CATION-EXCHANGE CAPACITIES OF ORGANO-MINERAL PARTICLE-SIZE FRACTIONS IN SOILS FROM LONG-TERM EXPERIMENTS

Soils 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: 489-813 mmol, kg-1, coarse clay: 367 749 mmol, kg-1, fine silt: 202-587 mmol, kg-1, medium silt: 63-345 mmol(c) kg-1, coarse silt: 12-128 mmol, kg 'and sand: 10-156 mmol, kg-1. The CECs varied with genetic soil type, mineralogical composition of the <6.3-mum particles. and the C and N contents of the size fractions. In a pot experiment examining the role of various organic materials in the early stages of soil formation, the clay-size fractions had the largest CECs (85-392 mmol(c) kg-1), followed by the medium-silt (19-222 mmol, kg-1) and fine-silt fractions (23-192 mmol, kg-1). The effect of organic amendments on CEC was in general: compost > fresh farmyard manure = straw + mineral fertilizer = mineral fertilizer.