In a field experiment with conservation tillage for onion production the influence of various methods of pre-winter and pre-sowing tillage (conventional tillage, no tillage, and disking) as well as the biomass of the intercrop plants on the soil organic matter content and soil aggregate formation were studied. Secale cereale and Vicia sativa grown as intercrop cover plants favourably influenced the soil aggregation. It was expressed by lower cloddiness and pulverization of soil in comparison with conventional tillage, without plant mulches. Leaving the mulch from cover plants on the soil surface from autumn to spring increased the proportion of macroaggregates ( 0.25-10 mm). The positive influence of plant mulches was mainly observed after winter and in the initial period of onion vegetation. The changes in soil aggregation under the influence of mulching cover plants and simplifications of the soil tillage system for vegetables occurred mainly in the soil arable layer (0-20 cm).