Soil organic matter in cropping systems
Increasing soil organic matter (SOM) benefits both crop production and the environment. On this webpage, we share factors that affect the amount of organic matter in soils, and strategies for accumulating SOM while avoiding losses.
Understanding soil organic matter
The Soil Science Society of America defines SOM as the organic fraction of soil after removing undecayed plant and animal residues. However, organizations and individual researchers differ in their opinion about whether undecayed plant and animal tissues (e.g., stover, dead bugs, earthworms, etc.) should be included in the definition.
Here, we’ll adopt a broader definition that considers SOM to be the diverse organic materials—such as living organisms, slightly altered plant and animal organic residues—and well-decomposed plant and animal tissues that vary considerably in their stability and susceptibility to further degradation.
Simply put, soil organic matter is any soil material that comes from the tissues of organisms (plants, animals or microorganisms) that are currently or were once living. Soil organic matter is rich in nutrients such as nitrogen (N), phosphorus (P), sulfur (S) and micronutrients, and comprises approximately 50 percent carbon (C).
Why it’s complex
Soil organic matter (SOM) has been called “the most complex and least understood component of soils.”
Soil organic matter is complex because it’s heterogeneous (non-uniform) and, due to the biological factors it was formed under, doesn’t have a defined chemical or physical structure. Soil organic matter isn’t evenly distributed throughout the soil and breaks down at various rates by multiple agents that are influenced by the unique environmental conditions they’re found in.
Despite research efforts, our understanding of the functions that SOM affords to soil quality and crop productivity still remain primarily descriptive in nature. The puzzle of SOM becomes even more complex given the varied responses of soil organisms to their environment and to farmers’ management efforts.
Role of SOM
Soil organic matter is present in all soils all over the world. Just as all life on Earth depends on the processes of microorganisms, SOM is transformed and degraded as a result of soil microorganisms.
The transformations discussed on this page—decomposition, mineralization, immobilization, denitrification and nutrient cycling—depend, at least in part, on soil organisms. The same factors that regulate the SOM pool, especially precipitation and temperature, also control the activity and community composition of soil organisms.
In fact, SOM and soil organisms are so interdependent that it’s difficult to discuss one without the other. As the U.S. naturalist and explorer John Muir said, “When we try to pick out anything by itself, we find it’s tied to everything else in the universe.” Soil organic matter and soil organisms are an excellent example and a testament to the wisdom of Muir’s teachings.
Soil organic matter accumulates to higher levels in cool and humid regions compared to warm and arid climates. In addition, SOM associated with different soil textures (sand, silt and clay) will differ in susceptibility to decomposition.
Many studies have shown that SOM associated with the sand-sized fraction is more susceptible to decomposition, and thus a higher turnover, than the silt- or clay-sized fractions.
Soil organic matter has been directly and positively related to soil fertility and agricultural productivity potential. There are many advantages to increasing or maintaining a high level of SOM:
Reduced bulk density.
Increased aggregate stability.
Resistance to soil compaction.
Reduced nutrient leaching.
Resistance to soil erosion.
Increased biological activity.
Reduction of greenhouse gases by soil C sequestration.
How to increase SOM
In most agricultural soils, you can increase organic matter by:
Leaving residue on the soil surface.
Rotating crops with pasture or perennials.
Incorporating cover crops into the cropping rotation.
Adding organic residues such as animal manure, litter or sewage sludge.
To describe soil organic matter, materials are divided into pools based on how easily the material is decomposed. The pools are:
Active or labile.
Slow or intermediate.
Passive or stable.
Classification methods: Fractionating vs. pools
The method of classifying SOM into pools is far more commonly used than the method of fractionating SOM into humic acid, fulvic acid and humin.
Outdated measurements of humic and fulvic acid separation are no longer considered meaningful because the fractions are artificially defined and don’t exist in soils per se. While this classic method has been known for hundreds of years, pools are more biologically and agriculturally meaningful.
Pools have measurable organic matter components, are theoretically separate entities and are more concisely designated by fractions. Rather than producing chemically discrete SOM fractions, pools produce heterogeneous and non-reproducible fractions.
In general, younger organic material from recently deposited roots and residue, dead organisms or waste products is the most biologically active fraction of the SOM.
