Structure is the most striking, visual aspect of vertisol morphology. The type and degree of structure development in a vertisol provides a digest of its genesis, constituent properties and agriculture management potential. The present study includes the results of the investigations of aggregate composition and water stability of structural aggregates in all horizons (Ap, A, AC, C) of 7 vertisol profiles formed over tertiary clayish and pyroclastic sediments in the central part of Macedonia (Štip, Probištip and Ovče Pole valleys). The main objectives were to determine and compare aggregate composition and water stability of structural aggregates of vertisols which have been used as long-term arable fields (6 of the analyzed profiles) or as pasture (1 profile). The most represented fraction of aggregates in the process of dry sieving of the soil samples is the fraction of cloddy macro-aggregates (10 mm), which is the most unfavourable from the agricultural point of view.
Micro- aggregates fraction (0.25 mm) is barely present, which means that in the dry state nearly all micro-aggregates and mechanical elements are associated in larger structural aggregates. The researched soils are characterized by different water stability of the structural aggregates. In the process of wet sieving, except surface horizon of the vertisol under natural vegetation, the cloddy macro-aggregates showed the greatest instability. With decrease of the dimension their stability increases so micro-aggregates have the highest water stability.
