Soil is only ever really soil when it’s teeming with organic components. Otherwise, it’s just dirt. And when you compare the land and fertile soil, the rich colour of healthy soils is a primary indicator of health.
What imparts this darker hue is the organic matter in the soil. Soil organic matter is derived from decomposing and already decomposed plant and animal material, as well as excretions and soil microbes, that form a reliable mixture with soil. When soil is rich in organic matter, it holds nutrients like nitrogen, phosphorus, and carbon.
Soil carbon or soil organic carbon is the fraction of the element carbon found in soil organic matter. It is the most abundant element, estimated that the carbon content in organic soil matter is about 58%.
Fig: 1: Dark-colored topsoil showing high levels of SOC due to abundant plant roots and their associated soil fauna and microbes in a cultivated soil
Carbon impacts the chemical, physical, and biological properties of the soil. And because soils store large amounts of carbon, this ecosystem service is seen as one of the ways to limit global warming; by turning the soil’s natural capacity to hold carbon through soil carbon sequestration.
How does organic matter enter the soil?
Organic matter enters the soil in a process called “Soil Organic Matter (SOM) cycling.” Each component of SOM turns over at a vastly different rate than the others due to climate, soil type, and management.
SOM cycles move continuously between living, decomposing, and durable components in the soil. The cycles fall into four categories:
Inputs: Animals and plants become part of SOM by creating by-products or through death (dissolved organic matter).
Transformation: Soil organisms break up and consume organic matter, creating smaller organic residues and materials (particulate organic matter).
Nutrient Release: Whatever nutrients and other compounds microorganisms don’t use are released and made available to plants (humus).
Stabilizing Organic Matter: As organic residues decompose, they become more resistant to further change (resistant organic matter).
Carbon Pools and Cycling
Cycling organic carbon is critical for sustaining the earth’s healthy ecosystems and maintaining food security. The number of fresh organic materials added yearly to the soil directly impacts the total amount of SOC.
Carbon exists in soils in various “pools,” categorizing anything from freshly deposited plant residues to smaller structures absorbed in clay and soil aggregates. The most stable carbon product attributed to residue decomposition is humus.
Unfortunately, agriculture practices often negatively impact carbon pools by depleting plant residues, mainly organic carbon and humus components. Carbon losses from agricultural systems are a significant cause of global warming and the loss of natural ecosystems.
Figure 2: Carbon balance within the soil is controlled by carbon inputs from photosynthesis and carbon losses by respiration.
Soil organic carbon in the present-day context
Soil organic carbon content is a crucial indicator of soil health. It supports ecosystem processes such as water storage capacity and availability to plants, nutrient and waste cycling, soil structure and biodiversity. Healthy soil underpins food and fiber production. Soil carbon is highly complex, and depending on how land is managed, it can be a significant source or sink for greenhouse gases.
Through natural processes over time, soils have been depleted of carbon and are poor in nutrients. Land clearing for agriculture has been the primary driver of soil carbon loss since European settlement. Typically, converting native vegetation to agriculture reduces soil organic carbon by 20–70 per cent. It has been projected that approximately half of the original soil carbon has been lost in agricultural soils. Uncertainty remains, especially, over the potential of agricultural soils to store carbon, the rate of carbon accumulation in soil and the permanence of captured carbon.
Why is soil carbon important to a farmer?
While soil carbon is present in all agricultural lands, overall soil carbon stock and soil carbon levels have decreased due to natural processes and land management. Soils that have become less productive are often dealt with impermanent fixes in soil amendments.
But how does soil carbon benefit farming?
Farm soils rich in carbon is critical to soil productivity. Carbon-rich soils contribute to yield and long-term sustainability, positively impacting profit.
How does soil carbon affect soil health?
Healthy soils teem with life, soil minerals, and life-nourishing properties crucial for flourishing plants, animals, and microorganisms. The soil’s properties also benefit from beneficial soil carbon, ultimately improving a farm’s productivity.
Below are some of the ways soil carbon contributes to healthy farm soils.
Improves resilience to drought and extreme rainfall
Extreme weather, such as drought, lasts longer and becomes more frequent. This makes the effects on yields harder to control and predict.
When the soil is healthy, water absorption and moisture retention improve. Because organic carbon increases the soil’s water-holding capacity and clay content, it is estimated that soils can hold up to 10-20 times their weight in water.
Soils that are healthy and rich in carbon can effectively absorb rainfall and support crops during droughts. More water can be retained in healthy soils during wet seasons with frequent rain. This feature can act as a water reservoir that plants rely on during dry months or droughts.
Influences nutrient availability and plant uptake
This is where the life found in soils, such as tiny organisms and plants, becomes beneficial for nutrient uptake. The nutrients from decaying matter become a source of nutrition for crops and other life forms in the soil once microbial life gets to them. Microbes must break down nutrients like nitrogen before plants can uptake them.
Healthy soils are rich in organic matter, which releases various nutrients when broken down by microorganisms. Due to microbial processes, essential nutrients such as carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulphur are made available to plants.
Reduces soil density
Compacted and dense soils are indicators and drivers of soil health. An abundance of organic matter provides an excellent environment for all sorts of life to prosper in the soil. As microbes, earthworms, and other organisms move around in the soil to take in carbon as their energy source, these create pockets of space that break up soil density.
Similarly, soil organic matter helps form and maintain the air passages and channels that protect the soil from compaction. Less compact and less dense soil encourages plant root development and penetration. The improved root system of plants indicates better nutrient and water absorption. This aids the soil’s capacity to withstand erosion and other soil disturbance. It also makes the soil more friable, less sticky, and easier to work.
