Methods Comparison for Cation Exchange Capacity and Exchangeable Cations of Soils

Abstract

Cationic exchange capacity (CIC) is one of the most important properties when assessing soil fertility. The most commonly used method is by ammonium acetate 1 N, pH 7.0; however, this method presents errors during determination. Several methods to measure CEC were compared: ammonium acetate (NH₄OAc) 1 N pH 7.0, sodium acetate (NaOAc) 1 N pH 8.2, sum of cations extracted with NH₄OAc 1 N at pH 7.0 (CICE), barium chloride (BaCl2) 0.01 M and silver-thiourea (AgTU) 0.01 M. The soils analyzed had variable characteristics in pH, texture, organic matter and carbonates. The highest CIC values were obtained with the sum of cations, followed by ammonium acetate and silver thiourea. The sodium acetate method presented intermediate values due to errors in the method. There was an association between CIC through AgTU and NH₄OAc as opposed to CICE and NH₄OAc; the cost of determination  with  NH₄OAc  was  eight times  higher than that of AgTU.  The  Mg2+, K+ and Na+ cations extracted by barium chloride, ammonium acetate and sodium acetate were highly associated; however, there was no relationship between K+ and Na+ extracted with NH₄OAc and AgTU. Calcium was the cation that was most overestimated, followed by magnesium and potassium extracted with NH₄OAc 1 N at pH 7.0, where the overestimation varied from 10 to 498 times in relation to that originally at the exchange sites, while magnesium and potassium were overestimated from 11.2 to 141.7 and from 3.7 to 33.3 times. respectively. The AgTU method was the best alternative to determine CEC, as it is fast, low-cost, minimizes errors, and is applicable to soils with different characteristics of pH, texture, organic matter, and carbonate content.