Soil organic matter in cropping systems

Sometimes the terms soil organic matter and soil organic carbon are used interchangeably. That is because carbon (C) makes up the majority of organic matter mass. Researchers estimate that carbon makes up about 58% of soil organic matter. Hydrogen, oxygen, nitrogen, phosphorous, and other nutrients make up the remaining mass.  If you see a report that lists soil organic carbon (scientists often do this), you can convert it to organic matter by multiplying by 1.7.

Organic soils (Histosols)

The soil organic matter level in most mineral soils ranges from trace amounts up to 20%.  If a soil has 20% or more organic material to a depth of 16 inches, then that soil is considered organic, and is termed a peat or muck depending on the extent of decomposition.  These soils are taxonomically described as a Histosol (Figure 1). 

Histosols make up only about 1% of soils worldwide and most soils have a much lower content of soil organic matter. 

Soils of the Upper Midwest

Soils in the Northern Great Plains of the United States have some of the highest organic matter levels of all soils that aren’t Histosols, commonly ranging from 4-7% of the total soil mass (Figure 2).

Mollisols

Most of the Midwest soils fall into the Mollisol classification, which is distinguished by the accumulation of organic matter in the shallow horizons of the soil (Figure 3).

The soils of this region are relatively young, having only been exposed to the elements since the glaciers receded and the glacial lakes, such as Lake Agassiz in the present day Red River Valley (Figure 4), dried up.  Since glaciers covered the soil, these soils have not had as much exposure to wind and water. These erosive elements strip and weather the soil organic matter away over time. In areas that don’t have a history of glaciers, soils have had more time to weather and they are more depleted in organic matter.

After the glaciers receded, prairie vegetation occupied much of this area until about 150 years ago. Deep perennial roots contributed large amounts of organic matter to the soil (Figure 5).  In a prairie, the aboveground material may produce about 0.2 tons of biomass per acre, but the root yield is closer to 2.2 tons of biomass per acre.  The root yields of prairie grasses contributed greatly to the high soil organic matter levels of Great Plains.

Finally, the short growing season and frigid winters of the Northern Great Plains prevent decomposition of organic matter for part of the year. Soil organisms aren’t active during long periods of freezing conditions, which allows the organic matter in soils to accumulate and remain at high levels.

You can think of active organic matter as the portion that is decomposing. Active organic matter is a small portion - 10 to 20% - of the total organic matter in the soil, but it is an important portion, because it fuels microbial activity and releases nutrients into the soil.

Active organic matter contains nutrients that are easy for microbes to digest and use for their metabolism. These materials are quite young – usually less than five years in the soil. Fresh crop residues are a good source of active organic matter. The bits of roots, leaves, and insect carcasses that you see as you look at a handful of soil are good examples of this, even if they have already started to degrade (Figure 6).

"The microbial biomass is the eye of the needle through which all organic matter must pass"

Jenkinson, 1977

The active organic materials are different from stable organic matter, which makes up a much larger portion of the total soil organic matter - 60 to 90%. As soil organisms digest and decompose material, several things happen:

• The chemistry of the organic matter is modified
• Nutrients are removed as microbes decompose the material
• It sticks to soil particles

The most stable organic matter from Lamberton, MN (typical for a prairie-derived midwestern Mollisol) was 1,510 years old in the top 8 inches of soil. This ancient, stable C has likely survived thousands of microbial ingestions and transformations.

When microbes die, the nutrients and carbon in their bodies is released to be consumed by other microbes, or it may become fixed to clay particles which makes them more resistant to further decomposition.

Soil color and organic matter

The dark brown or black color of topsoil is caused by organic matter coatings on soil particles. The dark soils of the Northern Great Plains hold a lot of stable organic matter, and you can detect it by just looking at the soil color (Figure 7).

Stable organic matter accumulates when active microbes are continually decomposing organic matter. It provides a number of chemical benefits, and we’ll talk about those later.

Soil aggregation

Aggregation is another process that stabilizes organic matter in soil. As soil particles stick and bind together, they form aggregates (Figure 8).

