Method Improvement for Determination of Soil Exchangeable Calcium and Magnesium

PurposeTo optimize and simplify the operation procedures of the national standard determination method GB 7865—1987(2) for rapid evaluation of large quantities of soil samples by comparing the accuracy and efficiency of different methods in determining the contents of soil exchangeable calcium (Ca²⁺) and magnesium (Mg²⁺) in terms of extraction reagents, centrifugation times, and shaking time.

MethodRepresentative soils were taken from four regions of Northwest, Northeast, Southwest, and South China, and five extraction reagents deionized water (H₂O), calcium chloride (CaCl₂), potassium chloride (KCl), ammonium acetate (NH₄OAc) and Mehlich 3 (Me 3) were utilized for method comparison at different extraction-centrifugation time (1-6 times) and shaking time (15-60 min).

Result1) Exchangeable Ca²⁺ and Mg²⁺ contents for a given soil sample varied with different extractant in the order of Me 3>CaCl₂>KCl>NH₄OAc>H₂O. 2) Soil exchangeable Ca²⁺ and Mg²⁺ contents were positively correlated with the extraction-centrifugation times, and increasing the extraction-centrifugation times might give rise to significantly higher values. 3) Extraction-centrifugation times depended on soil types. Exchangeable Ca²⁺ and Mg²⁺ contents of soils in Jiangxi (standard sample), Yunnan, and Fujian stabilized after four times of extraction-centrifugation, while the Xinjiang and Heilongjianghe soil needed 6 (Ca²⁺) and 5 (Mg²⁺) times of extraction-centrifugation, respectively. 4) Concentrations of exchangeable Ca²⁺ and Mg²⁺ determined by 60 min of NH₄OAc extraction and oscillation was comparable to those derived from the national standard method, and the oscillation method was more suitable for large quantities of samples.

ConclusionDifferent extraction reagents affect final values of soil exchangeable Ca²⁺ and Mg²⁺ contents. Compared with the national standard method, the NH₄OAc oscillation method has relatively less procedures, lower equipment requirements, and reliable results, which is more suitable for high-efficiency testing requirements of large quantities of soil samples.