It has been shown that the found regularities of the interaction between lignosulfonate and polymeric cations in aqueous media can serve as a scientific basis for chemical modification of lignin by way of the formation of the interpolyelectrolyte complexes. This energy-saving and non-complicated method of modification was applied for obtaining new lignin-based soil improving agents. Water-soluble polymeric products of the interaction of lignosulfonate with three synthetic polymeric cations have been used in order to study the effect of their adsorption at the clay soil surface on the water stability of the artificially soil aggregates formed by the treatment of soil with lignin-based polyelectrolyte complexes (LPECs). The obtained results are explained from the viewpoint of the complex mechanism of soil structure formation, combining simultaneously the flocculation of soil particles and stabilization of the formed aggregates. The conducted field tests have shown that the application of the lignin-based polyelectrolyte complex as a soil conditioner at a deep loosening of heavy clay soil allows the improvement of the hydrophysical properties and the filtration ability of the soil sublayers and, thereby, to enhance the barley yield.