This overview of soil testing covers soil organic matter (SOM), mineral analysis, and cation exchange capacity (CEC) in detail.  When managing soil fertility, these three metrics are essential for understanding the health of a soil.  Additionally, Ocean Agro LLC is interested in these metrics as they have specific relevance to humic acid products and their impact to soil characteristics upon application, including our new formulation Dirt MD.

Soil Organic Matter (SOM)

Soil organic matter is the total accumulation of living, nearly dead, and completely dead (humus) components of soil.  For more information SOM, please reference our Overview of SOM composition, distribution, and content.

According to a great review of soil organic matter testing from the University of Delaware’s agriculture extension office, organic matter determinations are usually obtained through two main approaches:

1. Weight loss method: removes organic matter from the mineral fraction. The weight of the mineral remainder is compared to starting concentrations to calculate SOM levels.  There are three separate approaches to obtaining the desired weight loss.
   1. Oxidation with Hydrogen Peroxide (H₂O₂)
   2. Ignition
   3. Ignition after decomposition of silicates with Hydrogen Fluoride (HF)
2. Quantify specific compounds method: Another approach is to infer organic content based on levels of specific compounds (namely Nitrogen and Carbon) that are known to occur in relatively consistent percentages within SOM.
   1. Nitrogen estimation of organic matter by determination of total nitrogen is not widely used because of the relatively wide variation in nitrogen concentration of organic materials from different sources.  For the purposes of this discussion, we will not go into elemental Nitrogen estimation methods.
   2. Carbon levels are much more consistent in organic matter and are widely used for determining SOM. There are three key methods:
      1. Dry combustion and measurement of evolved Carbon Dioxide (CO₂) (after removal of carbonates)
      2. Chromic Acid (Cr₂O₇^2-) oxidation and measurement of evolved CO₂ (after carbonate removal)
      3. Chromic Acid oxidation to measure easily oxidized material (2 approaches: applying external heat or only using spontaneous heating from the acid dilution process).

When comparing the various approaches to determining SOM, the weight loss methods listed above are more error prone but less toxic.  For the purposes of this discussion, we will focus primarily on the chemistry-based approaches, as the toxicity dangers are manageable through laboratory protocols.

Dry combustion and measurement of evolved CO₂ is the most accurate method of inferring SOM content.  This is due to the chromic acid procedures not oxidizing the carbon in graphite and coal, leaving them out of the calculations.  Unfortunately, the combustion and measurement of evolved CO₂ requires special equipment and is not well adapted for rapid analysis of a large number of samples without large investments in automated and computerized carbon analyzers.  As a result, the chromic acid methods are more commonly used by soil testing labs than dry combustion.  Within the chromic acid approaches, the approach that measures evolved CO₂ is expected to be as problematic as well.  The alternative to the measuring the CO₂ expelled from the system is to measure the amount of chromic acid that reacts with organic carbon.  This is generally considered to be the most effective method of measuring SOM. When reacting chromic acid with the SOM sample, one can either apply direct heat or allow only for spontaneous heating from the acid dilution.  Applying external heat to the system is the method of choice, as it more quickly and thoroughly cooks the samples, more effectively measuring easily oxidized material.

Measuring easily oxidizable Soil Organic Matter (SOM) by quantifying chromic acid activity:

Chromic acid reacts readily with both organic carbon and organic hydrogen found in SOM (organic hydrogen accounting for the carbon lost when carboxylic acid reactive groups in SOM break down and evolve CO₂ – leaving organic hydrogen groups) .  The results of these reactions are used to quantify SOM by determining the amount of chromic acid used up in the reaction (difference of initial and final amounts).  In order to quantify the final amount of chromic acid present in the reaction, the excess chromic acid is quantified through a reaction with Ferrous iron (Fe²⁺).

Testing Methodologies in Practice – converting the organic carbon counts to SOM levels:

Once you are able to measure easily oxidizable organic carbon, the testing values must be converted to SOM levels.  Typically, two established average ratios can be used to derive SOM:  easily oxidizable organic carbon as a percent of total carbon (77%), total carbon as a percent of SOM (58%).  Thus, easily oxidized carbon makes up 44.7% of SOM samples. The equation for calculating SOM from the measured organic carbon is as follows:

% of SOM present = Measured EOOC x EOOC as % of Total Carbon {Avg. value ~ 1.30} x Total Carbon as % of SOM {Avg. value ~ 1.72}

Since both of these factors are averages from a range of values, the best you can get is a derived value of SOM, not a direct measurement.  While this method of inferring SOM is practical, keeping in mind that they are derived values and confirming them through other methods is prudent.

In practice, a standardized protocol for using the chromic acid approach discussed above for SOM testing is the Walkley-Black method (procedure found here).  In addition to the full process, a rapid colormetric method can be used for route testing, with the more complex Walkley-Black method being used to create reference standards.

Mineral analysis

Another important metric related to SOM and its CEC activity is the change in trace soil mineral levels.  Mineral Analysis is essential to understand how levels of micronutrients and toxins are effected by humic applications. Specially, monitoring uptake levels in the plant systems and assessing the stability of various micronutrients in the soil provides potentially useful information.

