Improved determination efficiency of the cation exchange capacity and exchangeable base cations in soil using stirring and extraction modifications

The hexamminecobalt trichloride ([Co(NH₃)₆]Cl₃) method extracts exchangeable (exch.) base cations needing 60 min. For soils containing calcium carbonate, a calcite saturated [Co(NH₃)₆]Cl₃ solution is used to determine the exch. Ca. The preparing time of calcite saturated [Co(NH₃)₆]Cl₃ solution requires to set overnight. This study evaluated the equivalence of [Co(NH₃)₆]Cl₃ method using stirring modification. The proposed modification can vastly save time and the whole extraction time can be shortened to 3–5 min. The preparing calcite saturated [Co(NH₃)₆]Cl₃ solution procedure can be omitted when measured soils containing calcium carbonate. For acidic soil, the measured CEC was below the certified range, the exch. Ca, Na and K were all in the certified value range; the exch. Mg above the certified range. For neural soil, the measured CEC, exch. Ca, Mg, Na and K were all in the certified range. For alkaline soil, the measured CEC was above the certified range, the exch. Ca, Mg, Na and K were all in the certified range. For saline soil, the measured CEC was above the certified range, the exch. Mg and K below the certified range; accurate exch. Ca and Na could not be achieved even in such a short extraction time because of the dissolution of gypsum and sodium salts. For sodic soil, the measured CEC was above the certified range, the exch. Mg and K below the certified range; the extracted Ca can be assumed the exch. Ca; the exch. Na can be calculated as CEC－(exch. Ca + exch. Mg + exch. K). The difference of CEC between measured and certified values were mainly because of the discrepancy of extractant pHs. The addition of CaCO₃ (≤ 80 %) had no effect on CEC or exch. base cations’ determinations. The CaCO₃ and CaSO₄∙2 H₂O combination only affected the determination of exch. Ca.