31 May 2015
Uncovering a Soil Mystery Using Micromorphology and Petrography
Posted by John Freeland
This is the second in a series (link to 1st) about the genesis of the Success soil, which was the topic of my master’s research. This part has to do with using petrography to identify soil constituents and examine soil fabric to help understand soil forming processes in a particular case. Soil fabric consists of soil plasma and skeletal grains, which can be distinguished under magnification. W.L. Kubiena was an early pioneer of examining soils thin sections under polarized light microscopes. More about that in a bit, but first, some background on soil texture.
Grain Size Ranging Over Six Orders of Magnitude
Soils in the environment have a wide range of particle sizes (particle size distribution) from colloidal clays and organics to boulders variously categorized as the fine earth fraction consisting of grains 2 mm in diameter or less, and rock fragments made up of larger pebbles, cobbles and stones.
Word-wide, there are many classifications that and differ somewhat on the size limits used to define each grain size. Here in the United States, the USDA defines them according to the following diameters:
Soil Separates in the Fine Earth Fraction
Very coarse sand (2.0 – 1.0 mm)
Coarse sand (1.0 – 0.5 mm)
Medium sand (0.5 – 0.25 mm)
Fine sand (0.25 – 0.10 mm)
Very fine sand (0.10 – 0.05 mm)
Silt (0.05 – 0.002 mm)
Clay (less than 0.002 mm)
Recognizing the importance of soil colloids and particle surface area to mass ratio in soil chemistry, the clay fraction can be further subdivided into even smaller categories this way:
Coarse clay (0.2 – 2 µm = 0.0002 – 0.002 mm)
Fine, or “colloidal” clay (0.02 – 0.2 µm)
Fine colloidal clay (0.002 – 0.02 µm)
Ultrafine clay or large “molecule” (less than 0.002 µm)
It’s All About the Colloids
The importance of colloids in soils can not be overstated. Because the surfaces of colloidal particles (be they inorganic clays, organic humus, or some combination of the two) have molecular boundaries with “ragged edges” consisting of unsatisfied static electrical charges, they control the availability of ions in the soil solution.
Colloids control the mobility of ions including beneficial plant nutrients, as well as ions such as heavy metals, pesticides and excessive fertilizers that can be nefarious actors in the environment. Scientists have had some understanding these effects on soil solutions since the mid-nineteenth century (Way, 1850).
Colloids, through flocculation, also determine soil structure. The mystery surrounding the Success series (and a significant aspect of my masters thesis) had to do with finding out the material that was responsible for the massive indurated structure of an otherwise sandy soil. But aside from solving the soil cement question, the larger goal was to understand the processes of weathering and soil formation. That requires not only knowing what materials make up a structured soil, but also how the arrangement or “architecture” of the soil affects the availability of water and ions in solution, and the role of soils in the broader environment. Again, from Jenny (1980):
“Kubiena pioneered soil micromorphology by impregnating small soil slabs with liquid resin, cutting thin sections from the hardened material, and observing pores, cavities, and channels microscopically in polarized light. The often vividly colored microscope images unfold a vast domain of microarchitecture…”
Cutans in Thin Section
The term “cutan” was coined by soil micromorphologist Roy Brewer. Cutans are coatings, including “clay skins” that coat coarser soil grains and corm linings in pore spaces within the soils. Cutans are made of soil colloids, including organic humus, clay, and sesquioxides that become deposited in soils by transfer mechanisms, particularly illuviation Cutans may be simple, comprised of one kind of material, or compound with multiple contrasting layers indicating separate phases of weathering and accumulation. Cutans made of specific materials are referred to by adding “ans” to a prefix designating the material. For example, “argillans” are cutans made of clay.
Allans and O-sesquans in Thin Section
Allophane is recognizable in thin section as a yellow isotropic substance coating grains or filling voids (Farmer et al. 1985). The O-sesquans appear as dark brown grain coatings or void fillings. Based on appearance of the cutans in our samples and supported by chemical analysis described (here) the cutans looked to be made of allophane and/or imogolite (allans), or organo-sesquioxides (o-sesquans). The following micrographs depict soil with and without cutans.
The compound cutans strongly suggest a sequential two-phase weathering and transport process with an initial inorganic phase depositing the allophane and a subsequent process dominated by organics. Tere are two (at least) competing theories to explain soil development in Spodosols (podsolization), one emphasizing the dominance of inorganic chemical processes (Farmer et al. 1985) and another “organo-chelate” model having organic substances as the key mobilizer of metallic ions (Schnitzer and Skinner 1964). Our finding suggest they might both be right. More about that later, in part 3.
Brewer, R. 1976. Fabric and Mineral Analysis of Soils. Robert E. Krieger Publishing, Huntington, NY.
Farmer, V.C., W.J. McHardy, L. Robertson, A. Walker, and M.J. Wilson. 1985. Micromorphology and sub-microscopy of allophane and imogolite in a podzol Bs horizon: evidence for translocation and origin. Journal of Soil Science 36:87-95.
Freeland, J.A. 1992. Soil Genesis of the Success Series, a Typic Haplorthod with Cemented Subhorizons (Masters thesis), University of New Hampshire, Durham.
Freeland, J.A., and Evans, C.V. 1993. Genesis and Profile Development of Success Soils, Northern New Hampshire. Soil Science Society of America Journal 57:183-191.
Jenny, H. 1980. The Soil Resource: Origin and Behavior. Ecological Studies #37. Springer-Verlag, New York.
Kubiena, W. 1938. Micropedology. Collegiate Press, Ames, Iowa.
Schnitzer, M. and S. I. M. Skinner, 1964. Organo-metallic interactions in soils: 2. Reactions between different forms of iron and aluminum and the organic matter of a podzol Bh horizon. Soil Science 98:181-185
Way, J.T. 1850. On the power of soils to absorb manure. Journal of the Royal Agricultural Society of England 11:313-379.