Particle-Size, Microaggregate-Size, and Aggregate-Size Distributions in Humus Horizons of the Zonal Sequence of Soils in European Russia

Experimental data on size distribution of micro- and macroaggregates and primary soil particles were obtained by laser diffraction analysis of the samples of humus horizons of the zonal soil sequence (from Retisols to Kastanozems) in European Russia. Both undisturbed native soils and agricultural soils were analyzed. The relationships between the parameters of the size distribution of aggregates, microaggregates, and primary particles were valuated. The parameters of the size distribution (content of the particles and their mean-weight diameter) were compared with data on the contents of water-stable aggregates, organic carbon, nitrogen, phosphorus, potassium, and pH. Size distribution of primary particles in the humus horizons of Endocalcic Chernozems had a bimodal pattern. A positive linear correlation between the contents of microaggregates and organic carbon was found to occur only when the latter exceeded 2–3%. An index of the size distribution of microaggregates—the minimum size of stable microaggregates—was proposed; it can be determined as an intersection point of differential curves of size distribution of microaggregates and primary particles. For this index, a negative correlation with the soil pH was found. It was supposed that the observed correlation reflects the coagulation of soil colloids, when their electric charge is compensated.