Active carbon (POXC): What does it measure?

Carbon is the currency of nature: the backbone of soil organic matter and the energy source for soil microorganisms. Therefore, much interest in soil health focuses on increasing carbon storage in soil. When you reduce tillage or increase crop rotation diversity, you expect soil organic matter to increase. However, soil organic matter often changes slowly for several years. In fact, less than 1% of plant biomass carbon returned to soil eventually becomes stable humus organic carbon.

Active carbon, also known as permanganate-oxidizable carbon (POXC), is a sensitive tool for measuring soil carbon change. This portion of soil organic matter is actively involved in nutrient cycling and changes more quickly when cropping systems are changed (e.g. reduced tillage, diversified crop rotation, cover crop inclusion). Active carbon is a quick, repeatable soil test that measures the easily oxidizable, biologically active carbon fraction.

Why is active carbon important?

Active carbon typically comprises about 1 to 4% of total organic carbon in soil. It represents the microbially available carbon energy sources, that is microorganism food. As one component of the total organic carbon pool, active carbon has a strong relationship with overall soil organic matter (r = 0.80, Fig. 1), but it responds more quickly to changes in crop and soil management. It helps explain why two soils with 3% soil organic matter, for example, may behave differently regarding biological activity or nutrient cycling.

​Active carbon should be utilized as a tracking tool to measure improvement in soil quality. In a 17-year tillage experiment in Mandan, ND, total organic carbon did not change much after conversion to no-till; however, active carbon increased significantly from 470 to 600 ppm (Weil et al., 2003). Recall that soil organic matter includes all forms of organic carbon (i.e. microbial biomass, recently decomposed plant material, stable humus) and requires large carbon inputs to change that total measurement. In contrast, the active carbon fraction increases much more quickly and detects improvements in soil quality sooner. This is why active carbon is considered a “leading” soil health indicator.

What do the numbers mean?

In agricultural soils of the upper Midwest and northern Great Plains, active carbon commonly ranges from less than 300 ppm to 1000 ppm (Fig. 2). As with soil organic matter, soil texture is a major factor controlling active carbon. Coarse-textured soils generally contain less active carbon than medium- or fine-textured soils. Native prairie soils may contain active carbon as high as 1500 ppm.

While these are the common active carbon values you may expect, each field and its history is different; therefore, making comparisons between fields is not advised. There are no index ranges for active carbon because different climatic regions and soil types cannot be judged by the same standard. Active carbon should be utilized as a tracking tool in improving the soil quality of that field or zone, rather than making broadscale comparisons.

Cropping systems that include reduced tillage or no-till, diversified crop rotations, and cover crops will help increase active carbon. In addition, any system with greater organic matter inputs (e.g., plant biomass, manure) helps build active carbon and soil organic matter. Under perennial grass, active carbon can easily exceed 1000 ppm.

With sensitive soil health tools like active carbon, you can more easily quantify positive changes achieved through better soil management.

How to soil sample

Active carbon can be added to any standard soil fertility analysis, simply requested as an analysis add-on. Collect the soil sample with standard soil probe. The standard soil sampling depth of 0-6 inches is most common. In reduced tillage and no-till systems, stratification may warrant 0-2 or 0-4 inch soil sampling depths. Like any soil health test, it is a tracking tool measuring soil quality improvement, so make sure you are using GPS-marked soil sampling points.

Frequently asked questions (FAQ)

Q: Does higher active carbon (POXC) indicate better crop yield?

A: Not necessarily. Crop yield is comprised of numerous determinants: genetics, climate, soil type, soil fertility, crop pests, etc. Active carbon measures the carbon sources available to microorganisms as food. This is only one factor within a wide range of various soil biological functions and yield-determining factors. Soil management practices that can improve crop production, such as no-till in water-limited environments, also work to improve soil properties like active carbon.

Q: Can I use active carbon to reduce fertilizer rates or modify plant population?

A: Active carbon has NOT been calibrated with fertilizer rate or plant population trials. So far, active carbon provides information on soil management . The relationships with crop productivity remain unclear. We must wait for more research to answer these yield-focused questions.


