Evaluation of Exchangeable Potassium Concentration in Sri Lankan Rice Paddy Soils with ICP-MS Using CaCl₂ Extraction

With the advancement of analytical techniques, multi-element extraction and detection have been developed. Universal extractans like CaCl₂ and detection technologies like inductively coupled plasma-mass spectrometry (ICP-MS) could facilitate simultaneous analysis of multiple-elements, and would be useful in commercial scale soil analytical laboratories. The type of extractant and detection method could influence the concentration of exchangeable potassium (ex-K) determined in different soils. This study evaluated the effectiveness of 0.01 M CaCl₂ as a multi-element extractant (CC) and ICP-MS and flame photometry (FP) as detection technologies when determining ex-K in soils. The widely used protocol for Sri Lankan soils, i.e. 1 M ammonium acetate (AA) as an extractant and FP as detection method, was used as the control for comparison of results. Altogether, 1,250 soil samples representing rice-growing regions in Sri Lanka were used. Soils were grouped under 16 categories based on soil type. Ex-K varied widely (26 to 1376 mg kg⁻¹ from AA-FP method). Ex-K determined using three methods were in the order of AA-FP>CC-FP >CC-ICP-MS. On average, the values obtained from CC-FP and CC-ICP-MS methods were 53% and 24%, respectively, of those from AA-FP. Soil type-dependent variation in the detection of K using three methods was evident. In the 16 soil categories, coefficient of determination (R²) ranged from 0.23 to 0.86 for AA-FP and CC-ICP-MS, and from 0.05 to 0.70 for AA-FP and CC-FP. As a result, soil ex-K determined using CC-FP and CC-ICP-MS methods cannot be related to the results generated through AA-FP method across different soil types.