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The Critical Role Of Soil Organic Matter (SOM) In Reclaiming Phosphate Mining Clay Settling Areas.

There is an unquestionable consensus of scientific opinion that post mining clay settling areas [CSAs] have varying levels of "damaged or incomplete" soil characteristics:

  • Highly Compacted Soils (bulk density of clays).
  • Lack of free oxygen (to support many micro-organisms).
  • Elevated pH levels (very high levels of calcium).
  • Very little soils organics (~0.5% of Soil Organic Carbon).
  • Severe deficiencies in nitrogen available to plants.
  • Soil Structure Instability (to support farming equipment).
  • Extreme seasonal variations in top-zone moisture
        (modeling clay in Summer to brick-hard crusting in
  • Relative Sterility (lack of native flora seedbanks).

    As a result of these soil characteristics, CSAs are often invaded and then dominated by cogongrass (according to the USDA, the 3rd most destructive invasive weed in the world).

    However, "damaged or incomplete" does not mean "worthless" as CSA soils are very high in potassium (K) and phosphorus (P) -- two key components of productive soils. Also, there is a consensus opinion from soil science research that clays have a unique ability to be repaired relatively quickly compared to other soils types (e.g., clay's cation exchange capacity for nutrients, clay's binding capacity with organic matter, etc.).

    For decades, the issue of clay settling area soils has been at the forefront in the debate over phosphate mining in central Florida. For example, a report performed by the Central Florida Regional Planning Commission (CFRPC) applied the USDA's Natural Resource Conservation Service's soil rating index to post-mining clay settling area soils (which classifies various soil types on a scale of 1 to 10). The higher the number, the more limited the farming -- with soils rated 5 or higher generally unsuitable for farming. The CFRPC rated clay settling area soils as a 5.

    The Phosphate Mining Industry disputes this CFRPC ranking, citing the high nutrient levels of K and P in the heavy clay soils.

    So who's right? In discussing clay settling area soils with the lay public, we use an analogy of cake mix. While a box of cake mix contains many essential ingredients, its not a cake in and of itself. Other ingredients have to be added and the mix must be baked. In order for clay settling area soils to become productive and truly reclaimed for sustainable agriculture and/or environmental use, Organic Matter (OM) must be amended into the heavy clay soils, initiating physical, biological, and chemical processes in soil building/creation.

    The good news is that through field research performed by the University of Florida, the Common Purpose Institute, and the U.S. Department of Energy's Oak Ridge National Laboratory:

    Clay settling areas have demonstrated a remarkable ability to be repaired to productive levels in a very short timeframe when high levels of OM have been incorporated into the clays. [1]

    The bad news is that "Best Management Practices" have yet to be established to add the necessary high volumes of OM to the heavy clays in a cost effective way. This explains why the vast majority of clay settling area uplands have been "reclaimed" for low-value land use cattle grazing rather than higher-value land use agriculture crops. The unbiased marketplace also reflects this opinion, with CSAs having some of the lowest real estate values in central Florida.

    Thus, while it is technically possible that clay settling areas can be highly fertile soils (e.g., the Phosphate Mining argument), the commercial viability of adding needed OM to the heavy clays has not been established   -- resulting in these lands not being utilized for agriculture crops (e.g., the CFRPC's argument).

    Soil Organic Matter: For the purposes of this discussion, Soil Organic Matter (SOM) is defined as all organic materials found in soils irrespective of origin or state of decomposition. Since SOM consists of C, H, O, N, P, and S it is difficult to actually measure the SOM content and most analytical methods determine the soil organic carbon content (which can be readily measured) and estimate SOM through a conversion factor.[2] For clay settling area soils, this conversion factor is believed to be ~2.

    For simplicity, SOM can be divided into two major groups: Particulate Organic Matter (active fraction) and Humus (transitioning organic matter with increasingly stable/complex compounds).

