This method for total soil OC involves production, purification and measurement of CO2 evolved when soil carbon is ignited in a stream of O2. Because all C compounds are converted to CO2, the C from carbonates, charcoal, undecomposed wood, etc, will be included, as no soil pre-treatment is specified. In the volumetric sub-method 6B2a, concentrated KOH solution is used to absorb the CO2 released. The difference between the original volume of gas in the burette and the volume produced after ignition equals the volume of CO2 evolved from the sample, after correction for gas temperature and pressure. Sub-method 6B2b is similar to Method 6B2a, except the CO2 produced by ignition is measured via infrared / thermal conductivity detection. Both 6B2a and 6B2b use finely-milled air-dry sample, with weights varying with expected C concentrations. Surrogate estimates can be obtained by NIR (method 6B4a) or MIR (method 6B4b) reflectance spectroscopy. The methods specify reporting as %C on an oven-dry (105oC) basis.