A New Priority for Sustainable Agriculture

Droughts that last longer. Soils that crack. Yields that drop. Across the globe, especially in tropical regions like Southeast Asia, farmers face growing uncertainty. Climate change is no longer a distant threat: it’s affecting the land we rely on, right now.

One of the most promising solutions lies beneath our feet: carbon stocks. Soils act as a major reservoir for organic carbon, playing a crucial role in regulating the Earth’s climate. When soils lose their carbon (through degradation, poor management or deforestation), this carbon is released into the atmosphere as carbon dioxide (CO₂).

The organic carbon stored in our soils, as agricultural soils alone, represents about one-third of the total loss of carbon from soil and vegetation [3].

Soil Organic Carbon (SOC) is a key to climate mitigation, agricultural resilience, and long-term food security. But before we can protect or increase it, we have to know how much there is. That’s the heart of the problem. SOC is difficult and costly to measure, especially across large or diverse landscapes.

This article explores why SOC matters, what it is, how we measure it, and the challenges of scaling up carbon monitoring in real-world farming systems. We’ll also highlight why this matters now and how efforts in countries like Thailand are gaining momentum (this will mainly be developed in the next article).

What is Soil Organic Carbon (SOC)?

Soil Organic Carbon (SOC) refers to the carbon stored within soil organic matter, including plant residues, microbial biomass, and decomposed organic materials [1][4]. It plays a critical role in maintaining soil health and agricultural productivity.

Healthy soils provide food security, support biodiversity, regulate water cycles, and contribute to climate stability [4]. Therefore, improving soil health and boosting SOC content are not only agricultural challenges but crucial environmental and societal imperatives.

Higher SOC levels contribute to:

• Biological benefits: improved microbial activity, enhanced nutrient cycling, and resilience against pathogens [6]
• Chemical benefits: better nutrient retention, pH buffering, and toxin immobilization [7]
• Physical benefits: improved structure, water-holding capacity, and erosion resistance [5]

The Carbon Cycle

Carbon is constantly moving between the air, the plants, and the soil. The diagram below shows how soils can capture and store carbon, turning farming landscapes into potential climate solutions.

It all starts with plants. Through photosynthesis, they absorb carbon dioxide (CO₂) from the atmosphere (Fig. 1) and use it to grow. But the story doesn’t end at the leaves. A large share of this carbon travels underground, through roots and tiny organic compounds called exudates, which feed the life hidden beneath the surface (fungi, bacteria, and other microbes).

As plants die, their leaves, stems, and roots become organic matter. In healthy soils, this matter is slowly broken down by microorganisms. Some of the carbon is released back into the atmosphere as CO₂, but some is transformed and stored as Soil Organic Carbon (SOC). This is a stable form of carbon that can remain in the ground for decades or even centuries.

This natural process plays a vital role in climate regulation. The more carbon we can help soils retain, the less CO₂ ends up in the air. But it doesn’t happen by chance: farming practices, soil health, and land management make all the difference. With the right tools, farmers can turn their fields into powerful carbon sinks.

SOC Variability

SOC levels are highly heterogeneous. They depend on climate, vegetation, soil texture, management practices, and more. For example, soils rich in clay, with high biomass production, and subject to regular rainfall tend to have higher carbon storage capacities.

However, in tropical regions such as Thailand, although rainfall is abundant, high temperatures accelerate organic matter decomposition. As a result, long-term carbon storage potential is often lower despite favorable moisture conditions.

Recent data highlights the strong spatial variability of SOC across Southeast Asia (Fig. 2). Built through a collaborative effort led by the FAO, this map provides a view of how SOC stocks vary from one region to another.

This spatial variability makes it even more important to adapt measurement and monitoring systems to local realities, which is a key challenge for governments and carbon markets.

Not all carbon stored in the soil is equally stable. Some forms of SOC are easily lost through decomposition or disturbance (tillage, deforestation, burning, erosion, drought…), while others are more resistant and can remain stored for decades or even centuries. The concept of “Vulnerability to Change” reflects how likely a soil’s carbon stock is to decrease when exposed to high environmental pressures.