This means it serves as a food source for the living soil biological community. The younger fraction is also referred to as the labile SOM fraction, indicating that it’s more readily decomposed than the passive/stable fraction. Generally, this fraction of the SOM is less than five years old.
There are many ways to measure the active fraction but one of the most commonly used methods is to measure the particulate organic matter (POM). POM is defined as the microbially active fraction of soil organic matter. The reason POM has become so frequently used is because it’s been shown to have a strong response to management decisions, such as tillage, residue handling and levels and crop rotation.
There are a number of different ways to measure POM, but they all rely on separation techniques based on SOM material size and density.
Many soil organisms assist in the process of decomposing plant and animal tissues. During the decomposition process, chemical transformations take place, creating new organic compounds in the soil.
After years or decades of these transformations, the original organic materials convert into chemically complex, nutrient-poor compounds that few microbes can degrade. These compounds are referred to as passive or stabilized, and can make up a third to half of soil organic matter. Such passive or stabilized materials are what we commonly refer to as humus or the stable fraction.
The stable fraction doesn’t contain many nutrients, so it isn’t directly important for soil fertility. However, soil’s stable humus fraction is very chemically reactive and contributes to the soil’s net chemical charge, known as the cation exchange capacity and anion exchange capacity.
In this way, humus temporarily and reversibly binds plant nutrients in the soil, preventing them from leaching, so they’re available for plant uptake. The stable fraction also modifies and “stabilizes” toxic materials so that they’re less reactive and/or dangerous. Finally, the stable fraction enhances soil aggregation that reduces a soil’s vulnerability to erosive forces, reducing soil loss by erosion.
Worldwide, SOM levels in mineral soils range from trace amounts to as much as 20 percent. In general, if a soil is 20 percent or more organic material to a depth of 16 inches, then that soil is considered organic and is taxonomically described as a Histosol.
Histosols only make up about 1 percent of soils worldwide. Soils in the northern Great Plains of the United States have some of the highest SOM levels of all mineral soils, commonly ranging from 4 to 7 percent of the total soil mass (Figure 1).
Why SOM levels are high
There are several reasons to explain the higher levels of SOM in this area relative to the rest of the world. The soils of this region are relatively quite young, having only been exposed since the recession of glaciers and the drying of glacial lakes, such as Lake Agassiz in the present day Red River Valley. These soils are only 11,000 to 14,000 years old.
Compare this with the ancient soils of the southern Appalachian Mountains, which formed during the Paleozoic Era, somewhere between 650 million and 350 million years ago. Appalachian region soils have had much longer to decompose, erode, weather and leach much of the SOM that was originally present. As a result, the SOM levels are less than 1 percent in most areas of southern Appalachia.
Because the northern Great Plains soils are quite young, they haven’t been weathered and stripped of their SOM and nutrients as older soils have. Additionally, the prairie vegetation that, until 150 years ago, covered much of the Great Plains region added large amounts of SOM to the soil.
In a mixed prairie, the aboveground material may produce about 1.4 tons of biomass per acre, but the root yield is more like 4 tons of biomass per acre. The root yields of prairie grasses greatly contributed to the high SOM levels of Great Plains soil and explain why land dominated by forest vegetation is relatively lower in SOM. Soils that form under prairie vegetation commonly have SOM levels at least twice as great as soils formed under forest vegetation.
Maximum agricultural productivity
There’s currently no universally accepted SOM threshold value for determining maximum agricultural productivity. Some suggest that different soil types have different organic matter levels at which they’re most agriculturally productive.
So, although a weathered soil in the southeastern United States may demonstrate maximum productivity at 1.2 percent, the same SOM value may indicate a degraded soil with limited soil productivity in the Northern Great Plains. Research suggests that soil organic C levels less than 1 percent may be unable to attain maximum agricultural yields, regardless of the soil type.
Predicting rates of SOM decomposition and release of plant nutrients is quite difficult because they’re controlled by many different, yet related, physical, chemical and biological properties. However, it’s easy to understand how to increase SOM. When the input of organic materials into the soil exceeds the rate of loss from decomposition, erosion and leaching, SOM will effectively increase.
