Lecture: Soil Properties
Objective
That participants gain a basic knowledge of soil properties pertinent to hydric soil assessment
References
Information and links provided on web page under "Soil Properties".
Sprecher, S.W. 2001. Basic concepts of soil science. p. 3-18. In Wetland soils: hydrology, genesis,
landscapes, and classification. J. Richardson and M. Vepraskas (eds.) . CRC Press, Boca Raton, FL.
Soil Survey Staff. 1993. Soil Survey Manual. USDA-SCS Agric. Handbook 18. U.S. Govt. Printing
Office, Washington, D.C.
I.
What is soil?
A point of departure in the understanding of soil properties is to "agree" upon what we are going to
call "soil". We will adhere to the following USDA definition for the purposes of this class:
USDA, 1998:
"Soil is a natural body comprised of solids (minerals and organic matter), liquid, and gases
that occurs on the land surface, occupies space, and is characterized by one or both of the
following: horizons, or layers, that are distinguishable from the initial material as a result
of additions, losses, transfers, and transformations of energy and matter or the ability to
support rooted plants in a natural environment."
Note that the phrase "...on the land surface" gets a little murky (no pun intended) along the
shoreline of a water body. This is particularly the case where the land gradient extending into the water
body is shallow and the shoreline expands and contracts large distances between wet and dry climatic
cycles (e.g., Orange Lake!). It is commonly accepted that areas of wetlands frequently covered by shallow
water should be considered soils if they support rooted plants. There is a growing interest in the processes
and properties of "subaqueous soils".
Note also that the lower limit of soil is not all that well established by the definition. By
convention, soils are normally classified based on a 2-m depth, though processes (additions, losses,
transfers, and transformations) can occur at depths greater than 2 m in some areas (Florida being such an
area).
Would you consider the following material to be soil? "Moon soil"; an artificial berm; potting
media; beach sand; dune sand; landfill cap.
You will discover that hydric soils near the upland-wetland boundary (where hydric soil
delineation is most critical and relevant) generally would fit the within the soil definition indisputably.
However, the wetland/water-body boundary (as alluded to above) is not always clear cut with respect to
what is soil and what is sediment.
2. What is the distinction between mineral- and organic soil material?
The relevant question is, how much organic C must be present for the properties of the soil to be
dominated by the influence of organic matter? There is not a discrete break when we are dealing with a
continuum, so the organic C amount must be arbitrarily set to differentiate what we would then define as
"organic soil material" and "mineral soil material". Likewise, the distinction between organic soils (the
order of Histosols in the USDA soil taxonomic system) and mineral soils (all other soil orders of the
system) is also arbitrary, and based on the relative thickness of what we define as organic soil material.
Mineral soils can have organic layers that don't meet the thickness requirements for Histosols. (more will
be said about soil taxonomy later).
The USDA has defined organic soil material as having 12% or more organic C if there is no clay,
and 18% or more organic C if there is 60% clay, with proportionate amounts of organic C required at
intermediate values. In effect, organic C ≥ 12% + .1 * % clay. Another important distinction relates to
degree of organic matter composition, which is reflected in the fiber content. Relatively un-decomposed
organic material tends to have a high fiber content, and are termed fibric if they have > 2/3 fibers by
volume, un-rubbed, by hand lens. Highly decomposed organic materials, on the other hand, tend to have
low fiber content, and are termed sapric if they have < 1/3 fibers by volume. Organic materials with
intermediate fiber content are termed hemic. Please note that organic horizons (designated as "O", see
section on horizons) are given subordinate designations (explained below) to indicate fibric ("Oi"), hemic
("Oe"), and sapric ("Oa") materials.
