1. Soil Organic Matter Determination SSoil organic matter quantity - Can be expressed as a percentage - Can be expressed as mg.g -1 The wet oxidation of SOM using acid dichromate (K 2 Cr 2 O 7 ) is followed by the potentiometric titration with ferrous ammonium sulfate – Tjurin method SSoil organic matter quality - Can be evaluated by C:N ratio - Can be evaluated by Humic acid and Fulvic acid ratio Measuring of soil extract absorbance

2. SOM Quantity Determination 1.Grind the soil and sieve it through 0.25 mm sieve 2.Weight 0.3 g into 100-150 ml beaker 3.Add 10 ml of K 2 Cr 2 O 7 into the beaker 3.Stir the soil with K 2 Cr 2 O 7 carefully 4.Cover the beaker using the watch glass 5.Insert the beaker into the oven (125°C) for 45 min. 6.Wash the beaker side and watch glass using distilled water 7.Titration with (NH 4 ) 2 Fe(SO 4 ) 2

3. Tjurin method Equation 2 K 2 Cr 2 O 7 + 8 H 2 SO 4 → K 2 SO 4 + 2 Cr 2 (SO 4 ) 3 + 8 H 2 O + 3 O 2 3 O 2 + 3 C → 3 CO 2 K 2 Cr 2 O 7 + 7 H 2 SO 4 + 6 Fe(NH 4 ) 2 (SO 4 ) 2 → K 2 SO 4 + Cr 2 (SO 4 ) 3 + 3 Fe 2 (SO 4 ) 3 + 6 (NH 4 ) 2 SO 4 + 7 H 2 O Acid dichromate Mohr salt

4. Calculation Factor of Mohr salt Cox Humus content...

5. Evaluation of Cox/Humus Content Cox (%)Humus (%)Evaluation < 0.6< 1.0very low 0.6 – 1.11.0 – 2.0low 1.1 – 1.72.0 – 3.0medium 1.7 – 2.93.0 – 5.0high > 2.9> 5.0very high