410 ORGANIC MATTER - WALKLEY-BLACK METHOD
Physio Chem: OM, Org C
Organic Matter by potassium dichromate reduction of organic carbon and subsequent spectrophotometric measurement (modified Walkley-Black).
Summary: 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%.
The method requires 2 g sample, but 10 g is requested to allow for quality control. Samples with concentrations greater than 80% OM are best tested by the Loss-on-Ignition (OM-LOI) method.
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.