Carbon Content

The carbon content varies from approximately 45% in certain carbohydrates to up to 60% in complex humus compounds. Other elements in organic matter are primarily oxygen (O), but hydrogen (H), nitrogen (N), and other elements (S, Ca, Mg, K, etc.) are also present.

Carbon stocks in soil

In a mor layer, the carbon content is generally about 35–40%. The content is not higher because some mineral particles are mixed in, meaning that a mor layer does not consist of pure organic matter. The incorporation of mineral particles (e.g. sand grains) may occur through burrowing animals, mixing caused by windthrown trees, or windblown material.

In humus forms with intensive mixing, such as that caused by earthworms in mull, the large proportion of mineral particles results in a carbon content of only about 5–10%.

The carbon content decreases with depth in the soil profile, but measurable amounts—on the order of 0.5%—are still found at depths of around one metre. The total carbon stock within the upper 1 m of soil in typical forest land varies between approximately 6 and 14 kg per m², with higher values found on fertile sites in southern Sweden. In wet and moist soil types, the carbon stock is usually higher.

Carbon content in air-dried soil samples

Since 1993, the carbon content of air-dried soil samples has been determined using a CNS analyser, in which the sample is combusted and the combustion gases are separated. The gas amounts are then measured using a thermal conductivity detector. The carbon content is expressed as a percentage by weight of the dry matter content.

The dry matter content is determined by weighing a subsample of air-dried soil (w₁), drying it at 105 °C for 16–24 hours, and then weighing it again (w₂). The dry matter content is then calculated by dividing w₂ by w₁.