Soil organic matter is the organic fraction of soil which includes plant, animal and microbial residues and soil humus. An estimate of soil organic matter content is often used as an indicator of soil fertility.

Organic matter in the soil is oxidized by treatment with a mixture of potassium dichromate and sulfuric acid. The amount of dichromate remaining after reaction with soil organic matter is estimated by colorimetry after removal of the soil by filtration. The dichromate reduced during the reaction with the soil is assumed to be equivalent to a proportion (~75%) of the organic carbon present in the sample. The method also assumes that organic carbon is the only substance present that reduces dichromate.

This method is applicable to mineral soils containing up to 0.1% to 7% organic matter. This method can be used to estimate organic matter content in the range 7-10%, but variability will be up to 25% of the value. The method should not be used for organic matter contents >10%.