Grava, JohnHalverson, Merle2015-10-102015-10-101956-10John Grava and Merle Halverson. University of Minnesota Extension Service. Extension Folder 210. October 1956.https://hdl.handle.net/11299/174655This archival publication may not reflect current scientific knowledge or recommendations. Current information available from the University of Minnesota Extension: https://www.extension.umn.edu.enNewsletter or Bulletin