Soil Organic Carbon (SOC)

A recent ICAR study revealed that climate change and unbalanced fertiliser use are speeding up the decline of soil organic carbon (SOC) in India’s farmland.

What is Soil Organic Carbon?

Soil organic carbon (SOC) is the carbon part of soil organic matter (SOM), expressed as a percentage of total soil weight.
Composition: About 50-60% of soil organic matter is carbon, and healthy soils typically contain between 1% and 6% SOC.
Forms: SOC exists in various forms, including fresh residues, humus, and living microorganisms.
It aids in nutrient and moisture retention, soil structure, pollutant degradation, and carbon sequestration.

Soil organic matter primarily consists of carbon, hydrogen, and oxygen, with smaller amounts of nitrogen, phosphorus, sulfur, potassium, calcium, and magnesium from organic residues.
It includes ‘living‘ components, like roots, fauna, and microorganisms, and ‘dead‘ components.

Temperature: Cooler climates maintain higher SOC as low microbial activity slows decomposition, while tropical regions lose carbon quickly due to rapid decay.
Rainfall: Moderate rainfall promotes vegetation growth and residue return, leading to increased SOC. Arid or flooded soils retain far less carbon.
Soil Texture: Clay soils preserve more SOC because fine particles chemically bind organic matter, whereas sandy soils lose carbon through leaching and poor aggregation.
Vegetation Type: Grasslands build greater SOC through deep-root carbon storage. Forests lose carbon faster as surface litter decomposes quickly.
Topography: Low and flat regions collect SOC from runoff and cooler microclimates, whereas steep slopes face erosion that strips away carbon-rich topsoil.
Soil Depth: Topsoil contains the most SOC from roots and microbial activity, with carbon levels steadily decreasing with depth.

Tillage: Repeated ploughing exposes buried organic matter to air, breaks carbon bonds and lowers the soil’s organic carbon content.
Fertiliser Use: Excess nitrogen fertiliser increases microbial decomposition, which accelerates carbon loss and gradually raises soil acidity.
Residue Management: Burning or removing crop residues depletes the primary source of new organic carbon, weakening the soil’s ability to replenish its fertility.
Land Conversion: Converting forests or grasslands into agricultural land disrupts natural soil processes and leads to a decline in carbon storage.
Temperature Rise: Warmer conditions stimulate microbial respiration, release more carbon dioxide and reduce the soil’s long-term carbon reserves.
Erosion: Runoff and wind erosion strip away carbon-rich topsoil, decreasing both fertility and carbon-holding capacity of the soil.

Dry Combustion: Burns soil samples at high temperatures to measure carbon dioxide released.
Wet Oxidation: This method uses an oxidising agent to convert organic carbon to carbon dioxide for measurement. It is less accurate but more accessible and cost-effective.
Spectroscopic Methods: Uses NIR and MIR spectroscopy to estimate SOC content through light reflectance or absorbance; rapid and non-destructive but requires calibration with reference methods.

Soil Structure: SOC binds soil particles into stable aggregates, enhances water infiltration, and boosts resistance to erosion.
Microbial Life: It fuels beneficial microorganisms that manage nutrient recycling, suppress soil-borne diseases, and decompose organic matter.
Soil Fertility: Higher SOC improves cation exchange capacity and nutrient availability, ensuring balanced and healthy plant growth.
Water Retention: Carbon-rich soils retain more moisture, improve irrigation efficiency, and support crop stability during prolonged dry periods.
Nutrient Supply: It acts as a slow-release source of nitrogen, phosphorus, and sulfur, reducing farmers’ reliance on chemical fertilisers.
Climate Regulation: SOC captures atmospheric carbon dioxide and strengthens the soil’s capacity as a stable natural carbon sink.