Weil, R.R., K.R. Islam, M.A. Stine, J.B. Gruver, and S.E. Samson-Liebig. 2003. Estimating active carbon for soil quality assessment: A simplified method for laboratory and field use. Am. J. Altern. Agric. 18(1):3–17.

Soil aggregate stability: What does it measure?

Soil aggregates are the building blocks of soil structure. Soil texture is the relative percentage of sand, silt, and clay in soil, but soil structure describes how those particles are arranged in the soil profile. Soil aggregates are glued together with soil organic matter, plant root exudates, and microorganisms like fungi.

We classify soil aggregates by their size: large macroaggregates (>2000 μm), macroaggregates (250-2000 μm), microaggregates (53-250 μm), and free particles (<53 μm). A large macroaggregate is bigger than a sand particle. A microaggregate has the thickness of one or two human hairs. A macroaggregate lies in between.

Multiple soil management practices come together to improve soil aggregate stability. These include reduced tillage or no-till, greater crop rotation diversity, more plant roots, greater earthworm and microbial activity, and more soil organic matter. This is what makes soil aggregate stability such an attractive soil health indicator. Stable soil aggregates take time to form, so you should consider measuring soil aggregate stability every 3 to 5 years.

Why is soil aggregate stability important?

Soil aggregate stability is a comprehensive soil quality measurement. Soil aggregates provide numerous soil ecosystem services:

● Resistance to water and wind erosion
● More pore space for air and water movement, allowing deep root exploration
● Faster water infiltration, reduced runoff
● Less surface crusting
● Improved equipment trafficability and reduced soil compaction, especially on wet soils
● Diverse habitat for soil microorganisms

Strong soil aggregates form naturally with plant root and microbial activity. However, disturbances like tillage quickly break soil aggregates apart. If soil aggregates are broken apart by tillage, the soil pores fill with small particles that can clog and restrict air and water movement. Loose soil particles can also plug the surface pore network to reduce water infiltration and cause surface crusting.

Cropping systems that include reduced tillage or no-till are necessary to improving soil aggregate stability (Figure 2). Tillage also reduces soil organic matter that is needed to bind soil particles into larger soil aggregates. Diversified crop rotations, cover crops, and manure help improve soil aggregate formation.

Soil texture is a major factor in soil aggregate formation. Coarse-textured soils may take longer to develop soil aggregates. Fine-textured soils tend to develop soil aggregates quickly. This is important because good soil structure with large pore spaces is essential to air and water movement through fine-textured soils.

What do the numbers mean?

You can see large soil aggregates (>2000 μm) with the naked eye, but quantifying the different water-stable soil aggregate sizes must be done in the laboratory. This requires special wet sieving equipment that sorts the water-stable soil aggregates by size.

If you see the water-stable macroaggregate fractions increasing, you know that you are making real improvements in soil quality. Soils with high soil aggregate stability have less soil erosion, better equipment trafficability, faster water infiltration, and more diverse habitat for soil microorganisms. Increasing soil aggregate stability should be a long-term goal in all cropping systems.

Soil aggregate stability should be utilized as a tracking tool in improving soil quality. Each field has a different cropping history and soil type (e.g. soil texture); therefore, making broadscale comparisons between fields is not advised.

How to soil sample

A separate soil sample must be taken for soil aggregate stability. Unlike routine soil fertility analysis, this soil sample cannot be dried and ground (destroys soil aggregates). Soil aggregate stability requires special equipment and more labor than routine soil fertility analysis; ask about turnaround time when submitting soil samples.

Collect an undisturbed soil slice with tiling spade or tulip bulb planter (Figure 3); this helps prevent destruction of soil aggregates. Do not collect the soil sample with a soil probe because the soil probe will destroy or compress soil aggregates. Place the soil sample in one-quart plastic bag and clearly mark with permanent marker “SOIL AGGREGATE STABILITY. DO NOT GRIND.” Like any soil health test, it is a tracking tool measuring soil quality improvement, so make sure you are using GPS-marked soil sampling points.


Mikha, M.M., and C.W. Rice. 2004. Tillage and manure effects on soil and aggregate-associated carbon and nitrogen. Soil Sci. Soc. Am. J. 68(3):809–816.