This vulnerability is a critical factor in the context of carbon credit systems, where long-term storage and permanence are key requirements. If carbon is quickly lost after being counted, it is no longer a benefit to the climate. That is why understanding the resilience of soil carbon stocks is essential for designing reliable, science-based mitigation strategies.

How Do You Measure SOC?

General Principles

SOC stock is usually expressed in tons of carbon per hectare (tC/ha). It’s calculated using:

[1]

It is essential to distinguish between organic carbon and inorganic carbon (such as carbonates) in soils. Only the organic component is considered relevant for soil health and climate change mitigation [5].

Accurate SOC measurements require careful consideration of soil heterogeneity, particularly in terms of texture (sand, silt, clay content), land use history, and topographical variation [9].

Typically, SOC represents 50–58% of Soil Organic Matter (SOM) [8].

Field and Laboratory Measurement Methods

The standard method relies on direct soil sampling followed by laboratory analysis [13].

Step 1 – Field Sampling

• Representative soil samples are collected (Fig. 3) from various locations across the study area.
• Sampling strategies include random, grid-based, or stratified sampling.
• Only the fine earth fraction (<2 mm) is analyzed.
• Care must be taken to avoid sampling biases, such as oversampling easily accessible areas or failing to cover the full spatial variability of the landscape.

Step 2 – Laboratory Analysis:

• Dry Combustion (Dumas method): Samples are combusted at high temperatures (900–1,200°C) and the released CO₂ is measured [11]
• Wet Oxidation (Walkley-Black): A chemical oxidation process. It is cheaper but it tends to underestimate total carbon by 10–30% [12]

Bulk Density: Measured using undisturbed core samples, oven-dried at 105°C

New Measurement Methods and Technological Developments

Traditional laboratory methods are accurate but costly and slow. Newer technologies offer rapid, scalable alternatives.

Proximal Sensing:

• Portable Vis-NIR spectrometers for field-based estimation of SOC [14]. These devices measure how soil reflects light, with specific reflectance patterns linked to organic carbon content.

Remote Sensing:

• Satellites like Sentinel or Landsat use multispectral and hyperspectral data to infer SOC properties, though mainly limited to surface soils [15].

Modeling Approaches:

• Machine learning models combine remote sensing data with soil covariates to predict SOC at landscape scale [10].

Table 1. Comparative Overview of SOC Measurement Methods

Aspect	Traditional Sampling	Proximal Sensing	Remote Sensing	Machine Learning
Accuracy	High (reference)	Moderate	Low (surface)	Variable
Cost	High	Moderate	Low	Low
Time	Slow	Fast	Very fast	Very fast
Spatial Coverage	Limited	Moderate	High	High
Main Limitation	Laborious	Calibration need	Vegetation/noise	Data dependence

A comparison of different SOC measurement methods shows that while new technologies provide speed, scalability, and lower costs, they still face significant limitations in accuracy and reliability compared to traditional field sampling (Table 1).

Limitations of New Measurement Techniques

While new methods such as proximal sensing, remote sensing, and machine learning offer exciting prospects for faster and more extensive SOC monitoring, they still face important limitations that prevent their widespread adoption.

First, proximal sensing technologies, like portable spectrometers, require careful calibration and validation against traditional laboratory measurements. Their accuracy can vary significantly depending on soil moisture, texture, or organic matter content, reducing reliability across diverse landscapes.

Remote sensing approaches are mostly limited to detecting surface soil properties, typically only the top few millimeters, and are highly sensitive to vegetation cover, soil moisture, and atmospheric conditions. As a result, they cannot accurately assess deeper carbon stocks, which are critical for evaluating long-term sequestration.

Finally, modeling approaches based on machine learning depend heavily on the availability of high-quality ground-truth datasets. Without sufficient field measurements for training and validation, models can produce biased or unreliable estimates, especially when extrapolating to new regions.

While these emerging technologies are extremely promising, they cannot yet replace the precision and robustness of direct field sampling and laboratory analysis, especially for carbon credit systems where the verification of carbon stocks must meet strict standards. In practice, they are best used as complementary tools to traditional methods, helping reduce costs and improve spatial coverage, not as standalone solutions.

