Rapid Saturation–Diffusion Method to Determine Cation- and Anion-Exchange Capacities in Soil

Soils have long been recognized for electrostatic retention of anions as well as cations, although this property is usually characterized by measuring cation- but not anion-exchange capacity. A technique is described that determines effective cation- and anion-exchange capacities (CEC_e and AEC_e) for a single soil sample. In this technique, 0.5 g of soil is saturated by treatment with 1 or 0.006 M ammonium nitrate (NH₄NO₃) using a vacuum manifold equipped with disposable syringes, the excess NH₄NO₃ is removed by leaching with 2-propanol, and sequential diffusions are performed to determine ammonium-nitrogen (NH₄⁺-N) as CEC_e and nitrate (NO₃^‒)-N as AEC_e. In evaluative studies, complete saturation of cation- and anion-exchange resins was achieved with 1 but not 0.006 M NH₄NO₃, and neither CEC_e nor AEC_e measurements were affected by entrapment of excess saturating reagent, cross-contamination of NO₃^‒-N diffused following ¹⁵NH₄⁺-N, or interference due to soil organic N. Good agreement was obtained in comparing CEC data collected with 1 M NH₄NO₃ and ammonium acetate for 10 soils dominated by permanent charge but not for seven highly weathered soils. For all soils studied, both CEC_e and AEC_e were significantly reduced by the use of 0.006 M NH₄NO₃, and the AEC_e data thereby obtained agreed closely with compulsive exchange measurements using 0.002 M barium chloride. The rapid saturation-diffusion method described measures CEC_e and AEC_e when using 1 M NH₄NO₃, and can also be performed to estimate electrostatic ion retention at a low ionic strength (μ = 0.006) that approximates field conditions.