Ignition Test for Soil Organic-Content Measurement

Problems with organic soils are often avoided by engineers because of uncertainties in organic-content measurement and lack of information on potential decomposition. High temperatures, used in the ignition test for measurement of the organic fraction, require that a correction be made for loss of surface hydration water from the mineral solids. Model soils, prepared from kaolinite and pulp fiber, and the dehydration curves for these materials have permitted development of a correction factor for use with the ignition test. A review of published data and an error analysis suggest that this correction factor may be suitable for natural organic soils. The ignition test does not distinguish between undecomposed organic matter and microorganisms formed during decomposition; hence, the degree of decomposition has not been available. A relationship—between microorganism cell content, undecomposed organic matter, and the mineral fraction—has been developed which permits computation of the degree of decomposition.