Humic Substances and Soil Carbon
This page looks at humic substances from the point of view of soil carbon: how much of soil organic carbon they represent, how stable they are, and what that means for soil carbon management. For definitions and the three fractions, see humic substances overview.
Short answer: Humic substances are a large part of soil organic matter, and therefore of soil organic carbon. They decompose more slowly than fresh plant residues, so they contribute to longer-term carbon storage in soil. Current research also shows that soil carbon stability depends strongly on how organic matter is protected by minerals and soil aggregates, not only on its chemical form.
Humic substances in the soil carbon pool
| Soil carbon pool | Typical turnover | Role of humic substances |
|---|---|---|
| Fresh residues | Months to a few years | Starting material for humification |
| Particulate organic matter | Years to decades | Partly decomposed; feeds microbes |
| Mineral-associated organic matter | Decades to centuries | Includes humic material bound to clay and silt |
What current research says
Older soil science treated humic substances as distinct, very stable molecules. More recent work describes soil organic matter as a continuum of compounds at different stages of decomposition, and points out that the alkaline extraction used to isolate “humic substances” changes the material. In this view, carbon persists mainly because it is protected inside aggregates and bound to mineral surfaces (Lehmann and Kleber, 2015). The humic fractions remain a useful working definition for products and laboratory tests.
What this means in practice
- Building soil carbon depends mainly on regular organic inputs (residues, roots, compost) and on protecting soil structure.
- Humic products are used as soil conditioners and fertilizer additives; the amount of carbon they add at normal rates is small compared with crop residues.
- Soil organic carbon is measured on soil samples over time; see soil organic carbon and carbon sequestration.
Frequently asked questions
Do humic substances store carbon?
They are part of soil organic carbon and decompose more slowly than fresh residues, so they contribute to longer-term storage.
Are humic substances stable in soil?
Their persistence depends largely on protection by minerals and aggregates, as well as on their chemistry.
Can humic acid products increase soil carbon?
At normal application rates they add little carbon directly. Soil carbon is built mainly through regular organic inputs and good soil management.
What is the difference between soil organic matter and soil organic carbon?
Soil organic carbon is the carbon contained in soil organic matter; organic matter also contains oxygen, hydrogen, nitrogen and other elements.
Reference
- Lehmann, J. and Kleber, M. (2015). The contentious nature of soil organic matter. Nature 528: 60–68.
