SUMMARYSoils with and without organic manuring from 10 long‐term manurial experiments in East Germany were fractionated into organo‐mineral particle‐size separates by ultrasonic disaggregation and sedimentation/decantation. The cation exchange capacities (CECs) buffered at pH 8.1 were determined for the size fractions fine+medium clay, coarse clay, fine, medium and coarse silt, sand, and for the total soil samples.In the samples from nine field experiments the CECs decreased with increased equivalent diameters (fine+medium clay: 489–8 13 mmolckg−1, coarse clay: 367–749 mmolckg−1, fine silt: 202–587 mmolckg−1. medium silt: 63–345 mmolckg−1, coarse silt: 12–128 mmolckg−1and sand: 10–156 mmolckg−1. The CECs varied with genetic soil type, mineralogical composition of thefresh farmyard manure