Soil Organic Matter Determination
 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 (K2Cr2O7) is
followed by the potentiometric titration with ferrous
ammonium sulfate – Tjurin method
 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
SOM Quantity Determination
1.
2.
3.
3.
4.
5.
6.
Grind the soil and sieve it through 0.25 mm sieve
Weight 0.3 g into 100-150 ml beaker
Add 10 ml of K2Cr2O7 into the beaker
Stir the soil with K2Cr2O7 carefully
Cover the beaker using the watch glass
Insert the beaker into the oven (125°C) for 45 min.
Wash the beaker side and watch glass using distilled
water
7. Titration with (NH4)2Fe(SO4)2
Tjurin method
Equation
Acid dichromate
2 K2Cr2O7 + 8 H2SO4 →
K2SO4 + 2 Cr2(SO4)3 + 8 H2O + 3 O2
3 O2 + 3 C → 3 CO2
Mohr salt
K2Cr2O7 + 7 H2SO4 + 6 Fe(NH4)2(SO4)2 →
K2SO4 + Cr2(SO4)3 + 3 Fe2(SO4)3
+ 6 (NH4)2SO4 + 7 H2O
Calculation
• Factor of Mohr salt
40
f 
a
• Cox
100
Cox  (12  0,3  S  f ) 
n
..
.
• Humus content
%humus  Cox 1,724
.
Evaluation of Cox/Humus Content
Cox (%)
Humus (%)
Evaluation
< 0.6
< 1.0
very low
0.6 – 1.1
1.0 – 2.0
low
1.1 – 1.7
2.0 – 3.0
medium
1.7 – 2.9
3.0 – 5.0
high
> 2.9
> 5.0
very high