GeneralSoilSoil CompositionStructure


Carbon cycle is one of the fundamental requirement of life on earth. Soil organic carbon (SOC) can be described as the amount of carbon that is stored in the soil as one of the components of organic matter which comprises the animal and plant materials and different stages of decay. Organic carbon (OC) mainly enters the soil by the decomposition of the animal and plant residuals, dead and living microorganisms, root exudates and soil biota. Soil organic carbon is heterogeneous structure that varies in it particle size, carbon content, turnover time and its decomposition rate. Soil organic carbon is the main energy source for the soil microorganisms. The organic matter in the soil has approximately 58% carbon.

Organic carbon stored in soil can be increased by the following practices

The processes that can be used to increase the amount of soil carbon in the soil include; things like increasing plant growth which generally increases the input of organic carbon to the soil in roots, shoot material and root exudates example is the increasing water use efficiency and optimal nutrition. Also growing plants for longer durations each year increases the inputs of the organic carbon to the soil. Also by improving the soil structure it can also help to increase the organic carbon stored in the soil by reducing the losses of organic carbon from the soil by erosion and decomposition and this can be done by maintaining the ground cover, retaining stubbles and reduction of compaction. In addition practices like conservation farming, improving crop management through rotation, maintaining and improving forestry management and also improving grazing management can help to increase the organic carbon in the soil.

Soil carbon and soil health

Carbon content in the soil is a major factor of the overall soil health. Soil carbon is meant to improve the physical properties of soil like increasing the cation exchange capacity (CEC). It also increases the water retention capacity of sandy soils and helps to enhance the structural stability of the clay soils where the carbon helps to bind the particles into aggregates.

Systems of the PRI Zaytuna Farm, Ingrid Pullen Photography.
Systems of the PRI Zaytuna Farm, Ingrid Pullen Photography.

The soil organic matter (SOM) in which carbon takes the larger part holds a greater percentage of the nutrient cations and the trace elements that are of major importance for plant growth. Soil carbon also prevents the leaching of the nutrients and is of major importance in the integral to the organic acids that make minerals readily available to the plants. Also helps to buffer the soil from high pH changes.

Losses of soil carbon

Carbon exchange between the soil and the atmosphere is of great significance in the carbon cycle. A number of factors affect the variation that majorly exists between the soil carbon and soil organic matter which are influenced by humans and agricultural systems. Loss of the soil organic carbon has been caused by a series human activities like the use of fire, which acts by removing the soil cover leading to immediate and continuing losses of the soil organic carbon. Tilling of the land and drainage also exposes the soil organic matter both to the oxygen and oxidation. Poor soil organic carbon leads to reduced microbial biomass and nutrient mineralization due to shortage of the energy sources. Scarce soil organic carbon can lead to less diversity in the soil biota causing a risk of the food chain equilibrium disruption and this causes disturbance in the soil environment.

Some practices can be carried out to reduce the losses of carbon in the soil that include practices like no-till can increase the soil organic carbon and are illustrated in figure one below. Other practices like the application of compost and manure can also help to increase the soil organic carbon.


Importance of carbon in the soil

Soil organic carbon is the key factor of the soil fertility by releasing the nutrients for the plant growth, promotes structure, physical and biological health of the soil.

Carbon also helps to manufacture the organic matter in the plants by use of the carbon dioxide from the air and water. Plants die and are decomposed and recycled in the soil. Minerals get released to the soil and carbon dioxide is released into the atmosphere. Soil organic carbon can vary depending on the soil and landscape type and hence it can also lead to the climate change over some period of time and methods of farming. Increase in the soil organic carbon helps improve crop and pasture yields.

If a notable quantity of carbon is stored in the soil as organic carbon hence it reduces the amount present in the atmosphere and this helps to reduce global warming. This process of storing carbon in the soil is called soil carbon sequestration.

Soil carbon sequestration

Soil carbon sequestration is a process by which carbon dioxide removed from the atmosphere is stored in the soil carbon pool. The process is primarily mediated by plants through photosynthesis, with carbon stored in the form of soil organic carbon. Mostly In arid and semi-arid climates soil carbon sequestration can also occur from the conversion of carbon dioxide from air found in the soil into inorganic forms such as secondary carbonates. The natural ecosystems to agriculturalist has resulted in the depletion of soil organic carbon levels through reductions in the amount of plant roots and residues returns to the soil. This helps to increase decomposition from the soil tillage and also in soil erosion. Therefore some factors may impact potential soil carbon example change in environment including current land management strategies, historic land use patterns, climatic controls and also the topographic heterogeneity. These factors are demonstrated as indicated in the figure below to show how the landscape has effects on the soil carbon. These combined effects of change in the carbon inputs and losses results in the variation in the carbon sequestration capacity across landscapes. As indicated in figure 2 below;


Some land managements have been known to have different effects on the addition or reduction of the soil organic carbon. These changes in soil carbon may take some years to occur, making actual measurements of changes in soil organic carbon stocks difficult.


Table 1: possible management practices for increasing soil organic carbon levels through reduces carbon losses and increased carbon inputs in agricultural systems.

Fundamentals of soil organic carbon

Soil organic matter is a heterogeneous mixture composed of materials that range in the stage of decomposition from fresh residuals of plant to the highly decomposed materials which are called humus. Soil organic carbon (SOC) levels directly relate to the amount of organic matter that is contained in the soil and therefore soil organic carbon is often the measures of organic matter in the soil. The soil organic matter results as a consequence of the interaction of a number of ecosystem processes which include respiration, photosynthesis and decomposition. Each of this process impacts differently on the soil organic carbon where photosynthesis is the fixation of the atmospheric carbon dioxide into the plant biomass.

The input rates are determined by the root biomass of the plant and also inclusive of the litter deposited from plant shoots. The soil carbon results by growth and death of the plant roots and indirectly from the transfer of the compounds which are enriched with carbon from roots to soil microbes. The process of decomposition of the biomass by the soil microbes results in carbon loss. This is because carbon dioxide from the soil is utilized by microbial respiration while a small portion of the original carbon is majorly retained in the soil through the formation of humus. Humus is a product that gives the carbon rich soils the characteristic of a dark color.

Various forms of the soil organic carbon differ in their resistance to decomposition, humus being much recalcitrant and their resistance to breakdown leads to a longer residence time in the soil. Plant leftover are less rebellious resulting in a much shorter residence time in soil. When carbon inputs from photosynthesis exceed Carbon losses soil organic carbon levels increase over time.

The figure below illustrates some of the processes that take place as carbon and the soil relate;


Figure 2: Carbon balance within the soil (brown box) is controlled by carbon losses by respiration and carbon inputs from photosynthesis

All these factors of respiration, decomposition, and photosynthesis rates are in a way determined by climatic factors mainly moisture levels and soil temperature. Also, the processes of deposition and soil erosion acts to redistribute the soil carbon according to the topography of the landscape, where the low-lying areas have an increased soil organic carbon relative to the upslope positions.

In conclusion, soil organic carbon is important in the soil with significant effects on the functioning of terrestrial ecosystem. Human should carry out practices that promote the soil organic carbon increase in the soil so as to benefit from these healthy relationships between the soil and carbon.

Feature Image by Ingrid Pullen.

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