What is Soil Organic Carbon?

What is Soil Organic Carbon?

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Soil Organic Carbon is one of the most important biological components in the soil. In this lesson of the Soil Organic Carbon Course we discuss what soil organic carbon actually is and the components of it.

First watch the video and then read the supplementary material for more information. 

Key Lessons:

  • Soil is made from the combination of weathered rock and decomposed organic matter.
  • Soil Organic Matter is decomposed material that was once living. 
  • There are five types of soil carbon, they are Soluble Organic Matter, Living Organisms, Fresh Residue, Humus and Resistant Organic Matter.  

What is Soil?

Before learning about soil organic carbon, it is important to understand what soil actually is. All soil starts as some type of parent rock material. Parent rock can include:

  • Sedimentary rock: Rock that forms when sediment is compressed under high pressures, typically under lakes and oceans. These rocks are lifted to the earth’s surface with uplift forming dry land. Some examples of sedimentary rocks include sandstone, limestone and shale.
  • Metamorphic rock: These rocks start as other types of rock but have physically and chemically changed when these rocks are placed under high temperatures and pressures. Due to the extreme conditions, the new rock is completely different to the original rock. Examples of Metamorphic rock include marble and slate.
  • Igneous Rock: These rocks come from crystallised and solidified molten rock. This can either occur on the earth’s surface or near the earth’s surface and then rise towards the surface. Some examples include: basalt and granite. 

Over time, these rock weather so that every small particles come off the rock, this forms our soil particles (Clay, silt and sand). Weathering can be caused from physical weathering, chemical weathering or biological weathering. The collection of these individual particles form the mineral component of our soil. 

The mineral component of the soil makes up about 45% of soil volume, while water and air make up a combine 50%. This is referenced to as the pore space. The remaining 5% volume is soil organic matter.

Soil Organic Matter is important for building stable aggregates such as illustrated in the diagram below. Soil should contain areas of pore space, minerals, roots, organic matter and microbes.

Soil Aggregate

What is Soil Organic Matter?

Soil Organic Matter (SOM) is define as any once-living material that has decomposed. Soil Organic Matter includes all elements within that material including Nitrogen, Sulfur, Oxygen, Hydrogen and much more. Soil Organic Matter can be separated into the following groups:

Types of Soil Organic Carbon

Dissolved Organic Matter (5% of SOM)

This refers to the fraction of organic carbon compounds that are dissolved in the soil solution. It consists of a diverse array of organic molecules, including sugars, amino acids, organic acids, proteins, and humic substances, among others. DOM originates primarily from the decomposition of plant and animal residues, microbial activity, root exudation, and organic inputs.

Dissolved Organic Matter plays a crucial role in the soil by serving as a source of energy and nutrients for microbial communities, supporting microbial growth, metabolism, and organic matter turnover. Dissolved Organic Matter has a very vapid turn over ranging from a few minutes to days meaning that it needs to be re-supplied very few days. This is way it is important to always have a growing root in the soil to ensure that root exudates are recharging this fraction of the soil carbon stock.

Fresh Residue (10% of SOM)

Fresh residue refers to recently added or partially decomposed organic materials derived from plant and animal residues. These residues consist of recognisable plant or animal tissues, such as leaves, stems, roots, and animal manure, that have undergone limited decomposition since their addition to the soil.

Fresh residue plays a crucial role in supplying organic carbon and nutrients to soil microorganisms, fueling the initial stages of decomposition and nutrient cycling processes. As fresh residues decompose, they release labile organic compounds, such as sugars, amino acids, and simple carbohydrates, which serve as energy sources for soil microbes and drive microbial activity and growth.

The decomposition of fresh residues is facilitated by various soil organisms, including bacteria, fungi, protozoa, and soil macrofauna, which break down complex organic molecules into simpler compounds through enzymatic reactions and metabolic processes. As decomposition progresses, fresh residues transition into particulate organic matter, contributing to the buildup of soil organic carbon and the formation of soil aggregates. Fresh residues are continually added to soil through various natural processes, such as litterfall, root turnover, and animal activity, as well as agricultural practices, including crop residues, cover crops, and organic amendments. 

Fresh Residue can have a turnover ranging from 2 years for high nitrogen containing residue (such as legumes or manure), and potentially up to 50 years for high carbon containing residues such as hardwood.

Living Organisms (10% of SOM)

Living organisms refer to a diverse array of microscopic life forms that inhabit soil ecosystems. These organisms play integral roles in organic matter decomposition, nutrient cycling, soil fertility, and ecosystem functioning. The living organisms present in soil organic matter include:

  1. Bacteria: Bacteria are ubiquitous in soil and are the most numerous and diverse group of microorganisms. They play essential roles in nutrient cycling, organic matter decomposition, nitrogen fixation, and disease suppression. Certain bacterial species form symbiotic relationships with plants, aiding in nutrient uptake and plant growth.

  2. Fungi: Fungi are another major group of soil microorganisms, including both saprophytic fungi, which decompose organic matter, and mycorrhizal fungi, which form mutualistic associations with plant roots. Mycorrhizal fungi facilitate nutrient uptake by plants, particularly phosphorus, in exchange for carbon compounds from the host plant. Fungi also contribute to soil aggregation and structure formation.

