Soil carbon: the soul of the soil

A farmer measures a harvest in kilograms or quintals, fertiliser in bags and irrigation in hours. Soil organic carbon usually arrives as a small percentage on a laboratory report: 0.30%, 0.50% or 0.80%. Understanding that number can change how we think about the land beneath a crop.

I call soil carbon the soul of the soil because of its connection with the living processes that support agriculture. Organic matter helps sustain microbial life, nutrient cycling and soil structure. Carbon is a major component of that organic matter. Protecting it deserves a place alongside the attention we give to crop nutrition and yield.^{1, 2}

To turn this idea into practical action, we must understand what carbon means, how much is present and whether our management is helping to maintain it.

Two forms of carbon in soil

Soil contains two broad forms of carbon. Soil organic carbon, or SOC, is carbon held in organic matter derived from living organisms and their remains. It includes carbon in microorganisms, decomposing residues and material that has become more persistent in the soil. Soil inorganic carbon occurs in minerals such as calcium carbonate. A total carbon result can include both, so it must not automatically be read as organic carbon.^{1, 9}

This article focuses on organic carbon. It is also different from total organic matter: organic matter contains carbon along with oxygen, hydrogen, nitrogen and other elements. The two percentages are not interchangeable.

Where soil carbon comes from

Plants obtain most of their carbon from atmospheric carbon dioxide through photosynthesis. Roots, root secretions and crop residues help transfer some of it into the soil; manure and compost can supply additional organic material. Microorganisms use and transform these materials, releasing some carbon back to the atmosphere and contributing to forms that remain in the soil. Not all soil carbon is equally accessible to microbes.^{3, 8}

What 0.50% organic carbon means

A laboratory result of 0.50% organic carbon means 5 grams of organic carbon in every kilogram of dry soil. The percentage describes a proportion by mass. It does not mean 0.50 kilograms per acre, and it does not describe the moisture in the sample.

The calculation is straightforward:

SOC (%) = (organic carbon mass ÷ dry soil mass) × 100

(0.50 ÷ 100) × 1,000 g = 5 g of organic carbon

If a report uses grams per kilogram instead of a percentage, 5 g/kg equals 0.50%; 10 g/kg equals 1.00%. Check that the report identifies organic carbon and states the analytical method.⁴

In simple words: read the OC figure on your report as grams of carbon in every kilogram of dry soil. 0.50% means 5 g per kg, and 1.00% means 10 g per kg.

From a percentage to carbon per acre

A percentage tells us the concentration. To estimate the stock, or quantity of carbon in a field, we also need the area, sampling depth and dry soil bulk density. Bulk density is the dry mass of soil occupying a known volume in the field, including its pore spaces. Stones and gravel require an appropriate correction.^{5, 6}

Consider an illustrative one-acre field. Assume an area of approximately 4,047 square metres, a soil layer from the surface to 15 centimetres, and a dry bulk density of 1,300 kilograms per cubic metre, equivalent to 1.3 g/cm³. Assume negligible stones and gravel. These are calculation assumptions, not measurements from a particular farm; 15 centimetres is the depth of this example, not a universal sampling prescription.

First calculate the dry soil mass by multiplying area, depth and bulk density:

4,047 m² × 0.15 m × 1,300 kg/m³ ≈ 789,000 kg

We will use the rounded mass of 789,000 kg of soil, or about 789 tonnes, for the remaining examples. At 0.50% organic carbon:

789,000 kg × (0.50 ÷ 100) = 3,945 kg of organic carbon

Under these assumptions, that small-looking 0.50% represents approximately 3.95 tonnes of organic carbon in the top 15 centimetres of one acre. It is the amount estimated to be present, not a quantity that the farmer must apply.

The following comparison uses the same soil mass and depth throughout. It illustrates quantities, not universal ratings of soil quality.

What a rise from 0.50% to 0.60% means

Now consider a change from 0.50% to 0.60%. The difference is 0.10 percentage points. Relative to the starting concentration, it is a 20% increase:

((0.60 − 0.50) ÷ 0.50) × 100 = 20%

If the same dry soil mass is being compared, the corresponding difference in stored carbon is:

4,734 kg − 3,945 kg = 789 kg of organic carbon

That is approximately 789 kg more organic carbon per acre. This arithmetic shows why apparently small percentage changes can represent substantial quantities. It does not predict how quickly the change can occur, or establish that two laboratory results demonstrate a real gain. Sampling and measurement uncertainty must also be assessed.⁵

In simple words: a rise of 0.10 on the soil report looks tiny, but in the top 15 cm of one acre it is close to 800 kg of carbon. That is why real gains take years, and why one good test result is not proof.

