter through a Whatman No. 42 or equivalent paper if Na is to be determined;
when determining the other elements only, use the Whatman No. 5 or
equivalent paper for sample filtration.
3. Ca, Mg, and Na
Using Custom Lab Equip. diluter dispenser pick up a .75 ml aliquot of each
sample extract or standard solution. Dispense these aliquots and 17 mis H 20
into 30 ml plastic vials making approximately a 25 fold dilution. Repeat the
above step another time placing the aliquot and dilutent in the same vial.
4. Adjust the dilutor mentioned above to make a 10 fold dilution and to use 0.08
N NaC1 as the dilutent. Repeat the same two step procedure described above
using a different set of vials.
5. Calibrate the atomic absorption spectrophotometer with the diluted standard
work solutions. Use distilled water for the zero setting when analyzing for Ca,
Mg and Na and the 0.1 NaC1 for K. Determine all the elements by atomic
adsorption.
6. Report K, Na, Ca, and Mg in milliequivalents per 100 g of soil, and also report
K in parts per million on a soil basis.
C. Calculation
 ppm of cation in the soil sample = ppm of cation in the soil extract solution x
20 ml of extract per gram of soil.
 meq of cation per 100 g of sample = meq of cation per liter of soil extract
solution x 2 liters of extract per 100 g of sample.
D. Comments
The procedure for determining extractable cations with neutral 1N ammonium acetate
is a modification of the procedure outlined by Pratt (18) for exchangeable K. The
modification concerns the single equilibration of the sample with the extracting
solution (1:20 ratio of soil to extractant) rather than three successive extractions
specified in the original procedure. A further modification is the dilution of the soil
extract with H20 or a 0.08 N NaC1 solution. The four cations are determined on the
same soil extract but with different dilutions. The single extraction technique for
cations in non-calcareous soil gives values which are equivalent to at least 95% of the
values obtained by the process of multiple extractions. For samples which contain
carbonates of Ca or Mg, the multiple extractions with ammonium acetate may
dissolve these carbonates and give higher values for Ca and Mg than are obtained with
a single extraction. For purposes of routine testing there is usually no interest in
determining the extractable Ca and Mg in alkaline samples which contain free lime.
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