Small pieces of organic matter—both active and stable forms—can be trapped inside of these aggregates. When this happens, the soil particles that make up the aggregate act like armor and protect the organic matter from attack by decomposers.

This physical stabilization (also called occlusion) is another way in which organic matter can accumulate in soil, but it is dependent on aggregate formation and stability.

Organic matter accumulation takes a long time

Soil organic matter accumulates over long periods of time—years to decades to centuries. The majority of soil organic matter is the result of decomposition and aggregation that has occurred over a long time.

In fact, most of the material added to soil as residue is consumed and respired through decomposition within weeks to a few years. Only a small portion of organic matter makes its way into the stable pool each year. An illustration of this cycle is in the next drawer.

Organic matter contributes to a healthy soil

Healthy soil has a mix of both active and stable soil organic matter. A steady supply of organic inputs –such as crop residues and manure - helps build and maintain both active and stable organic matter pools, which provide a wide array of benefits to the soil.

Even though we think about aboveground residues as contributing to soil organic matter pools, most of the active organic matter that cycles and functions in the soil comes from plant roots. In order for leaves and shoots to become stable organic matter, they need to be consumed by insects and microbes, which mostly reside near the soil surface.  

Crop residues that have been incorporated into soil using tillage are only considered organic matter after they have started to decompose and fragment into smaller pieces.

When organic materials, such as residues (leaves and roots) or organic amendments (manures and composts), are added to the soil, they provide a source of active organic matter. This then is consumed by soil organisms for metabolism and growth.

Through this process, the mass of the residue decreases: the residue feeds soil organisms’ growth and CO₂ is respired into the atmosphere.

The processes of decomposition and biomass turnover release nutrients, which are used by plants and microbes.

The stabilization of organic matter can occur through occlusion (entrapment in aggregates) and by sticking to soil particles.

Fresh residues will decompose over short time periods (days, months, years), while stabilized forms of organic matter have longer turnover times (decades).

Soil organic matter increases the ability of a soil to receive and hold more water. The particulate organic matter in soil serves as a lightweight, low density bulking agent, similar to a sponge.

This material helps the soil create and maintain large pore spaces and channels that allow water to infiltrate and drain and small pore spaces that hold on to water. In addition, plant residues that are spongy and absorptive can also swell and retain water.

Residues on the surface protect the soil surface from the atmospheric elecments of wind, rain and sun.

They can protect the surface against forming a hard crust and reduce erosion risks by slowing water and air movement across the surface (Figure 9).

Organic matter acts like a sponge

Because organic matter acts like a sponge, it can drain excess water as well as hold water that can be used for plant uptake. Figure 10 shows the water content of different textured soils at organic matter levels of one through five percent. 

As the organic matter increases, so does the ability of all soils to hold more water.  This is important for consistent moisture uptake by the growing crop. 

Researchers found that a corn crop used an estimated 0.25” of soil moisture a day while growing. In a silt loam soil, corn could go three more days before needing a rain event by increasing the organic matter content from three to four percent (Figure 11).

This figure also illustrates why the hilltops dry up much faster than the low-lying areas of the field.  The light tan hilltops of Minnesota and eastern North Dakota generally have an organic matter content less than 1%.  The darker, low lying areas of these same fields can have organic matter in the 3-5%.  Taking the average of 4%, corn in the low-lying areas of a silty clay loam soil can go five more days before needing a rainfall event.  Water stress during corn tasseling can reduce kernel set, resulting in a loss of corn grain yield from 3-8% for each day of stress.

Plant exudates and microbial byproducts - both considered active organic matter - can be sticky substances that help soil particles hold together to form and stabilize aggregates. This organic matter “glue” directly helps soil develop and maintain aggregate structure. Organic matter also feeds microbes that helps them grow and metabolize. These activities further promote soil aggregation.

Physical benefits of increased aggregation

Better aeration

Plant roots and soil organisms need oxygen too! By promoting air exchange with the atmosphere, the soil community can stay active and healthy, and important chemical reactions that impact fertility can occur.

