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 exp eriments in East Germany were fractionated into organo-mineral particl e-size separates by ultrasonic disaggregation and sedimentation/decant ation. The cation exchange capacities (CECs) buffered at pH 8.1 were d etermined 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 cl ay: 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 composit ion of the <6.3-mum particles. and the C and N contents of the size fr actions. In a pot experiment examining the role of various organic mat erials 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-sil t (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 f armyard manure = straw + mineral fertilizer = mineral fertilizer.