Comparison of Methods to Determine Soil Cation Exchange Capacity.

Cation Exchange Capacity (CEC) determines a soil’s ability to provide plants with sufficient nutrients, retain potential groundwater contaminants, and, along with soil organic matter content, can be used to estimate the types of clay minerals present in a soil.  The most widely used and regulatory agency’s accepted method for determining CEC is to extract exchangeable basic cations with neutral ammonium acetate and analyze the extracted NH₄⁺ using distillation or colorimetric method.  Recently, using ion-selective electrode or combustion methods to analyze NH₄⁺ have drawn more attention due to their improved accuracy and shorten analytical time/cost.  This study systematically compares various analytical methods in order to provide a guideline to select most appropriate and environmentally sustainable analytical methods for determining the CEC of various types of soil. 
Three different types of soil with low (4cmol_c/kg), medium (12cmol_c/kg), and high (32cmol_c/kg) CEC in neutral pH range were tested with the analytical methods of Kjeldahl Distillation (Kjelhahl), Ammonia Selection Electrode (ASE), and Combustion by Vario Max CHNS Auto-analyzer (Combustion) and compared based on the criteria of accuracy, precision, life-cycle time, safety, and wastes generated.  Accuracy is quantified by analysis of reagent blanks and comparison with a standard reference material.  Precision is determined through percent relative difference and statistical analysis of duplicate samples.  Life-cycle time is evaluated to include standard solutions preparation, NH₄⁺ analysis, and glassware cleanup.  Safety is evaluated with the use of strong acids/bases, operation at high temperatures, etc.  Waste generation is focused on the generation of hazardous materials such as strong acids/bases, heavy metals, etc.
The study indicates the Combustion method had the best accuracy for low and medium CEC soils and the Kjeldahl had the best accuracy for high CEC soil.  The Combustion, ASE, and Kjeldahl method had the best precision for low, medium, and high CEC soil, respectively.  For all three types of soil, the ASE had overall best precision.  The low CEC soil had low precision, but the precision increased as soil CEC increased.  The Combustion method takes the least amount of life-cycle time (followed by the ASE and Kjeldahl) and is the safest method due to the absence of utilizing strong bases which are needed by other methods to volatilize ammonia gas. The Combustion method generates no waste because samples are vaporized during analysis, whereas the Kjeldahl method waste contains heavy metals which must be properly disposed of and the ASE method non-hazardous waste must be dumped down the sink.
The benefits of utilizing the Combustion method outweigh using the Kjeldahl method in terms of sustainable evaluation although both methods have similar accuracy and precision for various types of soil. If the Combustion method is not available, the Kjeldahl method is recommended over the ASE method due to better accuracy.  However, the ASE method will be the most practical in time efficiency if the analysis of a large amount of samples is required and the data accuracy could be compromised.