Use of Diffusion to Determine Soil Cation-Exchange Capacity by Ammonium Saturation

Soil cation-exchange capacity (CEC) is often determined by NH₄ saturation, using a 1 M solution of NH₄C₂H ₃O₂ at pH 7. A study was conducted to ascertain whether this determination can be performed by employing a simple diffusion technique previously developed for direct inorganic-N analysis of soils. Values obtained for 10 diverse Illinois surface soils following overnight NH₄ saturation were correlated very highly with CEC data collected for the same samples on the basis of NH₄ analyses by steam distillation (r = 0.999, P < 0.001) or colorimetry (r = 0.999, P < 0.001), and also with data obtained by summing atomic absorption measurements of calcium (Ca), magnesium (Mg), and potassium (K) displaced during NH₄ saturation (r = 0.811, P < 0.01). As a much more rapid and convenient alternative to existing methods for preparation of NH₄-saturated soil, a simple saturation technique was developed whereby 0.500 g of soil was leached under vacuum with 15 mL of 1 M NH₄C₂H₃O₂ (pH 7) in a 10-mL disposable syringe containing a stainless-steel frit, and then with 30mL of 2-propanol. After drying for a few minutes, the soil sample was transferred to a 473-mL (1-pint) wide-mouth Mason jar, treated with 10mL of 2 M KCl, and analyzed for exchangeable NH₄ by diffusion with MgO for 1.75 h at 45-50°C on a hot plate. Except for a calcareous soil, CEC measurements by the rapid saturation-diffusion approach did not differ significantly (P < 0.05) from diffusion data involving overnight NH ₄ saturation. This approach allows CEC determinations to be accomplished in a few hours instead of days, and will be especially useful for routine soil characterization and testing.