The Meaning of Soil Health Testing and the Big Picture

This submission is courtesy of Dr. Caley Gasch, Assistant Professor of Soil Health, North Dakota State University, Fargo, ND. It was originally published in the AGVISE Newsletter Winter 2017.

Multiple definitions of “soil health” exist. In general, they all recognize that soil is a complex system, with interacting physical, chemical, and biological factors, which should be managed in a manner that sustains its function and integrity over the long term. Conceptually, soil health is a topic that is easy to understand and support.

Yet, it has been difficult to capture this definition with measurements. Soil scientists can choose from hundreds of field- or laboratory-based methods to quantify different properties in the soil, but there is no single best test for assessing soil health. Research and commercial labs and focus groups across the country have compiled lists of properties that are being used to assess soil health, but we still have a lot to learn about how these suites of measurements can guide management decisions.

As much as we desire hard numbers that support the concept of soil health, I argue that we should keep the big picture in mind. Last summer, I heard a perfect analogy that demonstrates the disparity between the concept of soil health and the push for soil health testing. The idea solidified for me on a recent doctor visit.

You visit your doctor for a routine check-up. The doctor will ask you about your lifestyle: How often do you exercise? Do you smoke? What do you eat? How many drinks do you have per week? Which medications are you taking? What is your family history? They’ll take your weight, pulse, blood pressure, and temperature. They may order a few more tests, and they’ll most likely give you some advice: Wear sunscreen, eat more oatmeal, and come back in a year.

The doctor learns something from your vital signs and can compare them to the past; however, the doctor learns more about your overall health after understanding your eating habits and levels of activity. After all, it wouldn’t be good practice for the doctor to prescribe blood pressure medication solely based on a single blood pressure reading, or claim to know your heart condition based on your pulse alone (what if you ran up a flight of stairs on your way to the doctor’s office?). You are not given a mathematical health score or rank. The doctor knows that your body is a complex system, and that to understand its overall condition, we need to consider how you care for yourself, any risky behaviors, and the results of some routine tests. Certainly, if you have a specific complaint, or if the doctor identifies a particular problem, appropriate tests are available to diagnose and monitor the malady.

I propose that we take a similar approach to soil health. First, understand the “lifestyle” of a soil: What is the rotation? What kind of tillage do you use and how frequently? Do you ever use cover crops? What is your fertility plan? What are the “genetic” limitations of the soil? Second, identify specific barriers to overall health: salinity, erosion, poor drainage, disease issues, etc. Third, identify areas for improvement, address them with a lifestyle adjustment, and develop a monitoring plan to track specific problem areas, realizing that it may take a few years to see change.

For humans and soils alike, we know that there are certain risk factors associated with health and condition. We know a great deal about how practices influence soil properties,
and we don’t usually need a test to detect whether or not a practice is healthy. The testing becomes meaningful only after we focus on a specific challenge area, which will differ from person to person and from field to field. Targeted testing and acute treatments are useful and necessary, but they should not be the basis for managing a complex system.

I’m grateful that soil health is a popular topic, because it is helping us all to think about soil in a way that recognizes its true value and potential. But I challenge us to remember our overall goal for soil health, rather than focusing solely on the vital signs. Approach each field with a broad view, understanding that it is a unique case that reflects its own history and management. From this perspective, we can identify and adopt practices that will protect our soil and improve its value throughout our region.

Soil Science Review: Organic Matter

Soil organic matter is a fundamental component of soil. It is comprised of living microorganisms, recently decomposed plant material, and stable humus organic compounds. Soil organic matter influences numerous biological, chemical, and physical properties of soil. It influences soil structure, water holding capacity, nutrient cycling, biological activity, and chemical fate and transport (e.g. pesticides). Soil organic matter is so important, you cannot really call something soil unless there is some organic matter present.

In the laboratory, soil organic matter can be measured via three different methods: Walkley-Black wet oxidation, estimation from organic carbon dry combustion, or loss-on-ignition (LOI) dry combustion. Each method has advantages and disadvantages. The LOI method is the routine method in commercial soil analysis.

