Comparison of five methods to determine the cation exchange capacity of soil

Background

Cation exchange capacity (CEC) is a routinely measured soil fertility indicator. The standard NH₄OAc (pH 7) extraction procedure is time-consuming and overestimates actual CEC values of variable charge soils. Unbuffered extractants have been developed to measure the effective CEC (eCEC), but they differ in the type of index cation and extraction procedures.

Aim

This study was set up to systematically compare CEC values and exchangeable cation concentrations among different procedures and evaluate their practical aspects.

Methods

Five procedures were compared for (e)CEC, that is, silver thiourea (AgTU), cobalt(III) hexamine (Cohex), compulsive exchange (CE, i.e., BaCl₂/MgSO₄), BaCl₂ (sum of cations in single-extract), and NH₄OAc (pH 7). We applied these methods to a set of 25 samples of clay minerals, peat, or samples from soils with contrasting properties.

Results

The CEC values correlated well among methods (R² = 0.92–0.98). Median ratios of eCEC (AgTU as well as CE) to the corresponding eCEC (Cohex) value were 1.0, showing good agreement between eCEC methods, but NH₄OAc exceeded Cohex values (ratios up to 2.5 in acid soil). For BaCl₂-extracteable cations, the ratio ranged from low (<1.0) in acid soils (acid cations not measured) to high (>1.0) in high-pH soil (dissolution of carbonates). Multiple-extraction methods (CE and NH₄OAc) yielded more variation and increased labor.

Conclusions

The chemical properties of the sample cause method-specific interactions with chemical components of extractants. We found the Cohex method with ICP-MS detection to be the most efficient and cost-effective technique for determination of eCEC and exchangeable cations.