    Particulate Organic Matter -- POM (active fraction): Many compounds (sugars, starches, certain proteins) in this SOM Group are quick and easy for fungi and bacteria to decompose, so the carbon and energy they provide is readily available. Most of the microbes living in the soil have the enzymes needed to decompose these simple compounds. Because of its short turnover time (usually a few months), many soil scientists refer to POM as the "organic fertilizer" property of SOM. An example of a way to increase POM in CSA soils is by planting legumes (clovers, alfalfa, soybeans, etc.) -- also called "green manuring".

    However, it is strongly believed that a single green manure crop is not going to sustainably address "damaged or incomplete" soil characteristics of clay settling areas. The vast majority of a green manure crop's soil benefits will be gone in a year or less. A long-term approach to OM management needs to be developed -- which includes humus accumulation.

    Humus (stabilized organic matter): This pool of SOM is resistant to biological degradation because it is either physically (e.g., lignins contained in woody biomass) or chemically (e.g., humic acids) less accessible to microbial activity. These compounds are more complex than fresh POM -- created through a combination of biological activity and chemical reactions in the soil. Humus is a critical component for the long term sustainability of a soils ability to provide plant nutrients, especially nitrogen[3]. Compared to the short half-life of POM, transitional humus can last in soils for years, with highly stable humus lasting up to centuries.

    SOM Functions; The functions of SOM can be broadly classified into three groups: Biological, Physical and Chemical. These groups are not static entities and dynamic interactions occur between these three major components.

    It is widely recognized that SOM plays an important role in soil biological (provision of substrate and nutrients for microbes), chemical (buffering and pH changes) and physical (stabilization of soil structure) properties. In fact, these properties, along with soil organic carbon (SOC), N and P, are considered critical indicators for the health and quality of the soil.

    Functions Ascribed to SOM & Interactions:

    Raising the Red Flag: A very significant obstacle in understanding clay settling area soils and developing reclamation/soil building techniques is their uniqueness compared to agriculture soils (where the body of soil science research has been performed). However, for typical clay settling area soils having SOC of ~0.5% (SOM content of ~1%) a red flag should be clearly apparent.

    For example, Körschens et al. (1998) proposed that lower and upper limits of total SOC for soils with different clay contents to maintain optimum crop production. For soils with 4% clay, the lower and upper limit was proposed to be at 1% and 1.5% and for soils with 38% clays (still significantly below that of clay settling areas) the respective limits were 3.5% and 4.4%.[4]

    However, irrespective of soil type it appears that if SOC is below 1%, it may not be possible to obtain potential crop yields with sustainability. Also, with SOC less than 2%, soil aggregates are considered unstable.[5]

    We believe that any CSA research, reclamation practice, or Regulation based on the postulate that CSA soils are "ready to go" after hydrology (drainage) efforts is an incomplete approach. While hydrology (drainage) is certainly an important factor in reclamation, soil science is quite clear that the resulting crusting is an indicator of poor soil health. Rather, a "bridge" is needed to repair the "damaged or incomplete" soil characteristics:

  • "Bridge Crops" (high density tree planting, legumes)
  • "Bridge Farming Techniques" (mulch matting,
          chemical amendments such as PAM, Humins).
  • Simply stated, if sustainable OM management can be achieved on CSAs -- end-use environmental (native flora and trees) and/or agriculture (high-value crops) applications will be greatly enhanced.

    In achieving this goal for OM management on what might eventually be over 225 square miles of CSA mined lands in Florida, we might also be surprised on what can be accomplished on other issues such as carbon sequestration (Global Warming), water quality (filtration and capture of nitrogen, phosphorus, and particulates), controlling invasive plants (e.g., cogongrass, Brazilian Pepper, Soda Apple, etc.) renewable energy (fuel crops for electricity production, ethanol, biodiesel), and rural economic development -- creating value-added benefits which have quantifiable economic value (e.g., selling carbon credits).

    Field Observations: In understanding the critical role of SOM in repairing clay settling area soils, its important to recognize that different groups, proportions of SOM, and even types of plants/residues (e.g., sources high in lignin content) achieve different soil functions. For example, while POM provides the critical energy component for microbial activity in all soil types, humus accumulation is much more critical in creating soil stability in clay soils compared to sandy soils.