Walkley-Black Wet Oxidation

The Walkley-Black wet oxidation method is the classic soil organic matter method developed in 1934. The method measures easily oxidizable carbon using sulfuric acid and potassium dichromate. The Walkley-Black method works very well on soils with low organic matter (<2.0%); however, the method is less suitable on soils with very high organic matter (>8.0%) as the dichromate reagent is consumed and may not oxidize all organic carbon. Soils with high chloride may interfere with the Walkley-Black method.

The Walkley-Black method has been phased out since the 1980s because the method requires hazardous chemicals and additional labor; therefore, the Walkley-Black method is more expensive than other methods. As the standard method, it is still required for some pesticide registration studies and regulatory soil characterization work.

Estimation from Organic Carbon Dry Combustion

Soil organic matter is a large complex organic compound containing hydrogen, oxygen, carbon, nitrogen, phosphorus, sulfur, and other elements. Soil organic matter contains about 58% organic carbon on average. We can estimate soil organic matter content from the amount of organic carbon measured in soil. Soil organic carbon is easily measured using a dry combustion carbon analyzer. The analyzer heats soil at high temperature to oxidize all carbon as carbon dioxide, which is then measured with an infrared detector. Since the method requires specialized instrumentation, it is more expensive than the LOI method.

The organic carbon dry combustion method is preferred in carbon sequestration research because organic carbon is measured directly. Estimating organic carbon from Walkley-Black or loss-on-ignition methods introduces unneeded calculation error.

For calcareous soils (pH > 7.3), inorganic carbon (carbonate) must also be analyzed. The dry combustion carbon analyzer measures total carbon, which combines inorganic and organic carbon. The inorganic carbon is measured separately, then subtracted from total carbon to calculate organic carbon.

Loss-on-Ignition Dry Combustion

The loss-on-ignition (LOI) dry combustion method is the routine soil organic matter method used in commercial soil analysis. The amount of soil organic matter is measured directly as the weight loss upon combustion at 360 deg C. The LOI method is simple, affordable, and safe. It also requires no hazardous chemicals. The method works well on soils with high organic matter content since there is no consumable reagent (like Walkley-Black method).

For soils containing hydrated salts (e.g. gypsum, CaSO4∙2H2O; Epsom salt, MgSO4∙7H2O), the LOI method may overestimate soil organic matter upon loss of water from hydrated salts. Soil is preheated at 105 deg C to remove structural water from clay minerals and hydrated salts, but some hydrated salts may retain water above the 105 deg C preheating process.


Soil organic matter affects various soil properties and processes. In return, various soil properties and soil formation factors affect soil organic matter.

Soil Texture

Soil clay particles protect soil organic matter from microbial decomposition (i.e. formation of protective clay-humus complexes). Soils with more clay generally have greater soil organic matter. In addition, soils with more clay also can store more plant available water, so plant biomass production and organic material addition to soil is greater.

Cation Exchange Capacity

Cation exchange capacity is derived from negative-charged soil particles, like clay minerals and soil organic matter. Soils with more soil organic matter will have higher cation exchange capacity.

Bulk Density

Bulk density is the amount of soil mass per unit volume. Soils with high bulk density may be compacted, which can inhibit plant root growth and exploration of the entire soil volume. A compacted soil also has less open pore space for air and water storage and movement in soil. Soil organic matter has low particle density, and it helps alleviate soil compaction and high bulk density. Soils with low bulk density likely contain high soil organic matter.

Water Holding Capacity

The amount of soil organic matter in soil is closely associated with soil texture and bulk density. The association also extends to water holding capacity. Like clay particles, soil organic matter has a lot of surface area on which water films can adhere, thus increasing the water holding capacity. Soils with more soil organic matter will hold more plant available water than soils with low organic matter.

Soil pH

Soil microorganisms breakdown soil organic matter into its constituent nutrient components; this is called nutrient cycling. The type and species of soil microorganism present depend on soil pH. In neutral to alkaline soils, the soil microorganism community is comprised of diverse bacteria and fungi to decompose soil organic matter. In acidic soils, the soil microorganism community is mostly fungi, so decomposition processes occur slower. In acidic soils, soil organic matter accumulates faster, producing soils with high soil organic matter. Soils with very alkaline pH also have reduced soil microorganisms activity and decomposition rates.

