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.

References

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.

High soil pH and calcium carbonate inflate base cation saturation and cation exchange capacity (CEC)

Soil pH is a soil chemical property that measures soil acidity or alkalinity, and it affects many soil chemical and biological activities. Soils of the northern Great Plains and Canadian Prairies often have high soil pH (>7.3) and contain calcium carbonate (free lime) at or near the soil surface. It is the calcium carbonate in soil that maintains high soil pH and keeps it buffered around pH 8.0. The calcium carbonate originates from soil formation processes since the latest glacial period.

Soils with high pH and calcium carbonate create analytical challenges in determining cation exchange capacity (CEC) and subsequent base cation saturation ratio (BCSR) calculations. Cation exchange capacity is the amount of positive-charged cations (e.g. ammonium, calcium, hydrogen, magnesium, potassium, sodium) held on negative-charged soil particles, like clay and organic matter. Fine-textured soils (high clay content) and organic soils (high organic matter content) have high CEC, while coarse-textured soils (low clay content) have low CEC. The BSCR is the relative proportion of base cations in soil.

The routine laboratory method to determine CEC is the summation method, where all extractable cations on soil particles are added together. The assumption is that all positive-charged cations extracted are held on negative-charged exchange sites on soil particles. The assumption falls apart in soils with pH > 7.3 because the soil test method also extracts calcium from the naturally occurring soil mineral calcium carbonate, which is not held on cation exchange sites. The resulting amount of extractable calcium is inflated. The summation procedure still sums together the inflated calcium result, producing an inaccurate and inflated CEC result. Any subsequent base cation saturation calculations are similarly flawed.

To illustrate the inflated CEC problem, AGVISE Laboratories conducted a laboratory experiment. The experiment showed that more calcium carbonate in soil increased the amount of extractable calcium (Table 1). Furthermore, the inflated amount of extracted calcium also inflated the CEC because all the extractable cations are summed together. The correct CEC is 24 cmolc/kg, yet the inflated CEC could be 150 to 200% higher with increasing calcium carbonate content. Adding calcium carbonate to soil did not increase the inherent CEC sources, i.e. clay and organic matter, yet the laboratory CEC result increased. In reality, the ability to hold more cations did not change. This highlights the analytical challenge in determining CEC via summation method on soils with high pH and calcium carbonate.

Table 1. Effect of calcium carbonate addition on extractable cations and cation exchange capacity.
Calcium carbonate equivalent (CCE) pH EC Ca Mg K Na CEC
% dS/m —————– ppm —————– cmolc/kg
0 7.5 0.43 3350 730 220 45 24
1 7.6 0.51 6150 730 220 50 37
5 7.7 0.52 7480 720 215 45 44
10 7.8 0.50 7240 650 180 40 42
Abbrev.: pH (1:1); EC, electrical conductivity (1:1); Ca, calcium; Mg, magnesium; K, potassium; Na, sodium; CEC, cation exchange capacity via summation of cations.

The subsequent base cation saturation calculations were also affected, showing much lower percent potassium saturation as calcium carbonate content increased (Table 2). This presents a major challenge in using the base cation saturation ratio (BCSR) concept to guide soil fertility and plant nutrition on soils with high pH and calcium carbonate. Without an accurate analytical method, the BCSR concept loses its grounding and any practical application.

Table 2. Effect of calcium carbonate addition on base cation saturation ratio and cation exchange capacity.
Calcium carbonate equivalent (CCE) Ca sat. Mg sat. K sat. Na sat. CEC
% ————————– % ————————– cmolc/kg
0 71 26 2.4 0.8 24
1 82 16 1.5 0.6 37
5 85 14 1.2 0.4 44
10 86 13 1.1 0.4 42
Abbrev.: Ca, calcium; Mg, magnesium; K, potassium; Na, sodium; CEC, cation exchange capacity via summation of cations.

In general, the routine CEC method via summation of cations is quite accurate on soils with pH less than 7.3 and no calcium carbonate. However, the routine method has clear challenges on soils with pH greater than 7.3. The resulting CEC and subsequent base cation saturation calculations are inflated and/or flawed. This is why the BCSR concepts is extremely misleading on soils with high pH.

