Soil Test to Find Organic Matter Content in Soil

Purpose

Deleterious contents of soil such as organic matter affect its characteristics and behavior adversely and must therefore be within 2%.

Procedure

1. Take an air dried sample of soil and determine its moisture content. Using another sample, produce about 100 g of soil passing the 10 mm I.S. sieve. Pulverise this sample to pass through 425 micron I.S. sieve.
2. In case the soil contains a high percentage of sulphides or chlorides it may be neutralised with dilute sulphuric acid or washed with distilled water to remove the salts.
3. Take 5g of soil from thoroughly mixed soil passing 425 micron sieve and place it in a glass weighing bottle. The weight of the sample should be less (upto 0.2g) for soils with higher content of organic matter.
4. Add 10 millilitres of potassium dichromate solution into the conical flask with a burette and add 20 ml of concentrated sulphuric acid. Swirl the mixture for about one minute and keep it on a heat insulated surface for 30 min to allow oxidation.
5. Add 200 ml of distilled water along with 10 ml of orthophosphoric acid and 1 ml of sodium fluoride as the indicator. Shake the mixture vigorously. If the indicator gets absorbed, add another 1 ml of the indicator.
6. Add ferrous sulphate solution from a second burette until the colour of the solution changes from blue to green. Add a further 0.5 ml of potassium dichromate to change the colour from green to blue.
7. Add ferrous sulphate solution drop by drop with continuous swirling until the colour of the solution changes with a single drop. Note the total volume of ferrous sulphate solution used in the experiment.

Calculations

The total volume of potassium dichromate used to oxidise the organic matter in the soil is given by the following formula

V = 10.5 [1.0 – (Y/X)]

Where

Y = total volume of ferrous sulphate used in the test

X = total volume of ferrous sulphate used in the standardization test

The percentage of organic matter present in the oven-dried sample may be calculated from the following formula

The organic matter, percent by weight = (0.67*W₂*V) / (W₁*W₃)

Where

W₂ = weight on oven-dry basis of the soil sample passing 10 mm sieve

V = total volume of potassium dichromate used to oxidise the organic matter

W₁ = weight on oven-dry basis of the total soil sample before sieving

W₃ = weight on oven-dry basis of the soil specimen used in the test

Reference

I.S. 2720 (Part 22) – 1972