Compost applied is not carbon stored

Compost applied and carbon retained in soil are different quantities. Suppose a farmer applies 1,000 kg of compost. For this example only, assume the compost has 40% moisture by weight and its dry material contains 20% organic carbon.

Dry compost = 1,000 kg × (1 − 0.40) = 600 kg

Carbon supplied = 600 kg × 0.20 = 120 kg

The tonne of compost therefore supplies 120 kg of organic carbon, not one tonne of carbon. The figures must come from the actual material’s analysis when making a farm plan; moisture and carbon content vary between products.

Nor can we assume that all 120 kg will remain as a lasting increase in soil carbon. Decomposition releases some carbon, while roots, residues, erosion and other processes also affect the field’s balance. A compost application rate cannot, by itself, establish a future soil-test percentage.³

The same distinction applies to biological products. Their performance should be evaluated for the function they claim to perform. A microbial inoculant’s package weight, or a greener crop after its use, does not establish how much soil organic carbon has accumulated.

How to take a soil sample for organic carbon

Measurement begins in the field, before the sample reaches the laboratory. A useful starting procedure for routine farm soil testing is:

1. Separate different parts of the farm. Areas with different soil types, slopes, drainage or management histories should be sampled separately.
2. Agree on the depth. Follow the laboratory or agronomist’s guidance for the crop and purpose. Take each subsample through the full specified layer and keep different depth layers separate.
3. Collect representative subsamples. TNAU’s routine guidance recommends at least 10–15 subsamples from a reasonably uniform sampling unit. Remove loose surface litter and avoid unrepresentative spots such as manure heaps and field bunds.
4. Prepare and label the sample properly. Mix the subsamples thoroughly and submit the quantity and packaging requested by the laboratory. Record the field, sampling depth, date and recent management. Follow the laboratory’s instructions for drying and handling.⁷

This routine composite sample can guide farm management. Demonstrating a small change in carbon stock requires a more deliberate sampling design, enough independent samples and an assessment of uncertainty. One composite sample cannot establish field-wide variability.⁸

What to ask the laboratory

Ask the laboratory specifically for soil organic carbon, reported on a dry soil basis. The Walkley–Black method estimates oxidisable organic carbon through a chemical reaction; recovery is incomplete and results depend on the procedure and correction used. Dry combustion measures carbon with an instrument, but inorganic carbon must be accounted for when carbonates are present. Results from different methods should not be compared as if they were automatically equivalent.^{4, 9}

If the objective is kilograms or tonnes of carbon per acre, arrange a separate bulk-density measurement. For example, if an intact core with a volume of 100 cm³ contains 130 g of oven-dry soil, with negligible stones:

Bulk density = 130 g ÷ 100 cm³ = 1.3 g/cm³

This is the density assumed in our acre calculation. A loose bag of disturbed soil cannot reproduce the original field volume simply by being poured into a container. The laboratory or a trained technician should collect suitable intact cores.⁶

Tracking change over the years

Repeat measurements at comparable locations, at the same depths and in the same season, using consistent laboratory methods. Record changes in tillage, irrigation, residue management and organic inputs. Where management alters bulk density, comparing carbon at the same depth may compare different quantities of soil. A specialist may need to adjust the comparison to an equivalent soil mass, so that the change reflects carbon rather than a changed mass of soil being counted.^{5, 8}

Soil colour, smell and earthworm activity can be useful observations, but they cannot assign a reliable organic carbon percentage. Nor does one percentage describe every aspect of soil health. Nutrient balance, salinity, acidity, compaction and crop performance also need attention. Targets should reflect local soils and farming conditions.¹

A responsibility we share

As an agricultural entrepreneur, I believe the farming community should make these numbers easier to understand and use. Farmers need dependable tests and practical advice. Scientists and laboratories must explain what results mean. Input suppliers should connect their claims to measurable evidence, while farmer organisations and public institutions can help make testing and suitable management affordable.

The next step is a field-specific plan: protect soil from erosion, return suitable organic materials where feasible, maintain living roots when water and the cropping system allow, and coordinate these practices with balanced nutrition. Review costs and crop performance as well as carbon. The objective is sustained improvement that the farmer can continue.^{1, 3}

Calling soil carbon the soul of the soil should lead us towards this responsibility. Measure it carefully, understand what the figures represent and give the biological processes beneath every crop the attention they deserve.

Editor's note: the summary box, subheadings, "In simple words" boxes and links were added by NanoKheti's editors, and the worked calculations are set out as separate lines. The article text is Amit Nalawade's.