Better friability or tilth

Better friability means that the soil is crumbly, rather than compacted and hard. The ability of the soil to crumble will make it easier to place seed and fertilizer and will create an ideal rooting medium for plants.

Less crusting

if the soil is well aggregated, it is less likely to crust. Crusting prevents water and air movement into the soil, it can prevent seedlings from emerging, and it promotes water runoff (Figure 12).

Better infiltration, drainage and water storage

Aggregated soils will allow water to enter and drain into the soil by creating a network of pores and channels throughout the soil profile. A well-aggregated soil has a variety of pore sizes and shapes.

After a wetting event, some pores will completely drain and fill with fresh air, others are small enough to hold on to the water, which will be available for plant uptake.

This structure helps a soil to buffer against extreme fluctuations in water content during long periods of saturation or drought.

Soil resilience refers to the ability of a soil to resist or recover its healthy state in response to destabilizing influences (i.e. drought, excess moisture and tillage).

Biological benefits of increased aggregation

A home

Soil microbes, worms, and insects all need a place to live and aggregates help provide habitat for these organisms. Because an aggregated soil has pores of all shapes and sizes, there’s a space for everyone (Figure 13).

Food storage

Since organic matter is incorporated into aggregates as they form, aggregates serve as a slow-release source of food for microbes and other soil organisms.

Even though they might need to wait until aggregates break apart after wet/dry or freeze/thaw cycles, aggregates guarantee that there is always a snack in the cupboard.

What about the nutrient benefits of active organic matter? Active organic matter is full of fresh, accessible nutrients (Table 1). As soil organisms break down and decompose soil organic matter, the nutrients will be both consumed by the soil organisms and released into the soil solution. There, it is either free for uptake by plants and other organisms or lost to leaching or volatilization. As long as there is active organic matter decomposing, it will provide a slow and steady supply of nutrients into the soil solution (mineralization).

Soil organic matter can impact your fertilizer recommendations

If you’re curious how much organic matter can contribute to your soil’s fertility level, check out the fertilizer recommendations from North Dakota State University and the University of Minnesota, which take soil organic matter content into account:

• North Dakota Corn Nitrogen calculator - Play with the organic matter levels to see how it affects the recommended nitrogen application rate.
• Fertilizing corn in Minnesota - Organic matter content is built into the recommendations. Also see the Corn nitrogen rate calculator.

The plant available form of most nutrients is an ion, an atom or molecule that has a charge. An ion that has a positive charge is called a cation, while an ion that has a negative charge is an anion. 

An ion’s charge will determine how it behaves in soil.  The cation exchange capacity (CEC) measures the soil’s ability to temporarily hold on to different cations and many nutrients in the soil are cations. 

Soil in the Northern Great Plains has a net negative charge and can hold onto positively charged ions such as calcium (Ca⁺⁺), magnesium (Mg⁺⁺), zinc (Zn⁺⁺), potassium (K⁺) and ammonium (NH₄⁺).  However, the soil has difficulty holding on to anions such as sulfate (SO₄⁼) and nitrate (NO₃^-).

Soil organic matter contributes to CEC

Soil organic matter provides between 20-80% of the CEC in mineral soils. The net CEC of a soil depends on its texture - percent of sand, silt, or clay in the soil - and the nature of the organic matter, but it is mostly that stable pool of organic matter that provides this function (Table 2). In general, the higher the organic matter content in soil, the higher the CEC, and the more likely the soil will retain nutrients.

Another benefit of CEC is that it exchanges hydrogen ions, which determines the pH of the soil solution. A higher CEC allows the soil pH to be more resistant to rapid and large changes, which also protects nutrient availability and plant health in soil.

Organic matter is the main food source for many organisms in the soil.  It is no surprise that a steady supply of active organic matter into the soil will support an active and diverse community of microbes. Soil microbes are important for driving nutrient cycles and influencing the availability of nutrients to the plant; we rely on their activities to make fertilizers available for plant uptake and produce healthy crops.