Adjusting high soil pH with elemental sulfur

Soil pH is a soil chemical property that measures soil acidity or alkalinity, and it affects many soil chemical and biological activities. Soils with high pH can reduce the availability of certain nutrients, such as phosphorus and zinc. Soils of the northern Great Plains and Canadian Prairies often have high soil pH (>7.3) and contain calcium carbonate (free lime) at or near the soil surface. It is the calcium carbonate in soil that maintains high soil pH and keeps it buffered around pH 8.0. The calcium carbonate originates from soil formation processes since the latest glacial period.

An unfounded soil management suggestion is that soil pH can be successfully reduced by applying moderate rates of elemental sulfur (about 100 to 200 lb/acre elemental S). Elemental sulfur must go through a transformation process called oxidation, converting elemental sulfur (S0) to sulfuric acid (H2SO4), a strong acid. Sulfuric acid does lower soil pH, but the problem is the amount of carbonate in the northern region, which commonly ranges from 1 to 5% CCE and sometimes over 10% CCE. Soils containing carbonate (pH >7.3) will require A LOT of elemental sulfur to neutralize carbonate before it can reduce soil pH.

To lower pH in soils containing carbonate, the naturally-occurring carbonate must first be neutralized by sulfuric acid generated from elemental sulfur. You can visualize the fizz that takes place when you pour acid on a soil with carbonate. That fizz is the acid reacting with calcium carbonate to produce carbon dioxide (CO2) gas. Once all calcium carbonate in soil has been neutralized by sulfuric acid, only then can the soil pH be lowered permanently. It is important to note that sulfate-sulfur sources, such as gypsum (calcium sulfate, CaSO4), do not create sulfuric acid when they react with soil, so they cannot neutralize calcium carbonate or change soil pH (Figure 1).

Figure 1. Soil pH following gypsum application on soil with high pH and calcium carbonate.

In 2005, AGVISE Laboratories installed a long-term demonstration project evaluating elemental sulfur and gypsum on a soil with pH 8.0 and 2.5% calcium carbonate equivalent (CCE). The highest elemental sulfur rate was 10,000 lb/acre (yes, 5 ton/acre)! We chose such a high rate because the soil would require a lot of elemental sulfur to neutralize all calcium carbonate. Good science tells us that 10,000 lb/acre elemental sulfur should decrease soil pH temporarily, but it is still not enough to lower soil pH permanently. In fact, this is exactly what we saw (Figure 2). Soil pH declined in the first year, but it returned to the initial pH over subsequent years because the amount of elemental sulfur was not enough to neutralize all calcium carbonate.

Figure 2. Soil pH following elemental sulfur application on soil with high pH and calcium carbonate.

A quick calculation showed that the soil with 2.5% CCE would require about 16,000 lb/acre elemental sulfur to neutralize all calcium carbonate in the topsoil. Such high rates of elemental sulfur are both impractical and expensive on soils in the northern Great Plains. The only thing to gained is a large bill for elemental sulfur. While high soil pH does lower availability of phosphorus and zinc, you can overcome these limitations with banded phosphorus fertilizer and chelated zinc on sensitive crops. All in all, high soil pH is manageable with the appropriate strategy. That strategy does not involve elemental sulfur.

Soil Salinity Analysis: Which method to choose?

This submission is courtesy of Dr. Heather Matthees, Research Soil Scientist, USDA-ARS, Morris, MN. It was originally published in the AGVISE Newsletter Fall 2017.

Salt-affected soils are a major problem for agricultural producers, resulting in $12 billion annual losses in crop production across the world. In the northern Great Plains and Canadian Prairies, soil salinity has always existed in some soils of the region, but the problem has become more widespread and severe since a hydrological wet period began in the 1990s.

Salinity is the overall abundance of soluble salts, which compete with plant water uptake and reduce crop productivity. The soluble salts pull soil water toward themselves in the soil solution, which leaves less soil water available for plant uptake. This causes an apparent drought stress, reducing crop productivity and sometimes may kill the plant. Soluble salts are naturally occurring and a product of regional geology in the northern Great Plains and Canadian Prairies. Since the 1990s, the hydrological wet period has raised the groundwater level and allowed saline groundwater to rise toward the soil surface, causing soil salinization. Saline soils are often called “salty,” “sour”, or “white alkali.”

