Abstract: The centrally located outlet in the bottom portion of a silo for a flowable solid building material or fertilizer discharges directly into a first chamber of a mixing unit having a vertical shaft which is coaxial with the outlet and rotates one or more first tools in the first chamber to uniformly distribute the inflowing material and to reduce its tendency to become separated into two or more ingredients. The thus pretreated material descends into a second chamber and into the range of one or more rotating second tools which are driven by the shaft and intermix the material with one or more liquids before the finished product enters an evacuating pump which conveys it to one or more locations of use or to further processing units. The utilization of a vertical shaft and of parts which rotate about one or more vertical axes reduces the wear and prolongs the useful life of such moving parts.