From Measurement to Action

Measuring Soil Organic Carbon isn’t just a technical task, it’s a strategic necessity.

Accurate SOC data helps governments shape sustainable land use policies. It allows NGOs and consultants to build local carbon projects. It enables farmers to participate in carbon markets with confidence. And it gives everyone, from consumers to policymakers, a way to act against climate change.

But numbers alone won’t change the soil. We must go further.

To truly build SOC, we need farming systems that protect and regenerate soils, systems like Conservation Agriculture, which we have explored in another article.

Curious to learn more?
Conservation Agriculture: Future Farming

Sources

Foundational Reports and Databases

• [1] FAO, Carbon sequestration in dryland soils, 2004. https://www.fao.org/3/y5738e/y5738e00.htm
  
• [2] FAO GSOCmap v1.5, Global Soil Organic Carbon Map, 2022. https://www.fao.org/soils-portal/data-hub/soil-maps-and-databases/global-soil-organic-carbon-map-gsocmap/en/
  
• [3] IPCC, Land Use, Land-Use Change, and Forestry: Special Report, 2000. https://archive.ipcc.ch/ipccreports/sres/land_use/index.php?idp=0
• [4] IPCC, Climate Change and Land: An IPCC Special Report, 2019. https://www.ipcc.ch/srccl/
  

Scientific Articles and Reviews

[5] R. Lal, Soil carbon sequestration to mitigate climate change, Science, vol. 304, no. 5677, pp. 1623–1627, 2004. https://doi.org/10.1126/science.1097396

• [6] J. Six, R.T. Conant, E.A. Paul, K. Paustian, Stabilization mechanisms of soil organic matter: Implications for C-saturation of soils, Plant and Soil, vol. 241, pp. 155–176, 2002. https://doi.org/10.1023/A:1016125726789
  
• [7] FAO, Soil Organic Carbon: the hidden potential, 2017. https://www.fao.org/3/i6937e/i6937e.pdf
  
• [8] D.W. Pribyl, A critical review of the conventional SOC-to-SOM conversion factor, Geoderma, vol. 156, pp. 75–83, 2010. https://doi.org/10.1016/j.geoderma.2010.02.003
  
• [9] B. Minasny, S. Malone, A.B. McBratney, Soil carbon 4 per mille, Geoderma, vol. 292, pp. 59–86, 2017. https://doi.org/10.1016/j.geoderma.2017.01.002
  
• [10] J. Sanderman, T. Hengl, G.J. Fiske, Soil carbon debt of 12,000 years of human land use, Proceedings of the National Academy of Sciences, vol. 114, no. 36, pp. 9575–9580, 2018. https://doi.org/10.1073/pnas.1706103114
  

Technical Methods

• [11] D.W. Nelson, L.E. Sommers, Total Carbon, Organic Carbon, and Organic Matter, in Methods of Soil Analysis: Part 3 Chemical Methods, 1996.
• [12] B.A. Schumacher, Methods for the Determination of Total Organic Carbon (TOC) in Soils and Sediments, EPA/600/R-02/069, 2002. https://archive.epa.gov/epa/sites/production/files/2015-06/documents/epa600r02069.pdf
  
• [13] B.H. Ellert, J.R. Bettany, Calculation of organic matter and nutrients stored in soils under contrasting management regimes, Canadian Journal of Soil Science, vol. 75, no. 4, pp. 529–538, 1995. https://doi.org/10.4141/cjss95-075
  

Emerging Technologies and Monitoring Approaches

• [14] E. Vaudour, C. Gomez, Y. Fouad, P. Lagacherie, Sentinel-2 image capacities to predict soil organic carbon variability in croplands, Remote Sensing of Environment, vol. 223, pp. 50–63, 2019. https://doi.org/10.1016/j.rse.2019.01.022
  
• [15] S. Chabrillat, E. Ben-Dor, J. Cierniewski, C. Gomez, T. Schmid, S. Sommer, A. Stevens, Imaging Spectroscopy for Soil Mapping and Monitoring, Surveys in Geophysics, vol. 40, no. 3, pp. 361–399, 2019. https://doi.org/10.1007/s10712-019-09542-3