The severity of soil salinity will control which plant species are suitable for crop or forage production. Some crop species like dry bean and soybean are very sensitive to salinity, whereas other crop species like barley and sunflower have good tolerance to salinity. For soils with very high salinity, the only practical forage option may be salt-tolerant perennial grasses. To assess soil salinity, there are two soil analysis methods: saturated paste extraction and routine 1:1 soil water methods.

Saturated Paste Extraction Method

The gold standard in soil salinity research is the saturated paste extraction method. The method requires a trained laboratory technician to mix soil and water into a paste, just reaching the saturation point, which is about the consistency of pudding. The saturated paste rests overnight to dissolve the soluble salts. It is then is placed under vacuum to draw the saturated paste extract. Soil salinity is then determined by measuring the electrical conductivity (EC) of the saturated paste extract.

The saturated paste extraction method is fairly straightforward, but it requires a trained technician, specialized equipment, and over 24 hours to complete the procedure. The procedure is labor intensive and difficult to automate, so it is considered a special analysis service in commercial soil testing. Therefore, it is more expensive than routine soil testing methods. Among soil salinity determination methods, it is considered the most accurate because the soil:water ratio at saturation controls for differences in soil texture and water holding capacity.

Routine 1:1 Soil:Water Method

The routine method for soil salinity assessment is the 1:1 soil:water method, which mixes standard mass of soil (10 g) and volume of water (10 mL) in a soil-water slurry. Soil salinity is then determined by measuring the electrical conductivity (EC) of the soil-water slurry. It is most commonly abbreviated EC1:1.

The method is fast and inexpensive (only 5-10% of saturated paste extraction cost). The low cost per soil sample allows a person to collect more soil samples from various soil depths and multiple locations within a field (e.g. zone soil sampling), which can create a more comprehensive and detailed soil salinity map to evaluate soil salinity presence, severity, and variability. Since soil salinity is so intimately related to soil water movement across the landscape, the soil salinity map also provides information about soil water accumulation and leaching, soil nutrient movement (e.g. chloride, nitrate-nitrogen, sulfate-sulfur), and crop productivity potential.

A general caveat about the 1:1 soil:water method is that the reported values will be lower than the saturated paste extraction method. Fortunately, the two methods are highly correlated. AGVISE Laboratories worked with soil science researchers at North Dakota State University and South Dakota State University to validate the correlation between the two methods using over 2,300 soil samples from the northern Great Plains. You can convert the two methods by multiplying the 1:1 soil:water result by 2.26 to estimate the saturated paste extraction result (Figure 1).

The simple method conversion enables you to quickly and cheaply monitor soil salinity using the 1:1 soil:water method and still utilize the historical soil salinity interpretation criteria based on the saturated paste extraction method.

Figure 1. Soil salinity method conversion between saturated paste extraction and 1:1 soil:water methods.

References

Matthees, H. L., He, Y., Owen, R. K., Hopkins, D., Deutsch, B., Lee, J., Clay, D. E., Reese, C., Malo, D. D., & DeSutter, T. M. 2017. Predicting soil electrical conductivity of the saturation extract from a 1:1 soil to water ratio. Communications in Soil Science and Plant Analysis, 48(18), 2148–2154.

Estimating soil texture with cation exchange capacity (CEC)

Soil texture is a basic physical soil property that describes the proportion of sand-, silt-, and clay-sized particles in soil. It controls the ability of soil to retain water and nutrients and the movement of water and nutrients through the soil profile. Soil texture is a fundamental soil property, but measuring soil texture requires time-consuming and expensive laboratory analysis procedures. An alternative option is estimating soil texture from cation exchange capacity (CEC), which is a rapid, routine procedure in commercial soil analysis.

Cation exchange capacity is the amount of positive-charged cations (e.g. ammonium, calcium, hydrogen, magnesium, potassium, sodium) held on negative-charged soil particles, like clay and organic matter. Since soil texture and CEC are intimately related, we can use each soil property to make predictions about the other. Soils with more large soil particles are coarse-textured (sandy), whereas soils with more small soil particles are fine-textured (clayey). Each soil particle has some surface area, which controls the CEC. Fine-textured soils have more soil particle surface area, so their CEC is greater (Table 1).

