Air dry soil should be ground to pass through a
0.5 mm sieve. A 2.5 g soil sample is suspended in 50
ml of NaHCO3 solution of pH 8.5 along with 1
teaspoon of carbon black. Fine suspension is shaken
for a period of 30 minutes. The solution is filtered
through a Whatman No.40 or other suitable filter
paper. A 5 ml aliquot of clear filtrate is pipetted in to
25 ml vol. flask. A volume of 5ml acid molybdate is
added and the flask is allowed to stand for the
generation of CO2. After that add 10 ml distilled water,
then 1 ml stannous chloride (working solution), by
immediate shaking and make up the volume to 25 ml
and mix thoroughly. Prepare a blank as above and
read the intensity of colour developed, at 660 nm,
after 10 minutes and within 20 minutes
