Walker, George WarrenUniversity of Minnesota. Agricultural Experiment Station. Agricultural Chemistry and Soils Division2017-02-062017-02-061912-04Walker, George Warren. The Relation of Different Systems of Crop Rotation to Humus and Associated Plant Food. 1912. Saint Paul, Minn.: University of Minnesota. Agricultural Experiment Station. Bulletin 128, pages 166-186https://hdl.handle.net/11299/183885This archival publication may not reflect current scientific knowledge or recommendations. Current information available from University of Minnesota Agricultural Experiment Station: http://www.maes.umn.edu/enNewsletter or Bulletin