    Importance of Different SOM Pools In Performing
    Different Functions In Different Soils.

    The following discussion is an "attempt" to explain and possibly link field observations that we've seen on our clay settling area treefarm in Lakeland, Florida to soil science theory.

    Soil Organic Carbon (SOC): One "bridge crop" approach used is high density tree planting (up to 4,000 trees per acre) of fast growing non-invasive Eucalyptus Grandis and Cottonwoods. Based on field research, ~10 tons of OM per acre, per year is being introduced into the heavy clays via the trees' root system using this specific "bridge crop" approach.[6]

    As previously stated, it is believed that post mining clay settling areas have typical SOC levels of ~0.5% (1.0% SOM). However, over a three year period since initial high density tree planting, dramatic changes in the clay have been observed. Soil color and texture has changed from pre-existing conditions of gray and plasticity to porous granulated dark soils ~4 feet in depth on treebeds. Soil analysis performed by the U.S. Department of Energy's Oak Ridge National Lab validates this observation:

    Soil Property:
    Soil Depth (cm)
    Avg. for Tree Areas
    Carbon %
    Carbon %

    Also, in comparing control areas (where no trees had been planted), nitrogen content has increased an amazing 1,150% during this 3 year period.

    Soil Property:
    Soil Depth (cm)
    Avg. for Tree Areas
    Nitrogen %
    Nitrogen %

    Interestingly, in areas where high tree density was not achieved (e.g., from high mortality at initial planting), soil characteristics have shown little, if any improvement. Also, surviving individual trees in these lower density areas show much lower growth rates compared to high density areas.

    Thus in achieving higher SOC levels, it is believed that there must be some threshold level of needed accumulation in OM to initiate a large domino catalytic effect (e.g., chemical, biological) in the heavy clays.

    It is uncertain what this threshold level is, but this observation raises questions on Florida Department of Environmental Protection (FDEP) mining reclamation regulations. Under existing Regulations, a minimum level of native trees and flora is required to be established before the CSA can be released. However, with low density per acre tree plantings of usually slow growing natives, it is questionable whether the necessary OM threshold level can be achieved.

    This comment is not intended to be critical of Lawmakers or the FDEP, but to raise a seemingly valid question of whether some threshold requirement for OM should be included in Reclamation Regulations.

    Soil pH: Soil samples prior to tree planting (or in un-planted areas) reflect a range of soil pH that is slightly alkaline (~7.6) to moderately alkaline (~8.2). This alkalinity is a result of the CSA soils being calcareous with a very high content of calcium carbonate.

    With respect to environmental reclamation efforts to restore native flora and trees to CSAs, the wide difference in pH levels on CSAs and native sandy soils should be noted. In environmental reclamation, there is perhaps no better soil indicator for the need of "bridge crops" to reduce pH to more acidic levels that native flora spent thousands of years adapting to.

    Comparison Between Florida Sand Soil
    and Phosphatic Clay (ppm)[7] :

    Soil Component:
    Typical Sand
    Phosphatic Clay
    7.6 to 8.2

    In high density tree planting areas, soil pH was reduced to a range of 6.5 to 6.8 in three years. Also, in field research conducted by the University of Florida, approximately 30 native species of flora have now appeared naturally in the tree understory -- where before tree planting, the CSA was a prairie of monoculture cogongrass.

    Thus, the theory of pH reduction is that through the introduction of approximately 10 tons of OM per acre per year via "bridge crop" trees' root systems -- humic acids and nitrogen (from microbial activity) dissolved calcium carbonates, lowering overall soil pH.

