Determination of Soil Cation Exchange Capacity by Rotary Mixing-fully Automated Kjeldahl Nitrogen Analyzer and Its Application in the National Soil Census

To determine the cation exchange capacity (CEC) of soil, ammonium acetate, ammonium chloride-ammonium acetate, barium chloride, and sodium acetate are commonly used as extracting, employing manual glass rod stirring, magnetic stirring, ultrasonication, or leaching as extraction/washing methods. These methods suffer from cumbersome operation, low efficiency, and poor reproducibility. Establishing a rapid detection method with high precision, automation, and universality is therefore of great significance. This study employs EDTA-ammonium acetate as the extractant, requiring only a single soil treatment step, reducing the number of extraction treatments by 67%-75% compared to traditional methods. Using rotary mixing for extraction/washing enhances solid-liquid contact efficiency, increasing sample pretreatment throughput to 192 samples/12h. When coupled with a single fully automated Kjeldahl nitrogen analyzer, it enables the detection of 110 samples/12h. Through investigation and optimization of key parameters-rotary mixer speed, EDTA concentration in the extractant, extractant volume, extractant acidity, extraction time, rinsing agent and its volume, and sample weight-the optimal experimental conditions were determined. The method was tested on two acidic, three neutral and five alkaline soil reference/standard materials for available component analysis, sourced from different geographical distributions, pH levels, textures, and preparation institutions. The measured CEC values all fell within their certified uncertainty ranges. The method demonstrated a precision (RSD) of 0.85%-2.05% and a trueness (relative error) of -3.47%-2.31%. The method was successfully applied to determine CEC in 1642 surface and profile soil samples from forest, grassland, cultivated, and garden land during the third national soil census, meeting quality control requirements. Compared to traditional manual stirring methods, this method offers a high degree of automation, simple operation, and is suitable for the rapid determination of large batches of acidic, neutral, and alkaline soils.