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Organic Matter - Walkley-Black Method

Alternate Methods: OM (Walkley-Black), Org.C (W-B)


This method quantifies the amount of oxidizable organic matter in which OM is oxidized with a known amount of chromate in the presence of sulfuric acid. The remaining chromate is determined spectrophotometrically at 600nm wavelength. The calculation of organic carbon is based on organic matter containing 58% carbon. The method has a detection limit of approximately 0.10% and, on homogeneous sample material, is generally reproducible within 8%.

Samples with concentrations greater than 80% OM are best tested by the Loss-on-Ignition (OM-LOI) method.

Sample amount requested:  10 g

Questions concerning limited sample size can be answered by the UC Davis Analytical Laboratory.

Nelson, D. W. and Sommers, L. E. 1982. Total carbon, organic carbon and organic matter. p. 539-579. In: A. L. Page et al. (ed.) Methods of soil analysis: Part 2. Chemical and microbiological properties. ASA Monograph Number 9.