The unique soil additive provides protection against important plant pests by combining the action of two bioagents. The complex way of acting on pests prevents the development of resistance and leaves no residues, making it suitable for organic cultivation and bio-dynamic agriculture. A number of chemical insecticides will be banned in the future due to stricter legislative conditions, which is why major chemical concerns are already incorporating biopesticides into their portfolios. Special growing or suppressive substrates are difficult to store and transport in a way that preserves their properties. Due to unsuitable conditions, it loses efficiency significantly. The logistics of high-volume goods such as substrates are quite complicated and costly. The highly concentrated formulation, where the substrate is only a carrier, allows for several times smaller packaging. Soil additive is also not subject to as strict legislative rules as biopesticides. It enriches the soil or growing medium with useful micro and macro organisms and takes advantage of their synergistic effect. Duoeffect works on the basis of two bioagens: the entomopathogenic fungus Isaria fumosorosea strain CCM 8367, the use of which is protected by a patent, and another bioagens, the combination of which achieves extended efficacy on multiple species of insect pests and harmful soil mites. The combination and synergy of both bioagens ensures an immediate effect, e.g. against fungus gnats, as well as long-term survival in the treated soil (substrate). In addition, the entomopathogenic fungus has an inhibitory effect on the growth of some pathogenic microorganisms. It allows to reduce the consumption of chemical pesticides and increase the quality of production with minimal impact on the environment. This soil preparation can be used especially in the cultivation of indoor and outdoor flowers and herbs grown in pots, boxes or beds, to protect greenhouse crops, especially vegetables and fruit and ornamental shrubs and trees and other crops. It is intended for application to soil, for enrichment of growing media and composts and for pickling seeds and seedlings. It is particularly suitable for small growers and organic and bio-dynamic agriculture.