Soil Microorganism Biomass

Soil organic matter is the primary carbon food source for soil microorganisms. To maintain a high amount of soil microorganism biomass and biological activity, a significant amount of soil organic matter is required. Soils with low organic matter generally have reduced biological activity.

Fallow Syndrome: Preventing Phosphorus Problems

Some crops that do not support mycorrhizal fungi (left to right: sugar beet, canola, radish).

Producers in the northern Great Plains and upper Midwest need to consider the risk of fallow syndrome in their crop nutrition plans. You are probably asking, what is “fallow syndrome” and why should I care? After all, summer fallow is not that common anymore! But the greater number of Prevented Planting acres in 2019 and 2020 meant that we have had many unintended fallow fields, making fallow syndrome a serious and widespread concern for the next year.

Fallow syndrome is an induced phosphorus deficiency caused by a lack of mycorrhizal fungi in soil. Some plant species, like corn and wheat, rely heavily on mycorrhizae to colonize the plant root system and help acquire important nutrients like phosphorus and zinc. If soil is lacking sufficient mycorrhizae to colonize plant roots, a case of fallow syndrome will increase phosphorus fertilizer needs and even cost crop yield potential.

Understanding mycorrhizae

Mycorrhizae fungi occur naturally in soils and readily colonize plant roots. Upon root colonization, mycorrhizae fungal filaments act as extensions of the root system and increase the soil volume available for plant water and nutrient uptake. The combined root-mycorrhizae surface area can be up to 10-fold greater than roots without mycorrhizae. Mycorrhizae depend on living plant roots to support stable mycorrhizae populations. However, not all plant species host and support mycorrhizae growth. Some common field crops are non-host species and their planting results in rapid drops in mycorrhizae populations.

Summer fallow or unplanted cropland, such as Prevented Planting in 2020, is a classic example of providing no or few living plant roots in soil to maintain mycorrhizae populations. In addition, some crop species do not support mycorrhizae, such as those in the goosefoot family (sugar beet) and mustard family (canola, radish, turnip). Following a classic case of summer fallow or a non-mycorrhizae supporting crop, the mycorrhizae population in soil will quickly drop. A cover crop mix that included a grass species (e.g. barley, rye) should still support mycorrhizae and prevent fallow syndrome concerns.

Preventing fallow syndrome

The easiest prevention strategy after fallow is planting a crop species without fallow syndrome risk like soybean, canola, or sugar beet. Avoid planting susceptible crops like corn and wheat. These crops are highly dependent on mycorrhizae to acquire phosphorus, and extra starter phosphorus will be required if fallow syndrome risk is present.

To reduce fallow syndrome risk in corn or wheat, extra phosphorus fertilizer must be placed with or near the seed. Applying more broadcast phosphorus or relying on high soil test P will not prevent fallow syndrome. The starter phosphorus rate should be 20 to 40 lb/acre P2O5. In some university research trials, up to 60 lb/acre P2O5 with 2×2-band placement near the seed was needed to prevent corn yield loss to fallow syndrome.

For wheat, these phosphorus rates are typically seed safe with monoammonium phosphate (MAP, 11-52-0). Most corn planters can safely apply 20 lb/acre P2O5 (5 gal/acre ammonium polyphosphate, APP, 10-34-0) in the furrow. For medium/fine-textured soils with good soil moisture at planting, you can generally apply up to 10 gal/acre 10-34-0 (40 lb/acre P2O5) safely in the furrow at 30-inch row spacing. Higher 10-34-0 rates may exceed seed safety limits on dry soils or coarse-textured soils and require 2×2-band placement to maintain seed safety.

Complete liquid fertilizers, such as 6-24-6 or 9-18-9, are not suggested for preventing fallow syndrome. Compared to 10-34-0, the products have lower P concentration that result in less applied phosphorus, even if used at maximum seed safe rates. The extra N + K2O in “complete” liquid fertilizers increases the salt index and lowers the seed safe rate.