Role of soil organic matter in crop productivity
The roles of soil organic matter can be classified into three broad categories: Biological, physical and chemical.
There are many and varied interactions that occur between these aspects of SOM, plus the active and stable fractions will play different roles in specific SOM functions.
Cation exchange capacity (CEC) is the total sum of exchangeable cations (positively charged ions) a soil can hold. Cation exchange capacity determines a soil’s ability to retain positively charged plant nutrients, such as NH4+, K+, Ca2+, Mg2+ and Na+.
As CEC increases for a soil, it’s able to retain more of these plant nutrients and reduces the potential for leaching. Soil CEC also influences the application rates of lime and herbicides required for optimum effectiveness. The stable fraction (humus) of SOM is the most important fraction for contributing to a soil’s CEC.
Different soil textures have differing CEC (Table 1). In most soils, organic matter contributes more to exchange capacity than the soil texture. The interaction of texture and organic matter components in soil has a tremendous influence on CEC potential.
In table 1, cation exchange capacity is measured in centimoles of charge, expressed in cmol per kilogram of soil.
Table 1: CEC ranges for each soil texture and organic matter
|Texture||Cation exchange capacity (CEC)|
|Organic matter||40-200 cmol per kg.|
|Sand||1-5 cmol per kg.|
|Sandy loam||2-15 cmol per kg.|
|Silt loam||10-25 cmol per kg.|
|Clay loam/silty clay loam||15-35 cmol per kg.|
|Clay||25-60 cmol per kg.|
|Vermiculite||150 cmol per kg.|
As stated in the previous section, humus plays an important role in regulating the retention and release of plant nutrients. Humus has a highly negatively charged soil component, making it capable of holding a large amount of cations.
The highly charged humic fraction enables the SOM to act similarly to a slow-release fertilizer. Over time, as nutrients are removed from the soil cation exchange sites, they become available for plant uptake.
Predicting the released amount
Predicting the amount of nutrients released from SOM is complicated, and no widely agreed-upon methods are in use. Predicting N release to the soil from SOM is difficult but can be estimated by the pre-plant soil profile nitrate (PPNT) or pre-sidedress nitrate (PSNT) tests.
Many land grant universities have conducted trials to estimate the N release from SOM for plant growth. In Minnesota, a soil with a SOM content greater than 3 percent will have a lower fertilizer N requirement compared to a soil with less than 3 percent SOM.
Soil structure refers to the way individual soil mineral particles (sand, silt and clay) are arranged and grouped in space. A variety of different binding agents stabilize soil structure. Soil organic matter is a primary factor in developing and modifying soil structure.
While binding forces may be of organic or inorganic origins, the organic forces are more significant for building large, stable aggregates in most soils.
Examples of organic binding agents include:
Plant- and microbially derived polysaccharides.
Inorganic binding agents and forces include:
Charge attractions between mineral particles and/or organic matter.
Freezing-thawing and wetting-drying cycles within the soil.
Compression and deformation forces.
Both the stable and active fractions of SOM contribute to and maintain soil structure and resist compaction.
Increasing SOM is an effective method for increasing drought-resistance in arid areas. Drought’s effect of reducing crop yields isn’t only due to irregular or insufficient rainfall, but also because fields lose a large proportion of rainfall due to runoff (Figure 2).
Growers can’t control some factors in inefficient water usage, such as slope, rainfall intensity, soil texture and water that moves below rooting depth. However, some factors—especially those that reduce SOM, such as burning crop residues, excessive tillage and eliminating windrows—reduce water infiltration and increase water runoff.
SOM affects the amount of water in a soil by:
Influencing water infiltration and percolation.
Influencing evaporation rates.
Increasing the soil water-holding capacity.
Factors that reduce water infiltration and percolation
Compaction in surface soils.
Lack of surface residue.
Poor soil structure.
Surface crusting due to salinity.
Steep slopes that facilitate high volumes of water runoff.
If water is running off a field at a high velocity, it can’t overcome the lateral force of water movement and won’t vertically move down into the soil profile. Erosion of valuable topsoil is a common result of water runoff.
Surface residues physically impede water runoff, resulting in reduced velocity of water movement. As water movement across the soil surface slows down, water has more time to move downward into the soil profile, rather than across the soil surface. In this way, increasing SOM and leaving residue on the soil surface can increase water infiltration.
