Determination of Soil Cation Exchange Capacity Using a Fully Automated Kjeldahl Nitrogen Analyzer with Rotary Mixing and Its Application in the National Soil Survey

To determine the cation exchange capacity (CEC) of soil, ammonium acetate, ammonium chloride-ammonium acetate, barium chloride, and sodium acetate are commonly used as extractants, while manual glass rod stirring, magnetic stirring, ultrasonication, or leaching are employed 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. In this work, EDTA-ammonium acetate was employed as the extractant, requiring only a single soil treatment step, thereby reducing the number of extraction treatments by 67%–75% compared with traditional methods. Rotary mixing during extraction/washing enhanced solid-liquid contact efficiency, increasing sample pretreatment throughput to 192 samples per 12h. When coupled with a single fully automated Kjeldahl nitrogen analyzer, the workflow enabled the detection of 110 samples per 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. All measured CEC values fell within the certified uncertainty intervals; precision (RSD) was 0.85%–2.05% and trueness (relative error) was −3.47% to 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 China’s Third National Soil Survey, 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 samples, including acidic, neutral, and alkaline soils.