Fundamentals of Soil Science (1+1)

Exercise 15 : Determination of Organic Carbon Content of Soils

Importance:

• All plant, animal and microbial residues, both decomposed and un-decomposed in the soil constitutes organic matter, most of which is of plant origin.
• Soil organic matter (SOM) is an index of productivity of soil since it is a store house of essential nutrients for plant growth. It is also a source of cation exchange capacity besides promoting desirable soil structure and influencing moisture retention and aeration of soils.
• The SOM is the seat of nitrogen in soil and its determination is often carried out as an index of nitrogen availability.
• Organic matter on an average contains 58 per cent carbon, the per cent organic matter can be obtained by multiplying per cent organic carbon with 100/58 or 1.724 which is also known as van Bemmlen factor.

Methods:

• Carbon is the chief constituent of soil organic matter that can be readily measured quantitatively. Hence estimates of organic matter are based on organic carbon which can be determined mainly by two methods:
• Dry combustion method
• Wet digestion method
• Rapid titration method (Walkley and Black, 1934)
• Colorimetric method.

Principle:
Dry combustion method:

• The dry combustion method gives absolute values and is useful for very accurate estimations. However, this method is tedious and time consuming. In this method, a known weight of soil, pre-treated with excess of 0.5 per cent sulphurous acid (H₂SO₃) to destroy CaCO₃, is ignited in silica tubes packed with copper oxide.
• The CO₂ evolved is then absorbed in weighed quantity of soda lime and the amount of CO₂ produced is found by the difference in the initial and final weight of the soda lime. From the amount of soil taken and CO₂ evolved, content of organic carbon and hence organic matter is calculated.

Wet Digestion method:

• For ordinary routine work, wet digestion method is generally used.
• In this method, organic matter is oxidized with chromic and sulphuric acid making use of the heat of dilution of strong sulphuric acid so as to oxidize the humus.
• In this method the excess of chromic acid not reduced by SOM is determined by titration with standard ferrous ammonium sulfate solution in the presence of phosphoric acid, sodium fluoride and diphenylamine solution as indicator.
• Phosphoric acid and sodium fluoride make the colour change distinct because of their suppression of Fe⁺⁺⁺ (Ferric) ion activity (flocculating effect) which is generated during the titration with ferrous salt.
• At the end point, colour of the suspension changes from violet through blue to bright green. High chloride content (in case of saline soils), which interferes in the estimation of organic carbon, can be prevented by adding 1.25 per cent Ag₂SO₄ to concentrated sulphuric acid.
• The recovery of organic carbon by this method varies with the nature of the soil. The values obtained using this method are only approximation because all the carbon which is oxidized is 60-90 per cent with a mean value between 75-80 per cent.
• In the colorimetric method the intensity of green colour of chromic acid obtained due to its reduction is measured colorimetrically. The intensity of green colour is directly proportional to the amount of organic carbon present in the soil.

The details of most widely used method (Walkley and Black) in soil testing laboratories are given here.
Reactions:

Apparatus and Reagents required:

• 500 ml conical flask
• Pipette (10 and 20 ml)
• Burette (50 ml)
• Sieve (0.5 mm)
• 1N Potassium dichromate (K₂Cr₂O₇): Dissolve 49.04 g of AR grade K₂Cr₂O₇ in about 500 ml of distilled water and make the volume to one litre.
• 0.5N Ferrous ammonium sulphate [FeSO₄(NH₄)₂SO₄.6H₂O (Mohr’s salt)]: Dissolve 196 g of FeSO₄ (NH₄)₂SO₄.6H₂O in distilled water and add 20 ml of concentrated sulphuric acid before making final volume to one liter.
• Diphenylamine indicator: Dissolve 0.5 g of diphenylamine in 20 ml of distilled water and 100 ml of concentrated sulphuric acid.
• Concentrated sulphuric acid.
• Orthophosphoric acid (86%) or Sodium fluoride (NaF)

Procedure:

• Weigh 1g of soil sample (passed through 0.5 mm sieve) into 500 ml dry conical flask.
• Add 10 ml of 1N K₂Cr₂O₇ solution and shake gently to disperse soil in the solution.
• Add 20 ml of concentrated sulphuric acid carefully from the side of flask, swirl the flask during the addition.
• Keep the flask on a dry tile or asbestos sheet for 30 minutes at room temperature.
• Add a pinch of NaF/10 ml of orthophosphoric acid and 100 ml of distilled water
• Add about 10 drops of diphenylamine indicator and shake vigorously to mix the contents.
• Take 0.5 N Ferrous ammonium sulphate solution in 50 ml burette and titrate the contents of flask till the colour changes from violet to bright green.
• Note the volume of ferrous ammonium sulphate solution used.
• Carry out a blank titration in similar manner, using all the reagents except soil.

Precautions:

• Add potassium dichromate very carefully without touching the neck of the flask.
• Handle the string sulphuric acid with caution as it can injure the skin and spoil clothes.
• Read upper meniscus of ferrous ammonium sulphate solution.
• In case the contents of flask turns green with the addition of indicator before titration, repeat the sample either with double amount of potassium dichromate or with lesser amount of soil.

Observation and calculations:

• Weight of soil taken = S g
• Volume of N/2 ferrous ammonium sulphate = X ml
• used for blank titration
• Volume of N/2 ferrous ammonium sulphate = Y ml
• used to titrate excess nascent oxygen (from 1N K₂Cr₂O₇)
• Volume of 1N K₂Cr₂O₇ used for oxidation of carbon = (X – Y) / 2 ml
• 1 ml of 1N K₂Cr₂O₇ = 3 mg or 0.003 g of carbon
• Per cent of organic carbon (A) = ((X – Y) x 0.003 x 100 )/(2 x S)
• Per cent organic matter = A x 1.724

Note:
To obtain actual percentage of organic carbon by Walkley and Black method, organic carbon should be multiplied by a factor 100/77 i.e. 1.3 (77 % being the average recovery factor)
Interpretation:

Organic carbon (%)	Rating
< 0.25	Very low
0.25 – 0.50	Low
0.51-1.00	Medium
1.00 – 1.50	High
> 1.50	Very high