This method is suitable for direct measurement of cation exchange capacity (CEC) on soils known to be dominated by minerals with permanent surface charge. In addition, it gives estimates of exchangeable bases (Ca2+, Mg2+, Na+ and K+) similar to those obtained by method 15A1.
Nowadays, the method is not used widely, but it provides a link to historical data on the ion exchange properties of Australian soils (e.g. Stace et al. 1968). It is appropriate for the measurement of basic exchangeable cations from non-calcareous, non-gypsiferous, near neutral soils that are low in soluble salts [EC (1:5, soil/water) <0.3 dS/m].
Extraction of exchangeable bases is achieved by initially shaking the soil for 1 h with aqueous 1 M NH4Cl at pH 7.0 at a soil/solution ratio of 1:10. The suspension is then filtered and the soil washed with additional Extracting Solution giving a final soil/solution ratio of 1:20. Exchangeable bases are determined in the combined extract.
CEC is determined after leaching soil in the filter funnel with a solution of 15% potassium nitrate/6% calcium nitrate to displace exchangeable NH4+. According to Tucker and Beattie (1974a), the KNO3 + Ca(NO3)2.4H2O solution should displace the residual Cl– completely in addition to removing >99% of NH4+ from soils of high exchange capacity.
Contemporary information on measurement performance within and across laboratories is unavailable.
Extracting Solution – 1 M Ammonium Chloride at pH 7.0
As for Method 15A1.
5 M Ammonium Chloride at pH 7.0
As for Method 15A1.
0.05 M Ammonium Chloride Solution
Dilute 50 mL 1 M NH4Cl Extracting Solution to 1 L with deionised water.
Strontium Chloride Solutions (only required if using an AAS analytical finish)
As for Method 15A1.
Wetting Agent – Brij 35
As for Method 15A1.
Lithium Chloride for Automated Na+ and K+ Analysis
As for Method 15A1.
As for Method 15A2.
K-Ca Displacing Solution (15% potassium nitrate and 6% calcium nitrate)
Dissolve 150 g potassium nitrate (KNO3) in 800 mL deionised water, warming as necessary. Add 60 g calcium nitrate [Ca(NO3)2.4H2O], dissolve, and make to 1 L with deionised water.
0.1 M Nitric Acid
For use as collecting solution.
Acid-washed Sand
This may be purchased. Alternatively, prepare as outlined by Hewitt (1965) or Blakemore et al. (1987). Preferred particle size range is from 0.25 to 0.5 mm (to pass 30–60 mesh sieves).
Standard Solutions for Exchangeable Bases
As for Method 15A1.
Extraction of exchangeable bases
Weigh 5.00 g air-dry soil (<2 mm) and transfer to a 100 to 250 mL plastic extraction container. Also add about 4 g Acid-washed Sand to facilitate subsequent leachings. Add 50 mL of 1 M NH4Cl at pH 7.0 Extracting Solution, cap extraction containers securely, and mechanically shake end-over-end at about 25°C for 2 h.
Prepare Whatman No. 1 filter papers in 75 mm plastic funnels, arranged to deliver into borosilicate glass or plastic containers graduated to 100 mL. Wash filter paper and apparatus with ≈20 mL 1 M NH4Cl Extracting Solution and discard the washings.
When shaking is complete, quickly (within ≈30 min) and quantitatively transfer the soil plus sand onto the filter paper, aided by a wash bottle filled with 1 M NH4Cl Extracting Solution. Continue washing the soil plus sand with the Extracting Solution until almost 100 mL of leachate has collected. Remove plastic funnels carefully and make volume of leachate to 100 mL with 1 M NH4Cl Extracting Solution. Retain leachate for determination of exchangeable bases.
Washing excess NH4 Cl from soils
For convenience, place the funnels on their respective empty soil extraction containers for washing of soil plus sand prior to releaching for CEC. With a minimum of delay (no drying of soil should occur), wash the soil on the filter paper with about 25 mL of 0.05 M NH4Cl solution to reduce levels of entrained NH4Cl. Discard this leachate. Further reduce levels of entrained NH4Cl by washing the filter paper and soil plus sand three times with 25 mL of 60% Aqueous Ethanol; discard these leachates and remove the collecting containers.
Displacement of NH4+
Place funnels containing the now washed soil plus sand over borosilicate or plastic flasks graduated at 200 mL, each containing 80 mL of 0.1 M HNO3 as a collecting solution.
Commence leaching with K-Ca Displacing Solution, ensuring contact with all soil particles, until 100 mL have passed through the soil. Make volume of leachate to 200 mL with K-Ca Displacing Solution. Stopper, mix well and retain for CEC determination by Method 15I3 or 15I4: discard the soil plus sand.
Ca2+, Mg2+, Na+ and K+
Determine as for Method 15A1.
Report exchangeable Ca2+, Mg2+, Na+ and K+ (cmolc/kg), expressed on an oven-dry soil basis. Use the air-dry moisture to oven-dry moisture ratio to make the oven-dry conversion. Refer to Method 2A1 for guidance with regard to this soil moisture calculation.
CEC
Determine CEC by either Method 15I3 or by Method 15I4.
Report CEC (with relevant method code) (cmolc/kg), expressed on an oven-dry soil basis. Use the air-dry moisture to oven-dry moisture ratio to make the oven-dry conversion. Refer to Method 2A1 for guidance with regard to this soil moisture calculation.