Enhances soil’s capacity to host life
Soil biodiversity boosts the biological and structural functions of soil health. Plant and soil processes contribute to overall soil functioning, but soil microorganisms are equally important. The microbial life in the soil feeds on carbon and organic matter components for energy. And in return, soil microorganisms transform organic matter into available nutrients that the plants and the ground can use.
What is soil carbon sequestration?
Soil carbon sequestration harnesses the capacity of soils to absorb and store carbon to limit the increasing CO2 concentrations in the atmosphere. Soil carbon sequestration comes in many forms and techniques.
In agriculture, soil carbon storage is called carbon farming or regenerative agriculture.
Globally, soils are the most significant carbon reservoirs, and agricultural soils have the potential to sequester more carbon. In the European Union, one estimate suggests that soil carbon storage in croplands has the potential to hold up to 90–120 Mt C per year.
Sequestering carbon from the atmosphere into agricultural lands is a climate mitigation strategy that directly impacts farm productivity. Carbon is a crucial component that makes up healthy soils. And the impacts of healthy soils on a farmer translate to yield improvements, long-term farm productivity, and increased profitability potential.
How to manage soils to increase soil carbon?
Minimizing soil disturbance
Tillage contributes the most to soil disturbance with implications on soil erosion as it also breaks up soil organic matter aggregates. However, tillage is used by many farmers to prepare seeds for cultivation and manage crop residue by breaking up the surface soil.
A best management practice for soil conservation is minimizing till or using no-till completely. No-till practices have been found to have 71% lower greenhouse gas intensity than conventional tillage.
Covering the soil
Soils need protection from the elements. And it makes sense that soil conservation techniques such as using a cover crop and leaving straws in the fields are found beneficial by farmers. Adding organic matter inputs can minimize losses and help prevent the soil from eroding while improving soil structure.
Cultivating crops in rotation
Adopting crop management practices that increase soil carbon inputs helps improve productivity and soil condition. Crop rotation methods such as seasonal cover crops or continuous cropping which reduces fallow frequency are some ways to boost soil organic carbon stock.
How can you increase soil organic carbon?
As soil scientists have proven, the benefits of soil organic carbon are numerous. However, the question remains of how farmers can increase the quantity of carbon in their soil.
One of the first few things to think about is the carbon cycle at a fundamental level. It’s important to note that carbon is always coming in and out of the soil. The amount of soil organic carbon is the net balance of how much organic carbon is put into the soil, mostly from plants, such as dead leaves, roots, and compounds released by living roots, and how much organic carbon is removed by harvest or returned to the atmosphere as carbon dioxide by microbial processes.
Therefore, the goal is to add more carbon to the soil than you lose to the air or remove through harvest.
So, how do you achieve that? This is done through the management choices that alter the carbon balance.
It is much easier to lose soil organic carbon than gain it. Research shows continuous no-till builds soil organic carbon in surface soils over time. Many cropland soils have lost 40-50% of their precious soil organic carbon. Much of this has been through tillage. No-tillage results in more organic carbon accumulation in the surface soil, resulting in the on-farm benefits as described above.
Cover crops are also excellent for incorporating more carbon into your soil. Cover crops also save the soil from erosion and help recycle nutrients from more profound into the ground back to the surface. Their residue provides a mulch to keep the soil cool and moist; as that residue decomposes, some end up as soil organic carbon.
Other essential management decisions also affect carbon in your soil. For example, the choice of crop rotation and residue management can dramatically affect the amount of carbon added to your soil.
Additionally, farmers who integrate livestock into their operations have a fantastic opportunity to increase carbon in their soils by directly applying animal manure. In contrast, livestock grazes cover crops or crop residue for forage.
SOC Practices and Methodologies
Several practices are available to help improve the quality of soil organic carbon and then issue certified carbon offsets, including livestock grazing management, cover cropping, reducing tillage, and switching to organic inputs.
Soil organic practices are not explicitly removals but also contain an avoidance component, which can be sizable and highly impactful given the significant emissions caused by traditional agricultural practices.
How can I build soil carbon?
Increased carbon sequestration in farms can be achieved by adopting carbon farming practices that benefit soil health and productivity. As land managers, placing the health of soils central to farm operations may seem unclear at first. And with varying purposes, developing carbon in the soils takes years, decades, and even centuries to form (referred to as active, slow, and passive carbon pools, respectively). Regenerative soil practice is all about playing the long game — building carbon in the soil to help future-proof farms. Regenerative agriculture (RA) is addressed as a solution towards sustainable food systems. Various actors perceive RA differently, and a precise scientific definition is lacking.
Soil management is crucial for the long-term sustainability of croplands. And soil carbon is an area with a significant potential for improvement and impact. Carbon affects the health of the soil, which can determine its productivity and profitability for farmers.
Many factors affect soil carbon, like temperature, weather, soil acidity, and geography, among other things. But management decisions on the farm play a significant role as well.
Managing soil health is about finding the right balance between carbon input and output. Minimizing soil disturbance, covering the soil, and cultivating crops in rotation are ways soil can be managed that account for soil carbon and, ultimately, soil health.
Building soil carbon gains takes time; long-term commitment is essential for healthy soils. Added incentives in carbon farming can provide short-term gains while maintaining a long-term outlook on the sustainability of farmland soil health.
To learn more about soil organic carbon and how you can begin your carbon sequestration journey, contact the experts at SoilOptix® today!