Table 1. Estimating soil texture from cation exchange capacity (CEC).
Cation exchange capacity (CEC), cmolc/kg Soil organic matter, % Particle size family Soil texture class
≤10 <20 coarse sandy
11-20 <20 medium coarse loam
21-30 <20 medium fine loam
>30 <20 fine clayey
>20 organic peat or muck

The soil texture estimation procedure is valid on soils with pH < 7.3 and no salinity. For soils with high pH or salinity, analytical challenges in routine laboratory CEC determination make soil texture estimation inaccurate.

Soil Science Review: Soil pH, Acidity, and Alkalinity

Soil pH is a basic soil property that affects many biological and chemical processes in soil. Simply knowing if a soil is acidic or alkaline can tell us a lot about how it behaves and how we can manage it. This is why soil pH is often called the master variable of biological and chemical reactions.

Soil pH is the activity of hydrogen ions (H+) in the soil solution, expressed on a logarithmic scale. A neutral soil has pH 7.0 and contains equal parts hydrogen (H+) and hydroxide (OH) ions. An acidic soil has more H+ ions. An alkaline soil has more OH ions. The relative acidity or alkalinity is shown in Table 1. In the laboratory, soil pH is analyzed using the 1:1 soil:water ratio routine method. Other soil pH methods include CaCl2, KCl, and saturated paste.

Table 1. Relative soil acidity or alkalinity range from soil pH.
pH (1:1) Relative Acidity or Alkalinity
≤ 4.4 Extremely acidic
4.5-4.9 Very strongly acidic
5.0-5.4 Strongly acidic
5.5-5.9 Moderately acidic
6.0-6.4 Slightly acidic
6.5-7.5 Neutral
7.6-7.9 Slightly alkaline
8.0-8.4 Moderately alkaline
8.5-8.9 Strongly alkaline
≥ 9.0 Very strongly alkaline

 

The optimal pH range for most plant species is near neutral or slightly acidic. In the optimal pH range, most plant nutrients are at or near their highest solubility in the soil solution. If soil pH is too low or too high, the availability of plant nutrients decreases; therefore, soil pH may be corrected with soil amendments or other strategies to mitigate reduced nutrient availability.

To demonstrate the importance of soil pH, let’s look at soil pH and aluminum. Aluminum is a natural component of soil clay particles, and it is insoluble above pH 5.5. In strongly acidic soils (pH < 5.5), aluminum solubility increases, so aluminum begins to dissolve and enter the soil solution. Soluble aluminum is very toxic to plant root growth and development, and it may cause reduced plant production or plant death. Soluble aluminum also binds with phosphate in the soil solution to create insoluble aluminum phosphate compounds, which then reduce soil phosphorus availability and plant uptake.

Soil acidity and aluminum toxicity is often the primary limitation of crop production in tropical and subtropical regions. Acidic soils are frequently amended with lime (calcium carbonate) to increase soil pH, improve nutrient availability, and increase crop production. On the glaciated plains of North America, soil acidity is not a common phenomenon. However, some localized areas of long-term no-till crop production on coarse-textured soils has produced more soils with very low pH (<5.0) and new aluminum toxicity problems.

Soil alkalinity similarly reduces the availability of plant nutrients in soil. In moderately alkaline soils (pH > 8.0), phosphorus binds with calcium to create insoluble calcium phosphate compounds, which then reduce soil phosphorus availability and plant uptake. Similarly, the micronutrients iron and zinc are less soluble. To improve nutrient availability in alkaline soils, farmers apply fertilizer in narrow bands. These bands decrease the volume of soil with which the fertilizer can react, thus keeping more nutrients available in the soil solution. It is generally uneconomical to lower alkaline soil pH to the optimal pH range in crop production.

Soil pH goes beyond inorganic soil chemistry. It also controls the biological activity of soil microorganisms that help create soil structure, cycle organic nutrients, and fix nitrogen in the nodules on legume roots. Soil pH also controls the degradation of many pesticides in soil. If there is something going on in soil, it probably starts with pH.

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.

