What Is Soil Carbon?

Soil carbon is a fundamental component of healthy, functioning soils and plays a central role in agricultural productivity and climate regulation. It exists in different forms within the soil, each with distinct properties and behaviour.

Forms of Carbon in Soil

Carbon in soil exists in two main forms, and the balance between them varies depending on soil type and mineral composition.

Soil organic carbon, commonly referred to as SOC, is derived from living and once living material. It includes plant roots, decomposing plant and animal residues, fungi and other micro organisms. In UK soils, SOC accounts for around 60 per cent of total soil carbon.

Soil inorganic carbon, or SIC, is found in mineral forms such as chalk and limestone, primarily as calcium carbonate. This form makes up approximately 40 per cent of carbon in UK soils. Through the natural carbon cycle, carbon is continuously exchanged between soil organic carbon and the atmosphere. In contrast, soil inorganic carbon is generally more stable and less actively involved in short term biological processes.

Soil Properties and the Role of Organic Matter

Soil organic carbon has a profound influence on soil function. It affects soil structure, aggregate stability, aeration, water infiltration and retention, as well as nutrient cycling and availability. These properties directly influence crop performance and resilience to drought or waterlogging, both of which are becoming more frequent under a changing climate.

Many agricultural soils that have been intensively cultivated for decades contain lower levels of soil organic carbon than they would under undisturbed conditions. However, SOC can be increased gradually through sustained additions of organic matter. Crop residues, cover crops and organic amendments such as manures and composts all contribute to rebuilding soil carbon over time. This is typically a slow process that occurs over years or decades rather than seasons.

Particulate and Mineral Associated Organic Matter

Soil organic matter, which contains soil organic carbon, is complex but can broadly be divided into two components that differ in how they form and how long they persist.

Particulate organic matter, known as POM, consists of relatively fresh plant and animal residues that are less protected from microbial breakdown. As a result, POM may only remain in the soil for a few years.

Mineral associated organic matter, or MAOM, forms when organic compounds bind to soil minerals. This association slows further decomposition and can stabilise carbon in the soil for many decades. POM generally has undergone less microbial processing than MAOM and typically has a different carbon to nitrogen ratio.

Different crop types and management practices can influence the balance between these components. Legumes, for example, may promote the formation of mineral associated organic matter through interactions with soil microbial communities. In practice, however, maintaining a diversity of crop types within rotations may offer the most reliable route to supporting both short term and long term soil carbon storage.

Measuring Soil Carbon

There are two main laboratory methods used to assess soil organic carbon or soil organic matter. Whichever method is chosen, consistency is essential when monitoring change over time or comparing fields. Representative soil sampling is critical in all cases.

The Dumas method is considered the standard approach for analysing soil organic carbon. It provides a direct and accurate measure of total carbon content. Laboratories must account for soil inorganic carbon when using this method, particularly in soils containing chalk or limestone.

Loss on Ignition is widely used to measure soil organic matter. It is often considered useful for assessing soil health and the impact of management practices. However, results can vary between laboratories, so changing testing providers from year to year should be avoided. Soil organic carbon values can be derived from soil organic matter measurements.

Sampling depth is another important consideration. Soils are frequently sampled to 15 centimetres or at most 30 centimetres when assessing SOC or SOM. However, between 30 and 60 per cent of soil carbon can exist deeper in the root zone. Ideally, sampling should extend to 60 to 75 centimetres to provide a more complete picture of total carbon stocks.

Changes in soil organic carbon occur slowly and are difficult to detect in the short term. Reliable quantification often requires long term monitoring over decades rather than years. Establishing a clear baseline measurement before changing cropping systems or management practices is essential in order to assess progress accurately.

Sequestration of Carbon in Soil

Where soil organic carbon levels have been depleted, appropriate management practices can lead to gradual increases and therefore enhanced carbon capture. The rate of increase typically slows as SOC rises, eventually reaching an equilibrium point at which annual carbon additions are balanced by losses.

The time required to reach this equilibrium, and the level at which it stabilises, depends on land use, climate, soil type and management. If beneficial practices are not maintained, SOC levels can begin to decline again, resulting in a net loss of previously stored carbon.

Because soil organic carbon is constantly turning over through biological processes, demonstrating permanent long term sequestration can be challenging. Nevertheless, increasing soil carbon remains an important objective. While it is not a single solution to achieving Net Zero, its contribution to soil health, resilience and productivity makes it a valuable component of sustainable agricultural systems.