Abstract: The invention relates to an apparatus for introducing water-soluble substances, especially plant treatment substances, such as fertilizers, plant protectives, weed killers and the like, into a water stream, the apparatus having a casing for carrying a water stream and a container for taking up said substances. The container is arranged within said casing for rotation around a rotational axis and has a completely closed peripheral wall, laterally limiting the interior of said container, and has a completely closed bottom wall section. At least one opening is provided at the upper side of said container, the edge of said opening being nearer to the rotational axis of the container than at least a part of the inner surface of the peripheral wall, said part being arranged in between the bottom wall section and the opening.