Surface residues also slow the rate of water evaporation from the soil and improve soil structure, which helps prevent soil crusting. Crusting can result in significant losses to crop stand.
Crusting is especially likely after tillage events, when the surface soils are exposed and disrupted. This happens after using spring tillage events to prepare the seedbed.
When heavy rains occur shortly after planting, the pounding effect of raindrops’ impact on disturbed soils can create soil crusts up to 1 inch thick. This prevents adequate water infiltration and also creates a physical barrier for seedling emergence, potentially reducing plant population (Figure 3).
You may have observed that soils with higher SOM are fluffier or have better tilth than soils with less SOM. This is because SOM is less dense than the mineral soil particles per unit of volume, providing greater pore space for holding water and air.
The result of increasing SOM is greater soil pore space, which provides an area for storing water during times of drought. A unique characteristic of the pore space in SOM is that the pores are found in many different sizes.
The large pores don’t hold water as tightly, and will drain more readily. The medium- and small-sized pores will more tightly hold water and for a longer period of time. Even during a dry period, the soil retains moisture and a percentage of that water is made available over time for plant uptake.
Strategies for success
The benefit of leaving residue on the soil surface and increasing soil organic matter is that it increases water infiltration and decreases soil crusting. It also enables the soil to hold more of the water that infiltrates it, and can eventually make available for plant use.
According to the soil formation model of Hans Jenny, the father of soil pedology, a natural body of degraded mineral or organic material can’t be considered a soil without soil organisms. This emphasizes the importance of soil organisms in the study of soil science.
When considering the life in soils, we evaluate soil microorganisms (bacteria and fungi), plants and fauna (nematodes, springtails, mites, earthworms and insects).
While microorganisms only make up a small portion of the SOM (less than 5 percent) they’re imperative to the formation, transformation and functioning of the soil.
They conduct indispensable processes such as decomposition, nutrient cycling, degradation of toxic materials, N fixation, symbiotic plant relationships and pathogen control.
About soil fauna, Jenny said, “They break up plant material, expose organic surface areas to microbes, move fragments and bacteria-rich excrement around, up and down, and function as homogenizers of soil strata.”
Soil fauna play an important role in the initial breakdown of complex and large pieces of organic matter, making it easier for soil microorganisms to release carbon and plant nutrients from the material as they continue the decomposition process.
Effect of agriculture on SOM
The loss of SOM resulting from the conversion of native vegetation to farmland has been extensively studied and is one of the best-documented ecosystem consequences of our agricultural activities.
Agriculture has affected the quality and quantity of SOM on many levels.
The greatest loss of soil organic carbon (SOC) associated with agriculture occurs during the first 25 years of cultivation, with losses of 50 percent being common. In the Midwest, the majority of soils converted from natural to agricultural systems have lost 30 to 50 percent of the original SOC level, or 11 to 18 tons of carbon (C) per acre.
How agriculture depletes soil organic carbon
Agricultural practices contribute to the depletion of SOC through:
Deforestation and biomass burning.
Crop residue removal.
Overuse of pesticides and other chemicals.
Cropland soils generally store less SOC than grazing land because cropland has:
Greater disturbance from cultivation.
A lack of manure being returned to the system.
Less root biomass.
Less biomass returned to the soil surface.
Factors that contribute to carbon loss
Many factors related to agricultural management can affect the rate and amount of C lost from the soil system. Factors affecting soil C loss from agricultural soils include:
Climate and soil type.
Tillage intensity and depth.
Crop rotation decisions.
Amount of organic inputs.
Amount of plant residue on the soil surface.
Quality of plant residues returned to the soil.
Soil biological activity.
Length and time of fallow.
Agriculture is thought to have developed about 12,000 years ago. Since then, it’s changed the face of the planet in a slow but relentless transformation.
However, researchers and ecologists of the 21st century have interests and concerns that weren’t even considered by scientists of any other time in history. Global climate change is one of the areas receiving a great deal of attention and research effort.
Soil organic matter in the global C cycle
Soil organic matter plays a critical role in the global C cycle. The importance of soil in the C cycle is due to its role as both a major source and sink for C in the biosphere.