    Soil Compaction: A review of soil science literature indicates that clay particles normally lay together flat, but are repelled by the negative charges across their face. Salt (Na+) is present in minor amounts:

    When the percentage of clay in the soil is very high, the positive charge on the edge of a clay particle combines with the negative charge on the flat surface of another, forming a tight three-dimensional structure:

    As humic acid penetrates compacted clay platelets, it segregates salts (positive ions) and removes them from the clay particle surface. This restores a negative charge to the face of the clay platelets, causing them to repel each other[8] :

    As noted earlier, soil compaction has noticeably been reduced on treebeds within high density tree planting areas. It must also be noted that between treebeds (alleys), little or marginal improvement have been observed.

    Thus, a potential linkage to soil science theory is that on treebed areas, sufficient volumes of humic acids are being introduced to effect the ion charge on clay surfaces.

    Microbial Activity: Two soil characteristics of CSAs are their high compaction (bulk density) and swelling capacity in holding water -- resulting in extremely poor aeration. Many soil microorganisms involved in decomposition of OM are aerobic (oxygen requiring) and will not function well under oxygen-limiting (anaerobic) conditions. Moisture levels in clay soils often reach the saturation point leaving little room for oxygen needed by aerobic life.

    Through a "bridge crop" approach of high density tree plantings, it is believed that: (1) larger soil aggregates are being created via tree root system OM (increasing pore space) and (2) topzone soil is being de-watered by significant water uptake by trees. According to UF, the following soil/ground de-watering rates are occurring per acre per year:

  • 65 inches of water, or
  • 1,765,229 gallons of water, or
  • 7,334 tons of water.
  • Soil Structure and Soil Stability: Admittedly, soil structure is perhaps one of the most perplexing issues that we've encountered (second only to our inability to eliminate cogongrass). A review of soil science literature suggests that soils with SOC levels of 5% should achieve soil stability. However, while this SOC target was achieved in our specific "bridge crop" approach, CSA soil stability has not resulted. Four years after initial tree planting, soils would still not support farming/harvesting equipment (even using low ground pressure options).

    Understanding the components of SOM and their different soil functions may explain that while soil heath has unquestionably improved (color, tilth, drainage, etc.), necessary soil stability has not yet been achieved. Soil science appears to be very clear that soil stability for clays is largely driven by the proportion of the humus fraction in SOC -- much more so than stability in sandy soils.

    As the below graph illustrates, as clay content in soils increase, increasing proportions of humus is needed to provide optimal structural support through physical and chemical binding.

    Thus, a theory of why soil stability was not achieved is that while a SOC target of 5% was attained, the proportion of humus to the active fraction was too low.

    In the process of chemical and biological degradation, humus creation and accumulation just takes time. Its important to note that even under the best of conditions, a relatively small amount of humus is created in comparison to the level of OM initially introduced.

    However, several potential options appear to be promising in accelerating the time required to achieve soil stability:

  • Incorporation of high volumes of woody biomass residues
        (high in lignins) into clay soils.
  • Soil amendments such as PAM to increase soil pore space.

    Basically, lignins (highly resistant to biological degradation) are humus when first amended into soils. An example of our lignin soil stability research is mulch matting research -- where we are applying ~800 dry tons of OM per acre on CSA soils.

    (1) Enhancing Soil Carbon Sequestration on Phosphate Mine Lands in Florida by Planting Short-Rotation Bioenergy Crops; Wullschleger, Segrest, Rockwood Garten; Oak Ridge National Laboratory, June, 2004.
    (2) Organic Matter Management; University of Minnesota Extension Service; 2002.
    (3) Albrech, William; Loss of Soil Organic Matter and Its Restoration.
    (4) Functions of soil organic matter and the effect on soil properties, Krull, Skjemstad, Baldock; CSIRO, October, 2004.
    (5) Functions of soil organic matter and the effect on soil properties, Krull, Skjemstad, Baldock; CSIRO, October, 2004.
    (6) Based on tree yield estimates of 32 green tons per acre per year, times a below ground conversion factor of .63, times a moisture rate of .5.
    (7) Stricker, Jim; High Value Crop Potential of Reclaimed Phosphtic Clay Soil, Florida Institute for Phosphate Research.
    (8) Humic Acid Structure and Properties, Nutranetics ProBio Systems.