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.

Determinants of Soil Organic Carbon

• 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.

Factors Affecting Soil Organic Carbon

• 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.

Traditional Methods for Measuring SOC

• 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.

Benefits of Soil Organic Carbon

• 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.