The distinction between organic- and mineral soil materials is given a lot of weight in hydric soils
assessment. It is a good idea for you to pay careful attention to the field discrimination. In addition, mineral
soil materials are further described as "mucky mineral" if they conform to the formula , organic C ≥ 5% +
.1 * % clay, and they will have a textural modifier designating that distinction (e.g., mucky fine sand;
texture will be addressed further in later sections) . The following graph depicts the relationship between
organic, mucky mineral, and mineral soil material. Technically, "mucky mineral" is still mineral soil
material, but a subset of it on the high end of organic C content.
3. What factors control soil properties?
A classical concept advanced by Hans Jenny called the "five factors of soil formation" has
withstood the test of time. Jenny's five factors - parent material, climate, organisms, slope, and time - still
elegantly capture the essence of soil genesis in most cases. It would be difficult to conceive of a soil whose
formation couldn't be explained by these factors. The factors are, of course, interactive. For example, a
more weatherable parent material could form mature soil horizons (layers formed by post-depositional
additions, losses, transfers, and transformations) in much less time than a resistant parent material under
the same climate. Also, climate has a major influence on vegetation (organisms), which in turn has a major
influence on soil formation (e.g., prairies soils of semi-arid climates differ markedly from forest soils of
humid climates.).
I sometimes wish that hydrology would be given the status of "soil forming factor". In Florida,
hydrology has a major influence on soils. However, it is generally argued that hydrology is covered already
by climate and topography.
4. Properties used in description
It is important for you, a prospective hydric soils specialist, to become very familiar with the
properties commonly used to describe soils, and the conventions for assessing these properties. We'll begin
with what is arguably the most pertinent property for assessment of hydric soils: color.
a.
Color
With color it must be emphasized from the outset - ALWAYS DETERMINE COLOR FOR MOIST
SOIL IN ASSESSING HYDRIC SOIL INDICATORS - NOT DRY - NOT WET - MOIST! Moisture has
a major effect on color, especially for soil layers with high organic matter. You will be shown in the field
how to adjust the moisture content of a soil sample to the "moist" state.
Wade Hurt will be covering the details of color determination using the Munsell color book.
However, I will also give you my own brief "spin" on color. Color is so important in soil interpretation that
a little repetition is probably beneficial. The Munsell notation characterizes color using 3 variables - hue,
value, and chroma, defined as follows:
.
Hue:
Dominant spectral color, e.g., red, brown, yellow, etc. (EX:" 7.5YR").
Value:
Degree of darkness or lightness; related to amount of light reflected; low value =
dark and high = light.(EX: "4").
Chroma:
Intensity of color; related to spectral purity; low chroma = almost gray (or white or
black); high chroma = hue readily evident. (EX: 4).
(Combined EX: 7.5YR 4/4)
These notations will become very familiar to you as you gain experience in describing soils. You will
immediately know, for instance, that a low value means "dark" (e.g., 10YR 3/2) and a high values means
"light" (e.g., 10YR 8/2). You will also recognize that the "2" indicates a low chroma and a gray
appearance. Hence, the notation 10YR 8/2 indicates a "light gray" appearance (high value, low chroma).
Color is a powerful interpretive property for hydric soils because it is influenced by organic matter,
Fe, and Mn which are in turn influenced by wetness. Saturation at or near the soil surface for long periods
favors the build up of organic matter, which imparts low value and chroma colors (e.g., 10YR 3/2) to the
surface horizon. Wetness retards the rate of organic matter decomposition, while generally providing for a
high rate of biomass production. Iron oxides are one of the most stable components in soils under
oxidizing conditions, but they are destablilized under reducing (wet) conditions and the dissolved reduced
Fe is subject to redistribution. Iron oxides, like organic matter, are a strong coloring agent in soils,
imparting high chroma colors with hues ranging from red to brown. However, reduced Fe is colorless.
Hence, the chemical reduction induced by oxygen depletion under saturated conditions (wetness) produces
or maintains gray (low chroma) colors by destabilizing or preventing the formation of Fe oxides.
Some additional points on the color influences of organic matter and Fe are worth noting. Both
organic matter and Fe tend to impart low chroma, but only organic matter imparts low value as well.
