The method involves directly introducing fluid systemic active medium e.g. insecticide and fungicide, into a root region of plants to be protected i.e. maize plants, in the soil by an injection device. An injection element is provided in the injection device and is designed in the form of injection wheel, injection band or injection teeth. The injection element is fluid-conductively connected with a storage container for the active medium. The active medium is produced together with a liquid fertilizer that is formed as a fertilizer containing ammonium.
