Determination of Soil Organic Matter by Walkley Method

Determination of soil organic matter (By wet method)

Organic matter

Remains of plants (roots, leaves and shoots) and dead animals and decay of animal products can build organic residues 9in soil.

Soil organic matter can have determined by soil organic kits and wet chromate method (Walkley, 1947). Soil organic matter gives you rough values. Soil cannot be used at present time. In humid regions the organic carbon is an indicator of % organic matter in soil. In humid region the organic carbon is high and at the same time in dry areas organic carbon iv low as compared to the humid region.

If organic carbon is greater or equal than 50% then it is muck soil and if the organic carbon is greater or equal than 20% then the soil is peat soil. In Pakistan the organic carbon is less than 1%.

Principle

This method involves or relies upon oxidation of potassium dichromate by organic compound and remaining reduced from K2Cr2O7 can further undergo oxidation reduction by ferrous sulphate titration.

Apparatus

Dispenser, pipette, burette, magnet stirrer, Teflon, capped stirrer.

Reagents

1.       1 N of K2Cr2O7

2.       0.5 M FeSO4 + 5 ml concentrated sulfuric acid

3.       Concentrated H2SO4 (98% pure specific gravity of 1.84 w/w)

4.       Phosphoric acid (H3PO4)

5.       Diphenyl-amine indicator (1g of diphenylamine in100 ml of concentrated H2SO4). Don’t make indicator in water.

Procedure

·         1g air dry soil in 500ml beaker.

·         10ml K2Cr2O7 in it by pipette.

·         Add 20ml concentrated H2SO4 with the help of dispenser and then add 10ml H3PO4 by dispenser.

·         Wait for at least 10 minute.

·         Now add 200 ml DI water.

·         Add 10-15 drops od diphenyl indicator.

·         Now titrate the solution with 0.5 M feslous sulphate until you don’t get green colour.

·         Always use two blanks samples for measurements of soil organic matter.

·         Further these blanks samples must undergo the same procedure expect the addition of soil.

Calculation

M= 10/ volume of blank sample

% oxidizable carbon (w/w) =V (blank sample) -- V (sample)

% oxidizable organic matter = 1.334 x %oxidizable carbon

% Total oxidized soil organic matter = 1.74 xoxidizable organic matter