  3. Archaea: Archaea are single-celled microorganisms that inhabit various environments, including soil. They participate in nitrogen cycling, methane oxidation, and other biogeochemical processes, contributing to soil fertility and ecosystem stability.

  4. Protozoa: Protozoa are microscopic, single-celled organisms that feed on bacteria, fungi, and other organic matter in soil. They play roles in nutrient cycling, microbial predation, and soil food web dynamics.

  5. Nematodes: Nematodes are small, multicellular animals that inhabit soil and play diverse ecological roles. Some nematodes are beneficial predators of plant-parasitic nematodes and other soil pests, while others are decomposers or feed on bacteria and fungi.

High Soil Organic Carbon soil

Soil Carbon Comparison

This is a great example of the effects of soil organic matter on soil characteristics. These two soil profiles were taken on neighbouring farms with the same soil type and climate, except that one farmer was using regenerative practices while the other wasn't. On the left side is a regenerative soil with 3.79% organic matter while the right side has only 2.24% organic matter. Notice how dark and aggregated the soil is further into the profile. Moreover, roots can be seen at the very bottom of the profile. The soil on the left is building soil organic carbon throughout the profile while the soil on the right isn't.

Humus (60% of SOM)

Humus is a complex, amorphous organic substance that forms a significant component of soil organic matter. It is the end product of the decomposition of plant and animal residues by soil microorganisms, undergoing a process known as humification. Humus is characterized by its dark brown to black color, spongy texture, and high carbon content.

Key characteristics of humus include:

  1. Stability: Humus is relatively stable and resistant to further decomposition, persisting in soil for extended periods, ranging from decades to centuries. Its stability is attributed to the formation of complex, recalcitrant organic compounds through microbial and chemical processes.

  2. Chemical Complexity: Humus consists of a diverse array of organic molecules, including humic substances, fulvic acids, and humin. These compounds vary in molecular size, structure, and solubility, influencing their interactions with soil minerals, nutrients, and microorganisms.

  3. Nutrient Retention and Release: Humus has a high cation exchange capacity (CEC), allowing it to adsorb and retain essential nutrients, such as calcium, magnesium, potassium, and trace elements. Additionally, humus serves as a reservoir of nitrogen, phosphorus, and sulfur, gradually releasing these nutrients through mineralization and microbial activity, thereby supporting plant growth and soil fertility.

  4. Soil Structure Improvement: Humus plays a vital role in soil aggregation, stabilization, and structure formation. It acts as a binding agent, promoting the formation of stable soil aggregates and pore spaces, which enhances soil porosity, water infiltration, and root penetration. Humus also improves soil aeration, drainage, and resistance to compaction.

  5. Biological Activity: Humus provides habitat and substrate for diverse soil organisms, including bacteria, fungi, protozoa, and earthworms. These organisms interact with humus, contributing to its decomposition, nutrient cycling, and microbial biomass production. Humus also supports symbiotic relationships between soil microorganisms and plant roots, facilitating nutrient uptake and plant growth.

Humus is considered the most biologically active and beneficial component of soil organic matter, exerting profound effects on soil fertility, ecosystem functioning, and environmental sustainability. Typically humus has a turnover ranging from 10s of years to 100s of years.

Resistant Organic Matter (15% of SOM)

Resistant organic matter, also known as refractory organic matter, is a fraction of soil organic matter that exhibits high resistance to decomposition and persists in soil over long periods, ranging from centuries to millennia. Unlike more labile organic compounds, resistant organic matter undergoes minimal microbial degradation and remains relatively stable under various environmental conditions.

Key characteristics of resistant organic matter include:

  1. Chemical Stability: Resistant organic matter consists of complex, recalcitrant organic compounds that are highly resistant to microbial and enzymatic degradation. These compounds are often rich in aromatic structures, such as lignin, and have undergone substantial chemical alterations, rendering them less accessible to microbial attack.

  2. Physical Protection: Resistant organic matter may become physically protected within soil aggregates, mineral surfaces, or organo-mineral complexes, where it is shielded from microbial activity and decomposition. This physical protection prevents the rapid breakdown of organic compounds and contributes to the long-term persistence of resistant organic matter in soil.

  3. Environmental Conditions: Resistant organic matter is influenced by environmental factors, such as soil texture, pH, moisture, temperature, and redox conditions. These factors can affect the rate and extent of decomposition, with certain environments promoting the preservation of resistant organic matter over extended periods.

  4. Sources: Resistant organic matter originates from a variety of sources, including woody plant tissues, lignified residues, charcoal, and microbial by-products. These materials contain complex organic compounds that resist degradation due to their chemical composition and physical characteristics.

  5. Importance: Resistant organic matter contributes to soil carbon sequestration, long-term soil fertility, and carbon cycling in terrestrial ecosystems. While it represents a relatively small fraction of total soil organic matter, resistant organic matter plays a crucial role in maintaining soil structure, water retention, and nutrient availability over geological timescales.

Particulate, Labile and Stable Soil Organic Matter

Particulate Organic Matter includes any organic matter that has an identifiable structure, this includes Fresh Residue and Living Organisms.

