'vermiculite' Determination on Whole Soils by Cation Exchange Capacity Methods*

-The proposed method is a modification of one by Alexiades and Jackson (1965). Calcium exchange capacity (CaEC) and potassium exchange capacity (KEC) are determined, after removal of organic matter and free iron oxides, by saturating the exchange complex with centrifuge washings of pH 7 acetate solutions of Ca or K, respectively. Excess salt in solutions remaining in contact with the soil after saturation is determined by measuring the weight and concentration of the excess solution. The exchangeable cations and excess salt are then replaced by centrifuge washings with 1 N acetate solutions of Mg (for CaEC) or~NH, (for KEC), after overnight 110~ oven-drying to enhance K fixation for KEC. The replaced cations are determined and CaEC and KEC values are calculated. Per cent 'vermiculite' is based on the difference between CaEC and KEC (expressed in m-equiv/100 g) and an assumed 'vermiculite' interlayerexchangecapacityof 154 m-equiv/100 g; percentage Vr = (CaEC-KEC/154) • 100. The 'vermiculite' interlayer fraction (VIF) of the CaEC may also be calculated; VIF = CaEC-KEC/CaEC. The measured 'vermiculite' is shown in quotation marks since the method is open to criticism regarding exactly what is being measured, the assumptions made, etc. and to emphasize that the determination procedure is an operational one for the characterization of cation exchange complexes. Removal of free iron oxides increased both CaEC and KEC values of several soils but percentage Fr was little affected. The amount of K fixation was affected by the drying treatment employed after K saturation (none vs air-drying vs oven-drying). Thoroughly crushing Montana and African vermiculites dramatically increased their CEC and measured 'vermiculite' values, but had little effect with two samples of saprolite from chloritic metabasalt. I N T R O D U L-q'ION Procedures that determine the content of minerals on a whole soil basis are useful. Carbonate minerals (as CaCO3 equivalent) and free iron oxides have commonly been determined in such a manner, but clay minerals have not. Whole soil analyses may give a more meaningful characterization in practical terms than a particle-size fractionation approach, especially if there are considerable amounts of the minerals of interest in coarse fractions and if the alternative approach would be to study only the mineralogy of the < 2 ktm clay. Whole soil methods may also, in some instances at least, eliminate the need to do particle-size fractionation and thus save much time. The Alexiades and Jackson (1965) method for vermiculite determination by cation exchange capacity methods has no inherent assumptions that prevent its application to whole soils. The method is based on the assumption that all the interlayer cation exchange sites * Scientific Article No. A1876 and Contribution No. 4791 of the Maryland Agricultural Experiment Station. f Former Graduate Assistant, Presently: Agronomist, USDA, Agricultural Research Service, Agricultural Environmental Quality Institute, ARC West, Beltsville, Maryland 20705, U.S.A. and Associate Professor, respectively. of vermiculite (but not of any other minerals) are blocked by potassium fixatioh. Thus by measuring the CaEC (calcium exchange capacity; i.e. Ca saturation and Mg replacement) and the KEC (K saturation and NH4 replacement), and by assuming an interlayer exchange capacity for vermiculite (154 m-equiv 100 mg), the vermiculite content may be calculated. The objectives of this study were to develop, by modification of the Alexiades and Jackson (1965) procedure, a rapid method for the quantitative determination of'vermiculite' on a whole soil basis and to illustrate the value of this method in characterizing soils and geologic materials. The mineral (minerals) measured by these procedures is (are) identified here as 'vermiculite'. This usage recognizes that what is measured may not coincide with ideal vermiculite (defined, for example, on the basis of layer type, interlayer material and layer charge in the AIPEA nomenclature committee report of A I P E A Newsletter No. 7). M A T E R I A L S A N D M E T H O D S Of primary interest in this work were some vermiculitic soils developed in saprolite from chloritic metabasalt in the Middletown Valley of Maryland. A larger