15A3 Exchangeable bases – 1 M ammonium chloride at pH 7.0, adjusted for soluble Na⁺

This method is identical to 15A1 except that a calculation based on the concentration of soluble Cl^– present is used to adjust for the apparent level of exchangeable Na⁺ in the soil. This approximate correction for soluble Na⁺ is an alternative to chemical pre-treatment when EC (1:5, soil/water) exceeds ≈0.3 dS/m.

Procedure

Determine the amounts of Ca²⁺, Mg²⁺, Na⁺ and K⁺ extracted by 1 M NH₄Cl at pH 7.0 (Method 15A1) and the amount of water-soluble Cl^– present in the soil (Method 5) at the same soil moisture status. Convert mg Cl^–/kg to cmol_c/kg by dividing by 354.5. On the assumption that all Cl^– is associated with NaCl, calculate corrected exchangeable Na⁺ by subtracting an amount equivalent to the soluble Cl^–. The measured values for exchangeable Ca²⁺, Mg²⁺ and K⁺ are not affected.

Reporting

Report exchangeable Ca²⁺, Mg²⁺, Na⁺ and K⁺ (cmol_c/kg), expressed on an oven-dry soil basis. Use the air-dry moisture to oven-dry moisture ratio to make the oven-dry conversion. Refer to Method 2A1 for guidance with regard to this soil moisture calculation.