Labile Organic matter includes any organic matter that is useable as an energy source for microbes, this is also called active carbon. Microbes are able to use this organic matter as a food source for activity, growth and development. This includes particulate Organic matter as well as dissolved organic matter.

Stable Organic Matter is highly resistant to microbial decay and oxidations and therefore does not undergo further decomposition. This organic matter is important for general soil fertility and includes resistant organic matter and humus.


Humus is a complex, amorphous organic substance that forms a significant component of soil organic matter. It is produced through the process of humification where mycorrhizal fungi convert root exudates into these long carbon complexes. Humus is characterized by its dark brown to black color, spongy texture, and high carbon content.

Humus is of paramount importance in soil and ecosystem health for several reasons:

  1. Soil Fertility: Humus serves as a reservoir of essential nutrients, including nitrogen, phosphorus, potassium, sulfur, and micronutrients. It acts as a source of slow-release nutrients, gradually releasing them as it decomposes, thereby promoting plant growth and productivity. Humus also enhances the cation exchange capacity (CEC) of soils, allowing them to retain and exchange nutrients more effectively.

  2. Soil Structure and Aggregation: Humus plays a crucial role in soil aggregation and structure formation. It acts as a binding agent, promoting the formation of stable soil aggregates and pore spaces. This improves soil porosity, water infiltration, and root penetration, while reducing soil erosion, compaction, and surface runoff. Humus also contributes to soil stability and resistance to degradation, enhancing soil fertility and productivity.

  3. Water Retention and Drainage: Humus improves soil water retention and drainage by enhancing soil structure and increasing water-holding capacity. It helps to maintain optimal soil moisture levels for plant growth, reducing the risk of drought stress and waterlogging. Humus also reduces surface crusting and compaction, allowing water to penetrate the soil more easily and reducing runoff and erosion.

  4. Nutrient Cycling: Humus plays a crucial role in nutrient cycling and organic matter turnover within ecosystems. It serves as a substrate for soil microorganisms, providing energy and nutrients for microbial growth and activity. Microorganisms decompose humus, releasing nutrients into the soil, which are then taken up by plants and recycled through the ecosystem. This cycle of nutrient uptake, decomposition, and recycling is essential for maintaining soil fertility and ecosystem productivity.

  5. Carbon Sequestration: Humus is a significant reservoir of organic carbon in terrestrial ecosystems, contributing to carbon sequestration and climate change mitigation. By storing carbon in stable organic compounds, humus helps to mitigate the accumulation of atmospheric carbon dioxide, a major greenhouse gas. Increasing soil organic carbon levels through the accumulation of humus can enhance soil carbon sequestration and contribute to climate change adaptation and mitigation efforts.

Humus can be separated into three different groups depending on solubility, these are call humic substances:

Humic Acid: Humic acid is the fraction of humus that is insoluble in acidic solutions (pH < 2) but soluble in alkaline solutions (pH > 2). It consists of large, high-molecular-weight molecules with complex structures rich in carbon, hydrogen, oxygen, and nitrogen. Humic acid contributes to soil fertility, cation exchange capacity, and nutrient retention.

Fulvic Acid: Fulvic acid is the fraction of humus that is soluble in both acidic and alkaline solutions. It consists of smaller, lower-molecular-weight molecules compared to humic acid and is characterized by its high solubility and mobility in soil. Fulvic acid plays essential roles in nutrient transport, chelation, and plant uptake, as well as soil aggregation and structure improvement.

Humin: Humin is the fraction of humus that is insoluble in both acidic and alkaline solutions. It comprises the most stable and recalcitrant organic compounds in humus, including highly polymerized materials such as lignin and melanin. Humin contributes to soil aggregation, water retention, and carbon storage, providing long-term stability to soil organic matter.

humic substances

Humates: Humates are humic substances that have bound with a mineral such as iron, copper, zinc, calcium, manganese, magnesium, potassium and more. Typically these bind at the COOH or COH sites on the humic substance. 

Humates have several beneficial effects on soil and plant health, including:

  • Enhanced Soil Structure: Humates improve soil aggregation, porosity, and water retention, leading to better soil structure and aeration.

  • Increased Nutrient Availability: Humates chelate and complex essential nutrients, such as nitrogen, phosphorus, potassium, and micronutrients, making them more available to plants.

  • Stimulated Plant Growth: Humates promote root growth, nutrient uptake, and overall plant growth and development, resulting in healthier and more vigorous plants.

  • Improved Stress Tolerance: Humates enhance plant resistance to abiotic stressors, such as drought, salinity, heat, and cold, by improving plant water relations and physiological processes.

  • Enhanced Microbial Activity: Humates stimulate beneficial soil microbial activity, including the proliferation of beneficial bacteria and fungi, which contribute to nutrient cycling, organic matter decomposition, and disease suppression.

What to see more of the SOC Course?

Currently we are converting the second edition of the course to an updated third edition (such as the section you just read) which has more information, better graphics and a superior format , to view the second edition for the next lessons follow this link: Second Edition Soil Organic Carbon Course

Learn More!

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