Hence, low chroma colors in the presence of high organic matter (indicated by low value) cannot be
unequivocally attributed to Fe. Generally, low chroma can be attributed to reduced Fe or Fe depletion at
values above 4. Note that I said reduced Fe OR Fe depletion! Either situation would produce low chroma.
More will be said about that when "redoximorphic" processes are discussed in greater detail. Dark or gray
colors don't always indicated high organic matter or Fe reduction. For example, these colors can be
inherited from parent materials of those colors. Also, even if the gray colors were the result of redox
depletions, they could be "relict" features from a past era when water tables were higher.
b.
Texture
"Soil texture" is the general term referring to particle size distribution, which is characterized by the
relative proportions of sand (0.05 - 2 mm), silt (0.002 - 0.05 mm), and clay (< 0.002 mm). Wade Hurt will
provide detailed instruction on soil textural classes and how to estimate them in the field.. However, I will
give you a little of my own spiel as well.
Texture is a different kind of property than soil color. Color, as I "see it", is a passive indicator of soil
conditions and processes. Texture, however, has powerful direct and indirect influences on a number of
characteristics, including hydraulic conductivity, moisture retention, capillarity, cohesion, resistance to
penetration, etc. Texture comes into play in hydric soil distinctions. For example, sandy soils are
distinguished from loamy and clayey soils by having a different set of indicators. HERE IS A HELPFUL
HINT: Once you determine that you are dealing with a sandy soil, you don't have to consider further the
indicators for loamy and clayey soils! This means that you should pay close attention during the texture
estimation exercise to the distinction between sand and loamy sand. Of course, you still have to consider
the indicators designated for "all soils".
Here is something else to ponder. When you read the "Hydric Soil Criteria" (covered by Wade Hurt)
you'll see that sandy soils are required to have saturated conditions at or above the surface during the
growing season, whereas finer textured soils can be hydric even if the saturated zone doesn't reach the
surface. What do you think is the reason for that distinction?
Now that I have played up the importance of soil texture, I will temper the impression with some
qualifications. Soil texture DOES influence hydraulic conductivity, but other factors come into play in
dictating the actual hydrological condition of the soil and landscape. For example, sandy soils may be
highly conductive and wet at the same time. That is because there are often sparingly permeable layers at
depths below the soil zone that back water up into the soil. In effect, the soil itself is not the rate limiting
material. Also, some soils with finer textures (more silt and clay) can have relatively high hydraulic
conductivity due to aggregation (soil structure, to be discussed next) and the macroporosity that results
from this aggregation. Therefore, it is not uncommon to have poorly drained sandy soils and well drained
clayey soils!
c.
Structure
Soil structure refers to the way that soil particles aggregate into larger units, with planes of weakness
between them. Individual aggregates are called "peds". Structure is assessed in terms of:
Shape - granular, blocky, prismatic, columnar, platy
Size - very fine, fine, medium, coarse, very course
Grade - weak, moderate, strong
Wade Hurt will elaborate on describing soil structure. I have already alluded to the way in which soil
structure can mitigate the effects of clayey textures on hydraulic conductivity. Strong structure at the soil
surface favors infiltration over runoff, and enables good root penetration.. Also, the distribution of
redoximorphic features (zones of Fe depletions and concentrations, to be discussed later) is influenced by
the "architecture" of peds in subsurface horizons. Structure is therefore an important soil property, but it
doesn't have much of a direct bearing on hydric soil assessments.
d.
Consistence
Soil consistence refers to the degree and kind of cohesion and adhesion; resistance to rupture and
deformation. The convention is to describe consistence for different moisture conditions, since moisture
have a big effect on the rheology of soils. The following consistence designations are used in soil
descriptions.
