Extractable Cations – Ammonium Acetate Method
Environmental Soils and Biogeochemistry Characterization Laboratory
Scope and Application
This method semiquantitatively determines the amount of soil plant available K, Ca, Mg, and Na residing on the soil colloid exchange sites by displacement with ammonium acetate solution buffered to pH 7.0. Cation concentrations are determined using atomic emission (AES), absorption spectrometry (AAS) or ICP-AES instrumentation. A chemical interference solution is used to minimize chemical matrix effects. It is based on a modification of the procedure outlined by Knudsen et al. (1982) for exchangeable K. Generally, these cations are associated with the exchange sites. The exception are soils that have high soluble salts and are saline, which requires a special pre-analysis treatment. The method doesn’t correct for calcium and magnesium extracted as free carbonates or gypsum. In the northern Great Plains the method has been used to determine available sulfur. The method detection limit is approximately of 25 mg kg-1 (on a dry soil basis) and is generally reproducible ± 7%.
Analytical balance: 250 g capacity, resolution (at least) ± 0.01 g.
Reciprocating horizontal mechanical shaker, 180 oscillations per minute (opm).
Repipette dispenser, calibrated to 25.0 ± 0.2 mL.
Extraction vessel and associated filtration vessel.
Whatman No. 42, No. 2 or equivalent highly retentive filter paper.
Atomic Emission/Absorption Spectrophotometer (A AE) (A AS) or Inductively Coupled Plasma Atomic Emission Spectrophotometer (ICP-AES).
- Deionized water, ASTM Type I grade.
- Ammonium acetate (1.0 N) extraction solution neutral @ pH 7.0: Add 570 mL of glacial acetic acid CH3COOH (99%) to 8000mL of deionized water. Add 680 mL of concentrated ammonium hydroxide adjust pH to 7.0 with 3.0 N glacial acetic acid or 3.0 N ammonium hydroxide and dilute to 10 L final volume. Check solution for possible contamination of K, Na, Ca, and Mg (see comment # 1).
- Chemical interference solution: Dissolve 5000 mg L-1 lanthanum oxide (La2O3) and 2000 mg L-1 cesium chloride solution: Dissolve 4.691 g La2O3 and 5.071 g CsCl in 1.5 L of deionized water and add 25 mL of HCLO4 and 25 mL of HNO3 and dilute to 2000 mL. Check solution for contamination of K, Na and/or other cations.
- Standard calibration solutions of K, Ca, Mg and Na. Prepare six calibration standard solutions of 1.0 – 20 mg L-1 of K, 1.0 – 70 mg L-1 of Ca, and 0.50 – 20 mg L-1 for Mg and Na prepared from 1000 mg L-1 stock solutions. Dilute calibration solutions with a solution containing the chemical interference solution and 0.04 N ammonium acetate solution.
- Weigh 2.50 ± 0.05 g of air dried soil pulverized to pass 10 mesh sieve ( < 2.0 mm) into a 120 mL extraction vessel (See Comment #2). Add 25.0 mL of NH4OAc extraction solution and place on orbital mechanical shaker for thirty (30) minutes (See Comment # 3). Include a method blank.
- Filter, refilter if filtrate is cloudy (See Comment # 4 and # 5).
- Dilute 1:25 an aliquot of the soil extract with the chemical interference solution.
- Adjust and operate A AE, AAS or ICP-AES instrument with six standards in accordance with manufacturer’s instructions.Calibrate using prepared calibration solutions. Determine K, Mg, Ca and Na concentration of extract, of method blank, and record results as mg L-1 of cation in solution.
Report as mg kg-1 of K, Ca, Mg and Na to the nearest 1 mg kg-1 (See Comment # 7 and # 8): mg kg-1 cations in soil = (mg L-1 cation in solution – method blank) x 25 x 10
- Extraction solution should be prepared monthly, subject to microbial growth.
- Soils high in soluble salts (ECe > 1 dS m-1) should be washed with deionized water before adding extraction with ammonium acetate to reduce potential errors of soluble salts. This can be accomplished by adding 50 mL of deionized water to 2.5 g of soil, placed in a 50 mL centrifuge tube and centrifuge at 5000 rpm for thirty (30) minutes and decanting excess water followed by the standard analysis protocol. For soils having pH > 7.4 and calcium carbonate > 0.5%, should be extracted with ammonium acetate pH 8.5 to avoid the dissolution of calcium carbonate.
- Check repipette dispensing volume, calibrate using an analytical balance.
- Extracts may be stored for one week under refrigeration.
- Filter paper should be checked periodically (monthly) for possible contamination of alkali metals. If contamination is > 5 mg kg-1 on a soil basis select an alternative supply of filter paper.
- Results may be expressed as meq 100 g-1 or cmol kg-1. Divide concentration of K, Ca, Mg and Na by 391 mg, 200 mg, 121.5 mg, and 230 mg respectively.
- Generally, soils having less than 100 mg kg-1 potassium will respond to applications of potassium fertilizers for most crops.
Doll, E.C. and R. E. Lucas. 1973. Testing soil for potassium, calcium and magnesium. p 133-152. In: L.M. Walsh and J.D. Beaton. (ed.) Soil testing and plant analysis. SSSA Madison, WI.
Knudsen, D., G.A. Peterson and P.F. Pratt. 1982. Lithium, sodium and potassium. In : A.L. Page (ed) Methods of soil analysis Part 2. Agronomy Monograph 9. 2nd ed. ASA and SSSA, Madison WI.
McKeague, J.A. ed. 1981. Extractable cations. In: Manual of soil sampling and methods of analysis. Canadian Soil Survey Committee, prepared by subcommittee of methods of analysis.
Munter, R. 1988. Laboratory factors affecting the extractability of nutrients. p. 8-10. In : W.C. Dahnke (ed.) Recommended chemical soil test procedures for the North Central Region, North Dakota Agricultural Experiment Station Bulletin No. 499 (revised).