Magnetite (Fe3O4) and maghemite with an admixture of hematite (γ-Fe2O3/α-Fe2O3) powders are synthesized via sol-gel process and characterized using XRD, SEM, low temperature nitrogen adsorption, small-angle X-ray scattering (SAXS) and small-angle polarized neutron scattering (SAPNS). The synthesized γ-Fe2O3/α-Fe2O3 and Fe3O4 samples are found to be porous systems featuring with a three-level hierarchically organized structure with different intrinsic scales and aggregation types for each of the structural levels. For both materials, the intrinsic size of the highest level exceeds 70 nm, and magnetic structure involves superparamagnetic particles with a characteristic radius of magnetic-nuclear cross-correlations RMN ≈ 3 nm. The biological activity of γ-Fe2O3/α-Fe2O3 and Fe3O4 aqueous suspensions in certain concentrations in respect to seed treatment, growth and productivity of plants was studied using the example of spring barley variety Leningradsky under adjustable optimal conditions and physical modeling of stress effects (irradiation with high-intensity UV-B radiation, soil moisture deficiency).