The total soil C pool is three times greater than the atmospheric C pool and 3.8 times greater than the biotic C pool. The soil C pool contains about 1.7 x 1012 tons of organic C and about 8.3 x 1011 tons of inorganic C to a depth of 3.3 feet.
How soil C can reduce greenhouse gases
Although the soil C cycle is complex, the concept of C sequestration for mitigating the release of greenhouse gases is relatively straightforward. Carbon stored in soils ties up C that would otherwise be released to the atmosphere as C-containing greenhouse gases, particularly carbon dioxide (CO2) and methane (CH4).
Scientists are keenly interested in determining the extent to which atmospheric carbon can be diminished by storing C in soils.
Tillage results in lost SOM, primarily through three mechanisms:
Mineralization of C due to soil aggregates breaking down and changes in temperature and moisture regimes.
Leaching of organic C.
Accelerated erosion rates (Figure 4).
Even in cropping systems that return almost none of the aboveground residue back to the soil—such as corn silage production and some biofuel systems—reducing tillage intensity can maintain or increase the soil organic fraction that’s most readily decomposable. Additionally, reduced tillage has been shown to increase soil microbial biomass levels before measurable changes in total soil C occur.
CO2 losses in different tillage systems
Tillage is responsible for substantial carbon loss from the soil. As carbon is released from the soil as a result of tillage, it leaves in the form of carbon dioxide (CO2). The deeper and more aggressive the tillage, the more CO2 is released to the atmosphere.
Near Jeffers, the Department of Agriculture’s Agricultural Research Service (ARS) measured the CO2 loss from three tillage systems in a continuous corn system over a 24-hour period:
Moldboard plow, the most aggressive system used, lost 579 pounds of CO2 per acre.
Disk-rip, the intermediate tillage system, lost 271 pounds of CO2 per acre (47 percent of moldboard plow).
Strip tillage, a system that tills less than 30 percent of the soil and leaves the rest undisturbed, lost 106 pounds of CO2 per acre. Strip-till only lost 18 percent of the carbon dioxide the moldboard plow system lost.
Crop rotations enhance the productivity of all crops in the rotation and benefit the soil too. Some advantages of a well-managed crop rotation:
Breaking plant-pest cycles.
Maintaining soil fertility.
Reducing fertilizer inputs.
Cover crops and green manures
Cover crops and green manures are also part of the crop rotation in many sustainable land management systems. A cover crop is any crop grown to provide soil cover, regardless of whether it’s later incorporated into the soil or not.
Cover crops are primarily grown to prevent soil erosion. When growing a cover crop to reduce nutrient leaching or retrieve nutrients deep in the soil profile, it’s referred to as a “catch crop.”
A green manure is a crop that’s primarily grown to improve soil fertility and is incorporated shortly after planting while it’s still green or soon after flowering.
Crop rotations that include cover crops, perennial grasses and legumes and reduced tillage are an important factor in SOM management and can be adapted to any cropping system.
Rotations also affect an agroecosystem’s biological diversity, which is important for maintaining a high-functioning, disease-resistant and stable ecological system. Crop rotations that maximize soil C inputs and maintain a high proportion of active C are important factors in establishing a sustainable cropping system.
The advent of affordable N fertilizers after World War II established a new era in crop production. With ammonium- and nitrate-based fertilizers, marginally fertile land could suddenly be profitably cropped and yields improved on already-fertile land.
Effect on SOM levels
How does fertilization affect SOM levels? There’s no clear answer because it appears many other factors must be considered, including vegetation present, soil type and climate. Also, fertilization affects different SOM fractions (the active fraction, stable fraction, etc.) differently.
Fertilization directly and indirectly affects the soil microbial community by supplying mineral nutrients for microbial use and allowing increased production of plant biomass to serve as a microbial food source.
A larger microbial community can result in either a net C increase or decrease to the soil system. This depends on how much C stays in the soil system as microbial biomass versus how much is lost as respired C gases, because a greater microbial community leads to more soil respiration.
Consider all relevant factors—vegetation, harvested biomass, microbial community biomass and plant and microbial respiration—when determining if adding fertilizer will accumulate or degrade SOM.
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Reviewed in 2018