Soil Science Review: Cation Exchange Capacity

Each year, AGVISE Laboratories delivers thousands of soil characterization reports with something printed on them called, “Cation Exchange Capacity (CEC).” Unless you have some background in soil science or surface chemistry, the number might be a mystery to you. Cation exchange capacity is the amount of positive-charged cations (e.g. ammonium, calcium, hydrogen, magnesium, potassium, sodium) held on negative-charged soil particles, like clay and organic matter.

Let’s start with the basics. Elements and compounds in soil usually exist as ions, which have either positive or negative charge. The positive ions are called cations. Some common positive-charged cations are calcium, magnesium, potassium, and sodium.

Soil particles have negative charge on their surfaces and edges. Since soil particles have negative charge and the cations have positive charge, the two are attracted together like magnets. If the positive-charged cations are held on the negative-charged soil particles, then the ion cannot leach through the soil profile with soil water. Therefore, the amount of positive-charged cations that are held on negative-charged soil particles is the cation exchange capacity (CEC). The CEC reporting units are centimole of charge per 1 kilogram soil (cmolc/kg) or milliequivalent per 100 gram soil (meq/100 g). The units are numerically equivalent, so a soil with CEC 100 cmolc/kg is equal to 10 meq/100 g.

Soils with high CEC are generally more fertile and can provide plants with more nutrients and water. A soil with high CEC (>25 cmolc/kg) can hold many cation nutrients and likely contains a high amount of clay and/or organic matter. A soil with low CEC (<5 cmolc/kg) cannot hold many cation nutrients, and it is likely sandy with little organic matter.

The CEC measurement can also provide information about the fate and transport of other charged compounds in the soil solution, like pesticides. Pesticides with positive charge are bound more tightly to soil particles if CEC is high. Sandy soils with low CEC often cannot hold onto positive-charged pesticides, meaning that pesticide may be prone to leaching.

There are different laboratory methods to measure CEC of soil. The most common method in commercial soil testing is the summation method, where all extractable cations on soil particles are summed together. The cations are extracted with ammonium acetate, then analyzed with atomic absorption spectroscopy (AAS) or inductively coupled plasma atomic emission spectroscopy (ICP-AES). The extracted cations are then added together to calculate CEC. The routine method works well on most soils; however, it does not do work well on soils with salinity or calcium carbonate (pH > 7.3). The ammonium acetate extraction also dissolves cations contained in soluble salts and calcium carbonate minerals. These cations are not held on cation exchange sites (i.e. cation exchange capacity), but they are still included in the CEC measurement, creating an inflated and inaccurate CEC result.

To obtain accurate CEC results on soils with salinity or calcium carbonate, the saturation-displacement method is appropriate. The method first saturates all cation exchange sites with one cation (either ammonium or sodium) and washes away all other cations. The second step displaces the target cation to obtain the accurate CEC measurement. The saturation-displacement method involves more work and cost than the routine summation method. You should talk with a soil scientist to help you decide which laboratory method is required to obtain an accurate CEC result.

Molybdenum: The Micro-est of Micronutrients

Molybdenum (Mo) is an essential plant nutrient, necessary for nitrate assimilation and biological nitrogen fixation. Legumes, relying on symbiotic nitrogen fixation, have greater Mo requirement than non-legumes. Nevertheless, the Mo requirement of plants is the lowest among all micronutrients, with critical deficiency concentrations ranging from 0.1 to 1.0 ppm in plant leaves. The very low Mo concentration lies near the detection limit for most laboratory instruments used in commercial soil and plant analysis, so you may see Mo concentration reported as “below instrument detection limit.”

Plant-available Mo in soil is present as molybdate (MoO42-). Unlike most other micronutrients, molybdate availability in soil increases with soil pH. On soils with pH greater than 6.0, Mo deficiency is exceptionally rare. In the northern Great Plains and Canadian Prairies where most soils have high pH, Mo deficiency is virtually unknown, and background plant Mo concentration in legumes ranges from 4 to 8 ppm, indicating that plants obtain sufficient Mo from soil naturally. In the upper Midwest where low pH soils are more common, crop response to Mo fertilization has been limited to legume crops grown on strongly acidic, sandy or peat soils.