Wet condition: stickiness, plasticity
Moist condition: loose, friable, firm
Dry condition: loose, soft, hard
Cementation: weak, strong, indurated
Consistence is not directly a distinguishing property in hydric soils assessment. However, it can provide
clues about potential wetness. For example, cementation of a subsurface layer could hold up (perch) water
during wet periods.
5.
Soil Horizon Designations
A soil horizon is a "layer of soil, approximately parallel to the earth's surface, distinguishable from
adjacent layers ... by properties produced by the soil forming processes". They are delineated by one or a
combination of the morphological properties discussed above. Soil horizons are also important properties
of soils. Soil horizons are designated according to the prevailing soil forming processes that produce them.
Soil descriptions include a designation for master horizons, a subordinate designation which conveys
important supplemental information, and, when applicable, numeric distinctions for vertical subdivisions
within the same horizon. We'll begin by defining the master horizons.
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Master Horizons
O
A
E
B
C
(i) Dominated by organic matter (OM). EX: Leaves and twigs on forest floor; OM water-deposited or
accumulated through decay of marsh or swamp vegetation. > 12% organic carbon (OC) with 0 clay;
>18% OC with 60% clay.
(i) At the surface or below O. (ii) Has either OM accumulation or evidence of plowing, or both.
Doesn't meet organic requirements for organic (O) horizon. OM is intimately mixed with mineral
fraction.
(i) Loss of clay, Fe, and Al from weathering and downward translocation. (ii) Concentration of quartz
and other resistant minerals. (iii) Sand grains usually stripped and appear light in color, therefore this
horizon tends to have a lighter appearance than overlying and underlying horizons.
Obliteration of rock (or sedimentary) structure and any of the following:
(i) Illuviation (enrichment via translocation from overlying horizons) of silicate clay, Fe, Al,
carbonates, humus, gypsum, or Si.
(ii) Loss of carbonates.
(iii) Residual Fe or Al oxide buildup.
(iv) Coatings of Fe and Al oxides to give darker, stronger, or redder colors than adjacent
horizons.
(v) Alterations producing silicate clay or metal oxides and that result in soil structure.
Little pedogenic influence; lack of roots and soil structure.
R Hard bedrock.
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Now, consider the subordinate distinctions within master horizons:
(1)
Designated by lower-case letters following the master horizon.
Ex:
Bt, Bw, Ap
(2)
Some important ones to know for Florida soils: p, t, w, s, h, k, b, g, m, i, e, a
Ex:
Ap
- plowed or disturbed surface.
Oi, Oe, Oa
- i for fibric, e for hemic, and a for sapric organic soil material
Bt
- accumulation of silicate clays
Bw
- development of color or structure with little pedogenic clay accum.
Bh
- illuvial accumulation of organic C, with AL or Fe.
Bk
- accumulation of carbonates.
Btg
- gray colors due to anaerobic conditions and reduction of Fe.
Be aware that some of the processes indicated by subordinate designations are associated with
wetness, in particular - a, k, g.
If you need to subdivide a horizon, use arabic numerals at the end, e.g., [Bt, Btg1, Btg2]; [Bt1, Bt2,
Btg]; etc.
Transition horizons are named with letters designating both horizons for which they display some
characteristics, with the most strongly represented horizon written first. If the transition horizon is a mixed
homogeneous blend of the overlying and underlying horizon, only the two letters would be written, e.g.,
AE or EA. However, if the transition is more of a heterogeneous mixture of the two horizons, that case
would be designated as A/E or E/A.
Horizon boundaries are described in terms of thickness and topography, as given below:
Distinctness (thickness)
-----------Abrupt:
<2 cm
Clear:
2-5 cm
Gradual:
5-15 cm
Diffuse:
>15 cm
Topography
---------Smooth
Wavy (vertical displacement < horizontal periodicity)
Irregular (vertical displacement > horizontal periodicity)
Broken (boundary is interrupted by a 3rd layer)
Horizon boundaries have little bearing on hydric soil determination, but the information could be useful to
you in reading soil descriptions.