Prevented Planting Acres in 2020: Maximizing Cover Crop Effectiveness

In 2020, there are again widespread acres of Prevented Planting (PP) in North Dakota and northwest Minnesota. Farmers are now making plans to plant cover crops on unplanted cropland in the next few weeks. It is important to establish cover crops on PP fields because growing plants help reduce the chance these fields will be PP fields again next year.

Let’s look at the major reasons why cover crops are valuable tools on Prevented Planting acres.

Soil Water Use

A field without any growing plants is a fallow field. Before no-till, summer fallow was a widespread soil water conservation strategy in dryland agriculture. Actively growing plants transpire (use) a lot more water than evaporation from the soil surface alone does. Cover crops help fill the water-use void by transpiring a lot of water, helping to dry the soil surface and lower the water table before the following year. This also opens space in the soil profile for summer and fall rains to leach soluble salts from the soil surface and reduce salinity in the root zone.

Soil Erosion Control

Tillage is a popular weed control tool, but it also destroys crop residue and leaves soil exposed and vulnerable to water and wind erosion. Planting cover crops protects the soil surface from rain and wind, keeping soil firmly in place. Just because you cannot grow a cash crop on the field this year, you should not let your soil blow into the next field, letting your neighbor farm it next year.

Weed Control

An established cover crop can compete with weeds, helping suppress weed growth and weed seed production. For fields with problematic broadleaf weed histories, a cover crop mix containing only grass species is preferred. In grass cover crops, you can still use selective broadleaf weed herbicides to control the problematic broadleaf weeds of conventional or no-till systems such as Canada thistle, common ragweed, kochia, volunteer canola, and waterhemp while not killing the grass cover crop. For fields with low weed pressure, a cover crop mix containing grasses, brassicas, and legumes will provide more soil health benefits.

Soil Biological Activity

Have you heard about “fallow syndrome” before? Fallow syndrome is an induced nutrient deficiency, often seen in corn following fallow, when the population of mycorrhiza fungi is insufficient to colonize plant roots and help them acquire water and nutrients. Mycorrhizae are especially important in plant uptake of phosphorus, so plants with fallow syndrome often show phosphorus deficiency symptoms. Fallow syndrome is a major concern in corn following summer fallow or Prevented Planting without cover crop.

During the Prevented Planting year, it is important to include grass species in the cover crop mix to support and maintain the mycorrhiza population through next year. Brassica species, like radish and turnip, are often included in cover crop mixes for their deep taproot architecture and high forage value for grazing livestock, but brassicas do not support mycorrhizae. You do not want a cover crop mix consisting of brassica species alone because fallow syndrome might occur next year.


In June 2020, excessive rainfall slammed some parts of the upper Midwest and northern Great Plains, drenching soils with 3 to 15 inches of rain over a couple days. On summer-flooded fields, cereal rye is an attractive soil management tool. You can plant or fly on cereal rye well into August or mid-September, and it will continue to use soil water through late summer and fall. Next spring, the overwintered rye will grow again, using more soil water and maintaining soil structure, providing you with a much better chance to plant the field. If soybean is the next crop, you can plant glyphosate-tolerant soybean into green cereal rye then terminate the cereal rye with glyphosate later. This practice has become more and more popular on difficult fields.

Do not forget about soil fertility and plant nutrition for cover crops. A modest application of nitrogen will help cover crop establishment, plant water use, and competition with weeds, as cover crops with adequate nitrogen will grow faster and larger than those without nitrogen. Around 46 lb/acre nitrogen (100 lb/acre urea, 46-0-0) should be enough to establish nice cover crop growth. Prevented Planting fields, being wetter than those successfully planted in spring, lost some, if not most, soil nitrogen via nitrate leaching or denitrification. Although additional nitrogen may have mineralized from soil organic matter during May and June, excess precipitation in June may have caused additional soil nitrogen loss. The best way to know is collecting 0-12 or 0-24 inch soil samples for nitrate-nitrogen analysis.

As you choose the appropriate cover crop mix on Prevented Planting fields, you must consider the pros and cons of each cover crop species and how each will help accomplish your goals. These are some helpful resources that will provide additional information on what cover crop options will work best on your fields.