Dr. Murali, you may take help of NaOH extraction method

Remove roots and sieve the dried soil sample to pass a 2.0-mm sieve. Equilibrate the sample to a pH value between 1 to 2 with 1 M HCl at room temperature. Adjust the solution volume with 0.1 M HCl to provide a final concentration that has a ratio of 10 mL liquid/1 g dry sample. Shake the suspension for 1 h and then separate the supernatant from the residue by decantation after allowing the solution to settle or by low speed centrifugation. Save the supernatant (FA Extract 1) for the isolation of fulvic acid using XAD-8.

Neutralize the soil residue with 1 M NaOH to pH = 7.0 then add 0.1 M NaOH under an atmosphere of N2 to give a final extractant to soil ratio of 10:1. Extract the suspension under N2 with intermittent shaking for a minimum of 4 h. Allow the alkaline suspension to settle overnight and collect the supernatant by means of decantation or centrifugation. Acidify the supernatant with 6 M HCl with constant stirring to pH = 1.0 and then allow the suspension to stand for 12 to 16 h. Centrifuge to separate the humic acid (precipitate) and fulvic acid (supernatant - FA Extract 2) fractions.

Redissolve the humic acid fraction by adding a minimum volume of 0.1 M KOH under N2. Add solid KCl to attain a concentration of 0.3 M [K+] and then centrifuge at high speed to remove the suspended solids. Reprecipitate the humic acid by adding 6 M HCl with constant stirring to pH = 1.0 and allow the suspension to stand again for 12 to 16 h. Centrifuge and discard the supernatant. Suspend the humic acid precipitate in 0.1 M HCl/0.3 M HF solution in a plastic container and shake overnight at room temperature. Centrifuge and repeat the HCl/HF treatment, if necessary, until the ash content is below 1%. Transfer the precipitate to a Visking dialysis tube by slurrying with water and dialyze against distilled water until the dialysis water gives a negative Cl- test with silver nitrate AgNO3. Freeze dry the humic acid.

Please also see the reference Swift R. S. 1996. Organic matter characterization, Chap. 35 pp.1018-1020 in D, L, Spark et al (eds) Methods of Soil Analysis, Part 3, Chemical Methods, Soil Science Soc. Am. Book .Series:5. Soil Sci. Soc. Am., Madison, WI.

Please have a look at this link.

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2389.1966.tb01453.x/abstract

Drs.Tarafdar and Dr.Arvind Singh provided good references for extraction of humus from soils.

How do you think about " Redissolve the humic acid fraction by adding a minimum volume of 0.1 M KOH under N2. ", what dose the "minimum volume" mean?

Thanks.