Standard Method for Micronutrient Analysis: EPA Method 3050 and the updated version 3050B:

Essentially, the 3050B methodology involves two separate sample prep procedures that are then can be run on two different pieces of analytical equipment per approach (see full 3050B for explanation of procedure).

1. Flame atomic absorption spectrometry (FLAA) or inductively coupled plasma atomic emission spectrometry (ICP-AES).
2. Graphite Furnace AA (GFAA) or inductively coupled plasma mass spectrometry (ICP-MS).

Each procedure set (regardless of the equipment selected) can quantify the levels of various elements present in soil, sediments, and sludge samples.   The table below outlines the elements quantified from each procedure set.

Table 1: Elements tested in both protocols for EPA 3050B mineral testing guidelines

In the table above, the toxicity rank and Geometric Mean Maximum Concentration (GMMC) for soil (mg/kg) is from the ATSDR 2011 List of Hazardous Substances.  These values are included to provide an objective measure of toxicity of the various compounds.  Toxicity is ranked from the most toxic (Arsenic at 1) to the most benign (large numbers of compounds are tied for 699 and 783).  GMMC is a measure of the average maximum concentration found in soils, thus providing an upper limit for safe level benchmarking.

All things being equal, the FLAA/ICP-AES procedure possess a wider range of elements covered.  The bottom two elements listed under GFAA/ICP-MS are the two elements tested that are unique to the methodology.  Among these elements measured only by the GFAA/ICP-MS method, Arsenic is a key toxin that may be of interest.  Recently in 2006, herbicides containing organic arsenic were denied registration by the EPA, these compounds include MSMA, DSMA, CAMA, and Cacodylic Acid.  The main rational was fear that the organic arsenic compounds would convert into the more toxic inorganic form.  This could present a rational for discounting the importance of measure arsenic in soil samples and promotes the FLAA/ICP-AES process as the preferred method.

CEC (Cation Exchange Capacity) and Base Saturation

In soils, cation exchange capacity is a key indicator of the general chemical state.  It is due principally to finely divided aluminosilicate minerals and natural organic matter (chiefly humic substances).  Cation exchange in soils is discussed in detail by Bolt (1982).

The content of a soil’s negative charges is defined as the cation exchange capacity (CEC), which has units of equivalents per gram dry soil.  The CEC of constant-charge soil (soils where minerals are the dominant source of negative charge – those containing aluminosilicates) depends on the surface charge densities (eq m^-2), the surface areas, and relative amount of the aluminosilicates.  Values of CEC for mineral soils dominated by aluminosilicates are generally on the order of 10^-4eq (g dry soil)^-1.

“Soil organic matter consists of a variety of entities, among which humic and fulvic acids contribute by far the most to the cation-binding properties.”(Tipping 2002)  Peat soils contain nearly total organic matter, 20% of which is typically present as humic acids, has CEC charge is approximately 10^-3 eq (g dry soil)^-1 at neutral pH, which as an order of magnitude greater than typical aluminosilicate-dominated soils.  Essentially, organic matter is an critical component of soil cation exchange capacity (CEC).

For soil containing less than 100% organic matter, the cation exchange capacity increases with organic matter content (Sposito, 1989).  The importance of organic matter is determining cation exchange capacity has been known for well over a century (Hargrove & Thomas, 1981).

Additional Resources on CEC:

Here is an additional overview of CEC and base saturation testing from the team at Soilminerals.com.

Additional Tests

• Macronutrients
• pH, Salinity and Presence of Lime
• Soil Texture Analysis: sand, silt and clay percentages and gravel
• Sieve Analysis
• Water percolation rate

References

Hargrove, W. L., G. W. Thomas, J. A. Ryan, V. V. Volk, and D. E. Baker. “Effect of Organic Matter on Exchangeable Aluminum and Plant Growth in Acid Soils.” Chemistry in Soil Environment. Ed. R. H. Dowdy. Madison: American Society of Agronomy, Soil Science Society of America, 1981. 151-66. Print.

Harmsen, K., and G. H. Bolt. Movement of Ions in Soil, II. Ion Exchange and Dissolution. 2nd ed. Vol. 28. Elsevier, 1982. 103-16. Geoderma. Web..

Schulte, E. E., and Bruce Hoskins. Recommended Soil Testing Procedures for the Northeastern United States. 3rd ed. Vol. 493. Northeast Coordinating Committee for Soil Testing, 2011. Web.

Sposito, G. The Chemistry of Soils. New York: Oxford UP, 1989. Print.

Tipping, Edward. Cation Binding by Humic Substances. 12th ed. Cambridge: Cambridge UP, 2002. Cambridge Environmental Chemistry Ser. 2002. Web. 22 May 2012..

United State of America. Environmental Protection Agency. Environmental Monitoring System Laboratory. By Kenneth Edgell. Web. 22 May 2012..

United State of America. Environmental Protection Agency. Environmental Monitoring System Laboratory. Web. 22 May 2012..