Microbial diversity is important because different species can perform the same function, such as converting organic matter into plant-available nutrients.  If the soil conditions are not comfortable for one species, another species that can tolerate those conditions may be able to step in to do the job. This is called “functional redundancy.” Soils are notorious for hosting a great deal of microbial diversity, making them resilient to a wide range of ever-changing conditions.

In addition to providing a source of nutrients and energy to microbes, soil organic matter is also important for creating and maintaining soil microbial habitat. Supporting species and metabolic diversity requires a variety of habitat conditions in the soil including:

• Aerobic and anaerobic conditions.
• Wet and dry conditions.
• Nutrient rich and nutrient poor conditions.
• Large and small pore spaces.

Organic matter helps create a mix of these conditions and a variety of homes to support the diversity that we rely on for soil function.

A recent review of yield and soil organic matter indicated that soil organic matter content can influence crop yield, but only to a point. In corn and wheat production systems, yields increased as soil organic carbon increased, but those benefits diminished after the organic carbon content exceeded 3.45% organic matter.

When organic matter levels are higher, other factors may become more important in influencing yield. These might include other nutrient limitations, climate, plant genetics, and other factors.

While increased organic matter may impact yield to a point, its collective benefits on soil productivity, structure, and health are substantial and should not be ignored.

Building soil organic matter depends on keeping a stable flow of carbon through the soil. As long as microbes are incorporating plant residues (active organic matter) into their bodies, the stable soil organic matter pool will grow too. Living roots are a great way to keep microbes happy, giving them a high-quality food source to boost their activity.

Food

As we learned earlier, active organic matter is a food source for microbes. This readily decomposable and diverse food source promotes a healthy, resilient biological community.  Several practices will create a food source for microbes as well as build soil organic matter:

• Leave crop residue on the soil surface.
• Add perennials or cover crops to maximize plant and root growth.
• Reduce soil disturbances.
• Add organic materials such as manure and organic by-products.

A strong house

A well-aggregated soil provides many housing options for microbes, while an aggressively tilled soil breaks apart soil pores and aggregates. Aggressive tillage isolates more individual soil particles with small pore spaces, limiting the types of microbes that can live in this environment.  To promote soil aggregation, the soil will need minimal disturbance and a variety of growth habits over the entire season (Figure 14).

Freedom from drastic changes

Tillage is an example of a drastic change that may inhibit microbial growth and diversity.  It moves, fluffs, and breaks apart soil aggregates. Rapid changes in soil moisture or temperature, and an increase of wind or water erosion, are direct effects of tillage. 

The more aggressive, deep, and frequent the soil is tilled, the more the microbes are affected.  To protect the soil from drastic changes in moisture and temperature, the soil needs to be covered. Crop residue or living crops - including cover crops - provide good soil protection.

Your choice of a tillage system can influence residue quantity. In a Minnesota study, three tillage systems ranging in soil disturbance were compared to no till (NT):

1. Moldboard plow (MP).
2. Disk harrow (DH.
3. Chisel plow (CP). 

In this study, wheat residue from the previous cropping season added 2,844 pounds of organic matter per acre to the soil. 

When the soil was moldboard plowed (MP), the soil lost over 3,800 pounds of organic matter per acre within 19 days of the primary tillage pass (Figure 15).  This is 1,000 pounds more than what was added by the previous crop’s entire organic matter contribution.

In addition, this system will continue to lose organic matter in the spring when the field is tilled to prepare for planting. 

If this system is used year after year, the organic matter content will decrease dramatically over time.  These organic matter loss values are substantially more than the no-till (NT) treatments, which lost only 767 pounds of organic matter per acre due to natural carbon cycling processes.  The disk harrow (DH) and chisel plow (CP) were in the middle range of organic matter lost.

The selection of crops in a rotation, use of cover crops, or added composts and manures can influence residue quality. It makes sense that more types of food will support more types of microbes, who will work hard to turn that food into stable organic matter. As humans, we can’t get all of our nutrients from one type of food, and we don’t all like to eat the same types of food - we need lots of variety to keep everyone healthy and happy. In the same way, providing microbes with a variety of foods — residues that range in C:N ratios — will help to build organic matter in the soil.

