APPLICATION OF A RAPID EXTRACTION METHOD FOR THE DETERMINATION OF EXCHANGEABLE CATIONS IN VARIOUS SOIL ORDERS

Cation exchange capacity (CEC) and exchangeable bases are important properties of soils and sediments. They relate information on a soils ability to sustain plant growth, retain nutrients, buffer acid deposition or sequester toxic heavy metals. Cation exchange occurs due to the negative charges carried by soil particles, in particular clay minerals, sesquioxides and organic matter. These negative charges are cancelled out by the absorption of cations from solution.

Several standard methods exist for the determination of cation exchange capacity (Sumner and Miller, 1996). Among these are two methods that estimate the CEC by summation of exchangeable bases (Ca, Mg, Na, K) and exchangeable Al. Both methods involve the extraction of exchangeable bases and Al using neutral salt solutions, and are labor intensive. A new method was developed using a Dionex ASE 100 accelerated solvent extraction system. This method entails the extraction of exchangeable cations using a 0.2 M LiNO3. This solution differs from the standard methods, but was selected to reduce NH4 and Cl interferences during ICP analysis. The new method was demonstrated on a Cazenovia series soil A horizon, and compared with results using the standard method. Replicate analyses of both the standard test and the new method agree within 5%.

To further demonstrate the applicability of the method, characterized soil samples were obtained from the USDA Lincoln Laboratory and the University of Delaware Soil Testing Laboratory covering 6 different soil orders. These samples were extracted using the ASE method. Initially, problems arose in applying the method to fine textured soils due to clogging of the filter. To provide better flow in the reaction cell, acid washed and LiNO3 rinsed sand was mixed with the soil samples. Extraction of blank sand samples for Ca, Mg, Na, K and Al showed all results to be below detection limit, indicating that the sand would not impact the CEC results. Results for extractions of the 6 different soils used in the study, including A and B horizon samples were favorable. Individual cation concentrations were found to vary by as much as 20% from the characterization data, but total CEC as determined by the sum of the exchangeable cations was found to vary by no more than 10%.