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By David Okul

The implementation of international carbon trading has made the measurement of carbon important to the global economy. In pastoral rangelands, no meaningful way of measuring carbon has been identified. Common carbon sinks include oceans, soils, and vegetation.

Soil organic carbon is an indicator of healthy and fertile soils. Carbon is the 15th most abundant element on the earth’s crust, and fourth in the universe (after hydrogen, helium, and oxygen). On the human body, it is only second to oxygen.

Massive amounts of carbon are stored in the soil in two general forms: inorganic and organic. Inorganic carbon arises from the parent weathered rock, while organic carbon results from living or decomposing carbon compounds. However, the measure of productive soils can be captured from the soil organic carbon content.

Changes in Soil Organic Carbon take a few years and it is difficult to keep track of the trend. Regardless, frequent monitoring using a consistent methodology may help detect changes.

Why should we measure soil organic carbon?

Carbon is perhaps the most important parameter for healthy soil. Keep in mind that plants don’t absorb carbon from the soil but the atmosphere. Instead, the carbon in soil contributes to soil nutrients by adsorbing and desorbing nutrients and providing habitats for microorganisms. In short, measuring SOC helps in assessing the soil’s health. Other reasons for measuring soil organic carbon are:

  1. It is an indicator of the water holding capacity of soils: More SOC means higher water holding capacity
  2. Fight against climate change: As the second most important sink for carbon (after oceans), measuring SOC would help in monitoring the absorption capacity of soils.

Analytical tests for determining total soil organic carbon in soils

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 test and dry combustion.

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 calcinations 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. The soil sample (often 30mg) is placed in a capsule and combusted at 975℃. It uses an automatic analyzer with a thermo conductivity sensor detector (TCD).

Key discussion points

Some of the takeaway points of the different methods of measuring SOC are:

  • Walkley and Black Method is relatively easy and cheap, but inaccurate
  • Generally, wet digestion methods tend to underestimate carbon stocks while gravimetric methods tend to overestimate.
  • Additionally, wet digestion methods use potentially dangerous chemicals such as chromium which could be toxic.
  • The most common measurement of SOC is the ignition test of soil organic matter. However, it is not an accurate measure as carbon makes between 50 and 58% of the soil organic matter.
  • Most organizations (including IPCC) favor dry combustion over the other methods of measuring soil organic carbon
  • Indirect methods of measuring soil organic carbon take less time but are less accurate.

Additional reading

Quantification of Soil Organic Carbon – Comparison of Wet Oxidation and Dry Combustion Methods

A protocol for measurement, monitoring, reporting, and verification of soil organic carbon in agricultural landscapes

David Okul is an environmental management professional with over 10 years experience on donor projects, conservation, forestry, ecotourism, and community-based natural resources management. When not working on  environmental projects, I spend my time writing for Silvica on a variety of topics.

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