<p>Available reports focus mostly on the effects of post-fermentation sludge (digestate) on soil organic carbon level, soil chemistry, and microbiology, and little is known about the impact on soil structural and mechanical properties. Therefore, the influence of different rates of digestate (1–15%) on the structure and strength of several soils, varying in grain size composition, pH, and organic matter content, was studied. The effects were analyzed by scanning electron microscopy, nitrogen adsorption, mercury porosimetry, bulk density, and mechanical stability tests. Organic sludge usually increased total porosity, average pore diameter, total pore volume, and diminished bulk density of all soil aggregates. Digestate addition significantly decreased the specific surface area of most clayed soils of the highest initial surface. The application of digestate increased the strength of initially most fragile sandy soil aggregates. The more intensive positive changes in the pore and surface characteristics and increase in mechanical strength of sandy soils highlighted the potential of the digestate application to enhance the stability and structure of less productive areas.</p>