Soil Lab Methods

Organic Matter

Loss on Ignition for Organic Matter.

Apparatus

1. Muffle Furnace
2. "High Form" Porcelain Crucibles

Procedure

1. Number crucibles with wax pencil.
2. Weigh crucibles to 4 decimal places.
3. Scoop soil samples into crucibles using 5 g scoop.
4. Put crucibles plus samples on tray in left-hand furnace.
5. Set dial to 100. (This corresponds to a temperature of 105^oC +/- 5^oC.)
6. Ash at this temperature for 2 h. (It takes 30 min for the furnace to reach 105^oC, so set the dial at 100 for a total of 2 h and 30 min.)
7. 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.
8. Replace crucibles plus samples in left furnace.
9. Turn dial up to 360. (This corresponds to a temperature of 360^oC +/- 5^oC.)
10. Ash at this temperature for 2 h. (It takes 1 h for the furnace to reach this temperature, so set the dial at 360 for a total of 3 h.)
11. Turn furnace off and open door.
12. Place samples in desiccator with tongs and allow samples to cool.
13. Weigh samples to 4 decimal places.
14. Calculate OM % and record on data sheet with one decimal.

Calculation

• crucible wt = a
• crucible wt + sample wt after 105^oC heating = b
• crucible wt + sample wt after 360^oC heating = c

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

Reference

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

Walkley-Black procedure.

Potassium Dichromate Solution (1.0N K₂Cr₂O₇)

1. Place 49.04 g of potassium dichromate (K₂Cr₂O₇) (dried at 105^oC in a one-liter flask and dilute to volume with demineralized water.  Ferrous Sulfate Solution (0.5N FeSO₄.7H₂0)
2. Place 139.01 g of ferrous sulfate (hydrateFeSO₄.7H₂0 in a one liter flask. Add about 200 mL of demineralized water and 15 mL of concentrated sulfuric acid. Dilute to volume. Indicator (if necessary):
3. Use 0.025 M O-phenanthroline ferrous sulfate (Ferroin) as an indicator (commercially available).
   
   1. 0.5946 g/100 mL phenanthroline = 0.01N
   2. 0.2780 g/100 mL FeSO₄.7H₂0 = 0.01N

Procedure

1. Place 1.0 g of soil in a 250-mL Erlenmeyer flask and prepare a Blank by adding no soil.
2. Add exactly 10 mL of 1.00N potassium dichromate and mix by gentle rotation of flask.
3. Add 20 mL of concentrated sulfuric acid and mix for one minute by gentle rotation of flask.
4. Allow to stand for 30 minutes.
5. Add 150 mL demineralized water.
6. Add 10 mL of concentrated phosphoric acid and allow to cool to room temperature.
7. Add 4-5 drops of indicator. (If solution turns dark green weigh 1/2 g and multiply answer by 2 or weigh 1/4 g and multiply answer by 4.)
8. Titrate immediately with 0.5N ferrous sulfate solution until end point is reached.
9. Determine % organic matter using given formula:
   1. % O.M. = (1 - T/S) (6.8)
   2. Where T = sample titration in mL of 0.5N ferrous sulfate
   3. Where S = standard blank in mL of 0.5N ferrous sulfate
   4. This formula is based on two important assumptions which may not be valid for all soils:
      
      1. only 76% of the organic carbon present is oxidized to CO2, and
         soil organic matter contains 58% carbon.
10. Record on lab result sheets for special analyses as % O.M. (1 decimal place).