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. Grind the soil and sieve it through 0.25 mm sieve2. Weight 0.3 g into 100-150 ml beaker3. Add 10 ml of K2Cr2O7 into the beaker3. Stir the soil with K2Cr2O7 carefully4. Cover the beaker using the watch glass5. Insert the beaker into the oven (125°C) for 45 min.6. Wash the beaker side and watch glass using distilled

water7. Titration with (NH4)2Fe(SO4)2

Tjurin method

Equation2 K2Cr2O7 + 8 H2SO4 →

K2SO4 + 2 Cr2(SO4)3 + 8 H2O + 3 O2

3 O2 + 3 C → 3 CO2

K2Cr2O7 + 7 H2SO4 + 6 Fe(NH4)2(SO4)2 → K2SO4 + Cr2(SO4)3 + 3 Fe2(SO4)3

+ 6 (NH4)2SO4 + 7 H2O

Acid dichromate

Mohr salt

Calculation

• Factor of Mohr salt

• Cox

• Humus content

40f

a

100(12 0,3 )Cox S f

n

% 1,724humus Cox

.

.

.

.

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