Magnetite (Fe3O4) nanoparticles were prepared by chemical coprecipitation from a ferrous/ferric mixed salt-solution in the presence of PVP used as a stabilizer and without it. The resulting magnetite nanoparticles were characterized by X-ray powder diffraction (XRD) analysis and scanning electron microscopy (SEM). Aqueous colloidal solutions of the prepared nanoparticles were used in the biotest. The biotest was conducted to determine the degree of growth of wheat seeds in the contaminated soil treated with nanoparticles Fe3O4/PVP and nano Fe3O4. The maximum stimulatory effect on the growth of wheat seedlings in a soil subjected to long-term contamination with pesticides, mostly trifluralin, was observed in the case of incubation of the soil with a solution of Fe3O4/PVP nanoparticles at a concentration of 50 mg/kg of dry soil for 4 days; the effect was evident from an increase in the length of both the roots (by 165.0%) and the seedlings (by 141.0%) relative to the reference sample.