Organic Matter

  1. Number porcelain crucibles with wax pencil.  Weigh crucibles to 4 decimal places.  Scoop dried and ground samples into crucibles using 5 g soil scoop.  Put crucibles plus samples on tray in furnace.  Set temperature to 105 degrees C.  Ash at this temperature for 2 h.  (It takes 15 minutes for the furnace to reach this temp, so set the dial at 105 for a total of 2 h and 15 min.)
  2. Remove samples from furnace with tongs, place in desiccator, and weigh to 4 decimal places immediately while still warm so moisture won’t be absorbed.  Replace crucibles plus samples in furnace.  Turn dial up to 360 degrees C.
  3. Ash at this temp for 2 hours.  (It takes ½ hour for the furnace to reach this temperature, so set the dial at 360 for a total of 2½ hour.)  Turn furnace off and open door.  Place samples in desiccator with tongs and allow samples to cool.  Weigh samples to 4 decimal places.  
  4. Calculate OM% and record on data sheet with one decimal.
  • Crucible wt = a
  • Crucible wt + sample wt after 105 heating = b
  • Crucible wt + sample wt after 360 heating = c

%OM = ((b-a)-(c-a))(100)/(b-a)


  1. E. E. Schulte, Recommended Soil Organic Matter Tests.