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What are The Different Methods of Measuring Soil Organic Carbon?

Generating three centimetres of topsoil takes 1,000 years, and if the current rates of degradation continue, almost the entire world’s topsoil could be gone within 60 years, mentions the United Nations. The FAO reported that soils are crucial in absorbing carbon and filtering water. Destruction of the soil layer creates a vicious cycle in which less carbon is stored, the world gets warmer, and the land is further degraded. Managing carbon levels in soils has always been essential for sustainable crop production. However, the importance of soil carbon is now seen differently as environmental factors become part of future agricultural policies and opportunities. This makes it necessary for soil organic carbon measurement.
Soil can store vast amounts of carbon recovered from the atmosphere. Agriculture is uniquely positioned to play a vital role in reducing atmospheric carbon dioxide through sequestration and increased storage. Soil organic carbon makes up over 50% of all soil organic matter, and the amount stored by the soil will depend on the soil type and the farming practice. From a production point of view, optimal organic carbon levels, with increased microbial activity and enhanced nutrient supply, can sustainably maintain and raise yields. In sandy soils, adequate organic carbon is essential to improve water and nutrient retention.

What is soil organic carbon (SOC)?

Soil organic carbon (SOC) refers only to the carbon component of organic compounds. Soil organic matter (SOM) is challenging to measure directly, so laboratories tend to measure and report SOC using the following formula:
SOM (%) = SOC (%) x 1.72
SOC is measured because it contributes to agriculture, climate change, food solutions, and soil health. In addition, as one of the earth’s most highly valued biopolymers, organic carbon is natural and renewable energy storage.
SOC improves the biological, chemical, and physical properties of soil, which in turn, increase water-holding capacity and structural stability. It is also integral in the formation of soil’s amino acids.
Finally, SOC acts as a “buffer” for soil against extreme pH fluctuations. Steady pH is critical for soil and ecosystem functions. Depletion of SOC degrades soil quality, reduces biomass productivity, and adversely impacts water quality.

soil organic carbon measurement

Why should we measure soil organic carbon?

Carbon is the most critical parameter for healthy soil. Soil organic carbon impacts crop productivity, soil health, the movement of water and the removal of contaminants. Remember that plants don’t absorb carbon from the soil but from the atmosphere. Instead, the carbon in soil contributes to soil nutrients by adsorbing and desorbing nutrients and providing habitats for microorganisms. In short, soil organic carbon measurement helps assess the soil’s health. Other reasons for measuring soil organic carbon are:

  • It indicates the water-holding capacity of soils: More SOC means higher water-holding capacity.
  • It fights against climate change: As the second most crucial sink for carbon (after oceans), measuring SOC would help monitor soil absorption capacity.

The different analytical tests for measuring total soil organic carbon

In general, methods of measuring soil organic carbon can be classified into two; wet digestion and dry combustion. In wet digestion methods, carbon is oxidized using chemicals, while dry combustion methods involve the thermal decomposition of carbonate materials to generate carbon dioxide. Examples of wet methods include Walkley and Black and Photometric methods, while dry combustion includes ignition tests.
1. Walkley and Black Method
The method relies on the oxidation of potassium dichromate (K2Cr2O7) that is acid catalyzed. The heat from the dilution raises the temperature to induce substantial oxidation of carbon to carbon dioxide.
A modified Walkey and Black Method called Meibus uses the same procedure but includes sulphuric acid with K2Cr2O7.
2. Photometric Method
Potassium dichromate (K2Cr2O7) and sulphuric acid are added to the soil. After cooling for 1 hour, distilled water is added. The solution is measured using a spectrophotometer with varying concentrations of sucrose solution
3. Gravimetric Method – loss on ignition (Ignition Test)
Soil samples are subjected to calcination for 5 hours at 400℃. Then, samples are weighed, and the difference in mass corresponds to soil organic matter.
4. Dry Combustion
Soils are pre-treated with hydrochloric acid to remove inorganic carbon. Next, the soil sample (often 30mg) is placed in a capsule and combusted at 975℃. Then, it uses an automatic analyzer with a thermal conductivity sensor detector (TCD).

Practices to promote soil carbon storage

Soil carbon storage is a very critical ecosystem service. In agricultural land, soil carbon loss occurs due to improper methods of soil management like excessive tillage, increased use of chemical fertilizers, increased rate of irrigation etc.
One of the most impactful methods for leaving the soil untouched is the practice of zero-tillage.
Soil fertility can be maintained by introducing a proper management strategy for grazing cattle and reducing chemical fertilizers’ usage. Replacing chemical fertilizers with the natural and organic fertilizers and manures will help to restore soil health.
The erosion of topsoil, which reduces the amount of carbon in the soil, can be controlled by maintaining the ground cover. In addition, growing cover crops like eucalyptus can reduce the wash away of topsoil.
Excessive irrigation can deteriorate soil health. So the amount of water supplied to the plants should be according to their needs, not more, not less.
Another method of increasing carbon storage is by growing high-yield, high-biomass crops.
The amount of carbon in the soil will increase if the crop frequency of a place is maximum.

In Conclusion

SOC is an essential component of soil with crucial effects on the functioning of terrestrial ecosystems. Storage of soil organic carbon results from interactions among the dynamic ecological processes of photosynthesis, decomposition, and soil respiration. Over the last 150 years, human activities have led to changes in these processes and, consequently, to the depletion of SOC and the worsening of global climate change. But these human activities also now provide an opportunity for sequestering carbon back into the soil. Future warming and elevated CO2, patterns of past land use, land management strategies, and the physical heterogeneity of landscapes are expected to produce complex designs of SOC capacity in soil.
If you are looking for soil carbon measurement, SoilOptix® can help. Visit for a detailed consultation.