To determine organic matter in agricultural soils applied the WALKLEY-BLACK method, but doesn't present similar colours per soil samples? Suggest other method?
I think, your sample is not attaining the complete oxidation by Walkley & Black method. As the heating step is very crucial , you may go for Sulfochromic acid method by Hauba et al. 1997. The difference in this method with Walkley & Black method is the heating step ( 135 degree Celcius, 1h) or you may try with Ignition method given by Reddy et al. 2009.
why not simply ashing, dear Ana? Its ok if you do not have free carbonates or very high clay content: perfectly simple!
Do you have XS chromate? Are your soils high in OM? That is the usual problem and it is solved by decreasing the amount of soil digested.
The action T is indeed critical so you need to follow the instructions exactly, down to container size and the surface they sit on. Otherwise the efficiency is affected and the conversion factor to OM is affected.
For environmental reasons I do not encourage the use of chromate
Walkley and Black method is most common method using worldwide for organic carbon determination. sometimes colour does not develop due to improper washing of glassware and expiry of chemical being used.
Many thanks for your answers. I am going to follow your recommendations. Particularly by Paul Milham, my soils are indeed high in OM.
The color variation on addition of Pot. dichromate and sulfuric acid is due to difference in OM content of soil. Use same size flasks for equal surface area and allow the complete oxidation of soil. Use Ferroin indicator in place of diphenyl amine. This gives red color end point, that is very distinctive. you can remove the carbonate carbon before proceeding for SOC estimation. Compare the results of SOC estimation by dry combustion and WB method
Dear Ana, the limitations of Walkley and Black method is:
1. For soils containing high organic matter ( >1% ) more than 10 mL potassium dichromate is needed.
2. For eliminate the chloride interference in the soil, silver sulphate should be used with oxidizing agent.
3. The factors 1.334 and 1.724 used to calculate TOC and OM are appropriate; they may vary with soil depth and between the soils.
Accordingly you may manipulate your analysis and calculations to get more accurate results.
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