Since Mo deficiency is so uncommon and most soils are limed above pH 6.0, no reliable plant-available soil test method for Mo has been developed in the region. The acid ammonium oxalate method was infrequently used in the southeast United States, but the prediction of crop response to Mo fertilization aligns more closely with soil pH than soil test Mo. If soil pH is less than 6.0 and Mo fertilization is necessary, a molybdate fertilizer seed treatment or foliar application is usually sufficient. Overapplication of Mo fertilizer is not a concern for grain production. In forage production however, overapplication is a serious concern because excessive Mo in forages can cause Mo-induced copper deficiency (molybdenosis) in ruminant livestock.

5 Things You Should Know About Phosphorus

1. The two accepted soil phosphorus tests in the North Central Region are the Olsen and Bray-P1 methods

The Olsen (bicarbonate) method is the standard soil P test in the North Central region. This method was developed to work on soils with low and high pH. The Olsen method works well in precision soil sampling, where the same field may have zones with acidic and calcareous soils. The Bray P-1 method is another accepted method in our region, but not always recommended. This method was developed in the U.S. Corn Belt, has a long history of soil test calibration studies and works well on acidic soils. The Bray P-1 method fails on soils with pH greater than 7, producing results with false low soil test P. Therefore, it has remained limited to the U.S. Corn Belt proper. The Mehlich-3 method was introduced as a multi-nutrient soil extractant. But like the Bray P-1 method, the acidic Mehlich-3 method does not perform well on calcareous soils; therefore, it has not gained approval by universities in the northern Great Plains and Canadian Prairies.

All soil P test methods are designed to predict the probability of crop response to P fertilization. The methods measure the plant-available P pool. Since the soil test method is an index of availability, the units are reported in parts per million (ppm) and ranked low, medium, or high based on university soil test calibration research. No soil P test method measures the actual pounds of available P in soil, they are only indexes of crop response.

 2. Most soils in the Northern Plains/Canadian Prairies region could use more phosphorus

Soils in the region are naturally low in P and historical P fertilizer use has been low, relative to crop P removal. As a result, many areas in the region still have low soil test P (below soil test critical level of 15 ppm Olsen P) after many decades of crop production. In other words, most farmers are not over-applying P. In fact, soils with low soil test P should receive moderate to high rates of fertilizer P each year to achieve good crop yield and maximize profitability.

Figure 1. Map developed using AGVISE soil test data. AGVISE has created regional summaries like this for the past 40 years. Check out the summary data for Montana and Canada and summaries of other nutrients and soil properties here.

3. You should use starter phosphorus fertilizer

Starter fertilizer placed near, or with the seed, is critical for crops like corn and wheat, regardless of soil test P level. A P fertilizer band placed near the seed will ensure soluble P near developing plant roots and results in vigorous early season growth, which is important in cold, wet soil conditions. Placing P fertilizer in bands also improves P use efficiency, especially in soils with relatively low or high pH. Phosphorus availability is greatest near soil pH 6.5. Since changing soil pH is difficult and costly, fertilizer P use efficiency is more easily improved with application in fertilizer bands to reduce the volume of soil involved in P fixation reactions.

4. Phosphorus source doesn’t really matter

No matter the starting material, all P fertilizers go through the same chemical reactions in the soil. It does not matter if the fertilizer starts as a poly-phosphate or ortho-phosphate. Within about one week in the soil, all P fertilizer sources react to form lower solubility compounds. What is more important than source is the placement of the fertilizer to increase availability (banding) and the rate of actual P fertilizer applied.

5. Phosphorus can be an environmental concern

Phosphorus entering surface waters can create algae blooms and fish kills. Since P is not mobile in soil, the P leaching risk is very low. However, P does move to surface waters with soil particles when erosion occurs. In cold climates like those on the northern Great Plains and Canadian Prairies, dissolved P released from vegetation can move with snow melt to surface water.

For more information about phosphorus and its reactions in soil, explore the links below:

Understanding Phosphorus in Minnesota Soils (Univ. Minnesota)

Understanding Plant Nutrients: Soil and Applied Phosphorus (Univ. Wisconsin)

Phosphorus Facts: Soil, plant, and fertilizer (Kansas State Univ.)