You can consider some of the following options to achieve these goals:

• Include three or more crops in a rotation in a three-year time span. A tight rotation offers lower quality and fewer food choices.
• Incorporate a perennial crop such as grass or legume forage into your rotation.
• Add cover crops and green manures during periods that would otherwise be fallow (Figure 16).
• Apply composts, manures, or other organic materials.

To illustrate how manure can impact soil microbes, two types of fields were sampled at the West Central Research and Outreach Center in West Central Minnesota:

1. Fields that had been traditionally manured.
2. Fields that had not received manure.

The manured fields had 45% more microbial biomass - an early indicator of changes in total soil organic carbon, 40% more fungi, and 43% more bacteria (Table 3). 

Manure offers a diverse food source for the soil microbes and their populations responded.  Any of these strategies, or a combination of them will help you build organic matter in soil.

Carbon-to-nitrogen ratios (C:N) influence decomposition speed. The ratio also determines whether nitrogen will be mineralized (released) as the material is decomposed or if nitrogen will be immobilized (tied up) by the decomposer community as they break down the material.

Why do decomposition speeds vary?

Different types of residues have different decomposition speeds, but why? The answer lies in the differences in residue chemistry and the nutritional requirements of decomposers.

Higher C:N ratios

A residue with a C:N of about 25:1 provides the perfect balance of energy and nutrients for soil microorganisms. Residues with a higher C:N, such as sawdust (about 400:1), wheat straw  (80:1), or corn stover (60:1), will not provide enough nutrients to support high microbial biomass and activity. In those cases, residues are slow to break down, as microbes depend on other sources of nutrients for their activity to proceed. These decomposer microbes will scavenge any free nitrogen in the soil - or immobilize it in their bodies - as they chew through high-carbon materials.

Lower C:N ratios

Residues with C:N ratios that are lower than 25:1 - such as manure (20:1) or alfalfa (12:1) - will supply plenty of nutrients to microbes—enough that surplus nitrogen is released (or mineralized) into the soil. In these cases, the decomposers have all the nutrients that they require in the residues, so they remain active and decomposition occurs quickly.

At any given time, soil organic matter is a mix of different types of residues, each with different C:N and decomposition speed (Figure 17).

The C:N levels of residues are different than those of a soil sample. Soil C:N ratios are typically low (around 12:1), which reflect the C:N of the mineral, organic, and living portions of the soil.

When a soil is physically disturbed, such as with tillage, the soil structure that holds and protects organic matter is broken. The combination of exposing that protected organic matter (carbon) to decomposers and soil aeration results in a rapid loss of organic carbon as carbon dioxide (CO₂).

Even one tillage pass will reduce soil organic matter. With more frequent and intense disturbances, soil organic matter will be reduced even further.  Additionally, as the home of soil microbes is destroyed and fungal hyphae are physically ripped apart, microbial activity slows down. It will take some time to recover after soil is disturbed.

Soil mixing also impacts soil organic matter levels. Picture how tillage mixes up that rich shallow topsoil with the deeper layers. Since the deeper layers have a much lower soil organic matter content, mixing has a dilution effect on soil organic matter in the rooting zone. This re-distribution and homogenization of the soil organic matter diminishes its benefits and sets the clock back on soil organic matter accumulation.

Since we know that a continuous supply of residue into soil is important for building soil organic matter, we need to be cautious about how much residue we remove from a field. The amount of corn residue that can be sustainably harvested in the absence of supplemental carbon - manure, organic by-products, perennials, or cover crops - depends on the crop rotation and tillage system (Figure 18). 

How tillage affects sustainable residue removal

Tillage affects the amount of residue that can be harvested. With more aggressive tillage, more residue and air are incorporated into the soil. This, in turn, promotes the decomposition of crop residues and soil organic matter by soil microorganisms (especially bacteria).

As decomposition takes pace, the majority of the carbon in crop residues and soil organic matter is released into the atmosphere as carbon dioxide. The potential for sustainably harvesting crop residue is much greater in a continuous corn system. If grain yields are consistently 200+ bu/ac, then 44% of the corn residue could be harvested annually without severely impacting soil organic matter (Figure 18). In comparison, only 19% of the corn residue could be sustainably harvested with a moldboard plow tillage system in continuous corn.

