total Soil Carbon

Method of data collection and data needed to compute the method:

Soil carbon is usually assessed as total SOC through combustion (oxidation) of the soil. This can be done through various methods including burning in a muffle furnace, wet chemistry (Walkley Black), or combustion in CHN analyzers.

Unit of analysis:

Total SOC is expressed as units of C per unit of soil (e.g., ug/g, g/kg) or as a percentage; it can also be converted to C soil stocks (t C/ha) when the concentration is multiplied by the bulk density of the soil and the depth of the soil sample.

Limitations regarding estimating and interpreting:

Due to the spatial and temporal variability of SOC, it is extremely difficult to detect differences among treatments unless they have been in place for many years – often 10 years or more. The total pool of SOC is quite large relative to the small changes in accrual (or loss) in SOC that can occur over time; when combined with the heterogeneous distribution of SOC, it is very challenging to detect meaningful SOC differences over time. It is also important to consider that soil compactness (bulk density) is problematic to accurately measure, and this poses a major problem for detection of differences in SOC, as the volume of soil sampled is impacted by soil compaction. The analyses are also expensive and require specialized equipment.

We thus do not recommend measuring total soil carbon for experiments or for farmer’s fields that have been recently installed. There are some other measurements that provide an indication of more readily decomposed fractions of soil carbon: so-called “Active Carbon,” which is described below.