106.3.2.55 TM-55, Determination of Calcium, Magnesium and Potassium in Soils

This method determines the pounds per acre of exchangeable calcium, magnesium and potassium in soils by atomic absorption spectrophotometry.

106.3.2.55.1 Reagents and Apparatus

1) An Atomic Absorption Spectrophotometer.

2) Acetic Acid (HC₂H₃O₂), 95.5%, 1.05 specific gravity.

3) Ammonium Hydroxide (NH₄OH), 0.90 specific gravity.

4) Hydrochloric Acid (HCl), 1.42 specific gravity.

5) Lanthanum Oxide (La₂O₃), low calcium grade.

6) Calcium Carbonate (CaCO₃), primary standard grade, dried at 105-110 ºC for several hours prior to use.

7) Potassium Chloride (KCl), reagent grade, dried at 105-110 ºC for several hours prior to use.

8) 1000 ppm Magnesium Stock Solution. This solution can be purchased from a number of sources, or it can be prepared in the laboratory from a suitable pure magnesium salt.

106.3.2.55.2 Preparation of Extracting Solution (1N NH₄OAc @ pH 7.0)

Pour 58 mL of acetic acid into approximately 500 mL of distilled water. Add 70 mL NH₄OH and mix. Dilute to approximately 950 mL and cool. Adjust the pH to 7.0 ± 0.05 with acetic acid or NH₄OH. Dilute to 1000 mL with distilled water.

106.3.2.55.3 Preparation of Standard Solutions

Lanthanum Stock Solution (10% La): Weigh 117.28 g of La₂O₃ into a 600-mL beaker and add 400 mL distilled water. While stirring, slowly add 500 mL HCl. Stir until solution is complete. Cool, transfer to a 1000-mL volumetric flask, and dilute to volume.

Ca, Mg and K Standard Solution: Weigh 0.5619 g dried CaCO3 into a 500-mL volumetric flask, dissolve in a minimum amount of HCl and add 0.0286 g dried KCl. Pipette a 25-mL aliquot of 1000 ppm Mg stock solution into flask and dilute to volume with distilled water. Pipette a 2-mL aliquot of this stock solution into a 100-mL volumetric flask. Add a 2-mL aliquot of 1N NH₄OAc extracting solution, a 2-mL aliquot of 10% La solution, and dilute to volume with distilled water. This solution is 9.0 ppm Ca (equivalent to 9000 lbs./acre Ca), 1.0 ppm Mg (equivalent to 1000 lbs./acre Mg), and 0.60 ppm K (equivalent to 600 lbs./acre K).

Note: If necessary, the range for Ca, Mg and K can be extended by preparation of a second standard prepared as follows:

Pipette a 4-mL aliquot of the Ca, Mg and K stock solution into a 100-mL volumetric flask, adding a 2-mL aliquot of 1N NH₂OAc extracting solution, a 2-mL aliquot of 10% La solution and diluting to volume with distilled water. This solution is 18.0 ppm Ca (equivalent to 18,000 lbs./acre Ca), 2.0 ppm Mg (equivalent to 2000 lbs./acre Mg), and 1.20 ppm K (equivalent to 1200 lbs./acre K).

Blank Solution: Pipette a 2-mL aliquot of 1N NH₄OAc extracting solution and a 2-mL aliquot of 10% La solution into a 100-mL volumetric flask and dilute to volume with distilled water.

106.3.2.55.4 Procedure

Place 1 g of soil into a 20-mL scintillation vial or other suitable container. Add 10 mL of extracting solution and shake for 5 minutes. Filter on Whatman #2 paper or equivalent. Pipette a 2-mL aliquot of the filtrate into a 100-mL volumetric flask, add a 2-mL aliquot of 10% La solution and dilute to volume with distilled water. Calibrate the instrument using the Blank Solution and the Ca, Mg or K Standard Solutions, then determine the concentration of the Sample Solution.

106.3.2.55.5 Calculations

The method of calculating the lbs./acre of Ca, Mg and K will vary according to the make and model of instrument used.

Report the results to the nearest lb./acre as follows:

Pounds per Acre Calcium (Ca)

Pounds per Acre Magnesium (Mg)

Pounds per Acre Potassium (K)