Abstract: A mobile machine and process are provided for milling, mixing, wetting and re-forming an initial windrow of vertically layered ingredients used in the pre-wet cycle for producing mushroom compost. In performing these actions, an assembly of toothed drums, rotating in the same direction and arranged along an upwardly and forwardly projecting inclined plane, progressively mills the windrow across its cross-section longitudinally and propels the milled material upwardly and rearwardly so that it drops and mixes to form a vertically blended, transverse product. The product is preferably cross-mixed by auguring it transversely and depositing it at a transfer point, from whence it is elevated and delivered by a conveyor belt, to form a new windrow parallel or collinear with the initial windrow. Water is sprayed on the ingredients in the course of propelling auguring and elevating.