SIZE DISTRIBUTION OF DISPERSED AND AGGREGATED PARTICLES AND OF SOIL PORES IN 12 DANISH SOILS

Soil texture, aggregates in different size classes and water retention at several water potentials were measured in the top layers of 12 Danish soils. Size frequency curves of dispersed and aggregated soil particles and of soil pores were calculated using numerical differentiation of sum curves, which were obtained from measured data through interpolation procedures. Soils which originated from water sediments had narrow peaks with approximately lognormal distribution of dispersed soil particles and of soil pores, reflecting the sorting action of the water. Moraine soils appeared to have broad and flat frequency curves of dispersed soil particles, some of which were bimodal or skewed. These soils exhibited a typical bimodal size distribution of soil pores. Degree of aggregation was determined primarily by the soil content of clay, 10% or more creating stable macroaggregates of 2-6 mm diameter. A comparison of the frequency curves for soil pore size to a generalized four-parameter mathematical expression relating matric potential and volumetric water content revealed that the model fitted the empirical data reasonably well for the well-sorted water-sedimented soils, while in the case of the bimodal pore size soils a deviation of differing magnitude was observed. From the investigation it is recommended that in most cases, quantities of particles and pores in soil should be related to size with frequency rather than cumulative expressions.