How yield affects sustainable residue removal

Typically, more residue is produced with higher grain yields.  For example, if continuous corn yields are over 250 bu/ac, three large round bales per acre could be harvested without diminishing soil organic matter, even with moldboard plowing. On the other hand, no residue harvest is recommended when grain yield is 150 bu/ac or less in this system. 

Since yield levels can fluctuate greatly from one year to the next, producers should take this into account, and possibly adjust the quantity of residue harvested from year-to-year (Figure 19).

Soil erosion is the wearing away of a field's topsoil by the physical forces of wind and water. This can be accelerated through forces associated with farming activities, such as tillage. 

The topsoil has the highest concentration of organic matter, nutrients and is where most of the soil's biological activity occurs.  Soil that is not protected by residue or living cover is subject to erosion.  Practices that increase soil erosion - excessive, aggressive tillage, low residue crops and compaction - also reduce organic matter.

Wind erosion

Soil samples collected in March in western Minnesota county ditches show an average of 9.1 Tons/ac (or 18,200 pounds/ac) of soil gathered in an area measuring a half a mile by 16 feet - which is equivalent to one acre (Table 4). 

Not only did the wind blow soil from the field (Figure 20), nutrients such as nitrogen, phosphorus, and potassium were blown away as well.

When soil blows or washes away, the best soil leaves the field—never to return - and the nutrients within it will not be available to the crops in that field.

Soil loss via wind erosion cuts your profits and the field’s productivity by removing the non-renewable resource -soil - and nutrients. Both of these resources are necessary for crop production and they are either irreplaceable or expensive to replace. 

Think of the soil as a bank.  We want to make more carbon deposits than withdrawals.  Deposits of carbon include

• Adding cover crops, perennials in rotation, manure, or compost. 
• Leaving crop residues on the field.
• Reducing tillage intensity by adopting no-till or strip till. 

Withdrawals include

• Making multiple tillage passes with aggressive equipment.
• Tight crop rotations.
• Wind and water erosion. 

Building soil organic matter is a balancing act and one that will be different for each farmer.

Have realistic expectations for how long it’ll take to build soil organic matter.  Organic matter buffers the soil from chemical, physical, and biological changes. 

This means that lower organic matter soils (0-2%) may see changes within 3-5 years of changing management, while higher organic matter soils (4-7%) may take 7-10 or more years to see a change. 

This process is also limited by a region’s climate and growing season length. While the organic matter may change slowly over time, other benefits will be more immediate.  These include

• Better water infiltration. 
• Improved crop resiliency with weather extremes. 
• Increased soil carrying capacity.

Organic matter is based on many factors.  Soils with higher levels of silt and clay usually have higher levels of organic matter than those with a sandier texture. For example, 2% organic matter in a sandy soil is very good and difficult to reach, but in a clay soil 2% indicates a depleted situation.

If you’re interested in measuring and monitoring soil organic matter status of your soil, you don’t need to go much farther than a standard soil test. Most fertility packages will include a measure of soil organic matter percentage alongside nutrient content.

Soil sampling tips

Be sure to remove surface residues and plant material from the soil sample before submitting the sample, since that material will inflate your soil organic matter values. Remember that soil organic matter levels do not change rapidly, and you may only see increases of a fraction of a percent over a few years.

Soil health tests

Commercial labs are also increasingly offering additional soil health tests including “active carbon” and “aggregate stability.” Based on what you’ve read here, you can use these tests to supplement your standard soil tests and to track changes in soil response to management, crop type, or new practices.

Expect active carbon measurements to fluctuate within a growing season, and across growing seasons because it is much more dynamic than total organic matter. You can expect aggregate stability to respond within 3-5 years, depending on soil texture (clay soils will aggregate faster and more strongly than loam and sandy soils). If you have difficulty interpreting these tests, reach out to the lab or university Extension personnel to help you.