|
Based on their formation, minerals are grouped into two broad classes: primary minerals and secondary minerals. Primary minerals have not been altered chemically since the time of their crystallization from molten lava and their subsequent deposition. The Bowen reaction series chart (Figure 1) lists several primary minerals in sequence based on resistance to weathering. The lower the minerals fall on the chart, the more they resist weathering. Secondary minerals form from the decomposition of primary minerals and a subsequent reprecipitation into a new, chemically distinct mineral. Layer alumino-silicates are the dominant minerals formed in most temperate region soils. These layer silicates are composed of various arrangements of silicon/oxygen sheets in tetrahedral coordination and aluminum/oxygen sheets in octahedral coordination. Kaolinite is composed of one silicon/oxygen tetrahedral sheet and one aluminum/oxygen octahedral sheet and therefore is called a 1:1 mineral. Kaolinite forms in warm to hot, subhumid to humid climates. This mineral crystallizes in acid soil where basic cations (positive ions) and some silicon have been leached. Vermiculite is a 2:1 mineral with two silicon tetrahedral sheets surrounding one aluminum octahedral sheet. It forms in subhumid to humid soils high in mica. Hydrous mica (illite) forms in subhumid cool areas as mica dissolves and recrystallizes. Smectites, including montmorillonite, form in arid to humid soils with low permeability and minimal leaching. As primary minerals dissolve, leaching does not remove their constituents and they are available for recrystallization as smectites. Illites and smectites are 2:1 minerals. The mineral chlorite (2:2) forms in marine sediments exposed to weathering. Within a warm to hot, subhumid to humid climate, well-drained soil containing parent minerals high in magnesium fosters chlorite formation. Iron and aluminum oxides and hydrous oxides, collectively called sesquioxides, dominate soils in the humid tropics. Sesquioxides form in hot wet regions where soils are subject to excessive weathering. High precipitation is necessary to leach silicon and basic cations from the soil leaving the relatively insoluble iron and aluminum compounds. A difference in the ratio of secondary minerals present in two soils with the same parent material indicates a difference in weathering intensity. As a soil becomes more intensely weathered, minerals with a ratio of two silicon tetrahedral sheets to one aluminum octahedral sheet (2:1) are converted to 1:1 minerals (one tetrahedral to one octahedral sheet). Still greater weathering converts 1:1 minerals to sesquioxides. Table 1 shows the sequence of clay mineral distribution as weathering increases. Soil formation is a dynamic process with material continually added, transformed, and/or removed. Beginning with soil from a relatively uniform parent material, windblown sediments and annual floods, for example, add new material to the surface. Physically weathered saprolite adds material to the bottom of the profile. Dissolved and suspended material can be deposited or redistributed within a soil profile by water flowing below the surface. Evaporite minerals commonly accumulate in the subsoil at the top of a water table. Developmentally, a high annual sediment input often characterizes young soils. At the other extreme, material is continually removed from a soil. Erosion by wind and water, even under dense vegetation, can remove five tons of soil per acre, per year. Whenever precipitation exceeds the field capacity of a soil, material can be leached below the soil solum (the root zone or an area active in soil pedogenesis). Biochemical degradation can remove organic matter. This can lead to a significant reduction in soil volume. Changes also occur within a soil profile. Material is converted from
one form to another and translocated within a profile. Clay, organic matter,
and iron/aluminum ions typically migrate out from surface horizons especially
in humid-region soils. The root zone provides a good environment for biogeochemical
activity. Two examples of this are melanization and gleization. Melanization
is the darkening of a soil layer by organic matter. This process gives
the surface (A horizon) its brownish color. Gleization is the reduction
of mostly iron-bearing minerals. It produces a gray to greenish color in
soil. Saturation of a soil layer for long periods within a year usually
causes this condition.
Soils are categorized by certain physical and chemical characteristics. Many physical characteristics, including color, texture, and structure, can be determined in the field through careful observation and hand manipulations. Others, such as bulk density, require simple laboratory procedures. The most obvious soil characteristic is color. Although color is not used as a quantitative measure, it does give a good indication of certain conditions. A black to dark brown color usually suggests staining with organic matter. Red indicates the presence of oxidized iron and is normally found in well-drained soils. In soil saturated for long periods during a year, oxides become reduced, yielding a gray or bluish gray color. Soil color is described by three attributes: hue, value, and chroma. Hue Hue is the dominant spectral color. It is related to the wavelength of light reflected by soil particles. Common soil colors are white, gray, black, yellow, brown, red, and their various mixtures. Value Value is the lightness or darkness of the color. It is a measure of the amount of light reflected. Since moisture affects how light is reflected, normally soil color determinations are reported at three different moisture contents. Chroma Chroma is the strength or purity of color. It indicates the degree of difference between white, black, or neutral color. ¤¤¤ Munsell Soil Color Charts Soil color is characterized by comparison to the Munsell Soil Color Charts, which contain several series of distinctively colored chips (Figure 2). Each page represents a different hue. The Munsell book normally has 15 pages, each with a number (10, 7.5, 5, or 2.5) followed by a letter or letters indicating red (R), yellow (Y), green (G), blue (B), or combinations of these. For example, the 10 Y/R page contains color chips yellow-red (Y/R) with more yellow than red (10). Value units range between 0 and 10. The numbers ascend vertically on the page from the lowest to highest numbers, indicating dark to light values. Thus, a 0 value is black with no light reflected, while 10 is white with maximum light reflected. Chroma units are arranged horizontally across the page from 0 to 10, increasing in numbers from left to right. Low numbers indicate an increase in grayness, while high numbers signify a pure color with little mixing with other hues. Hence, a designation of 10R 6/4 indicates a hue of 10R, a value of 6, and a chroma of 4. On careful observation, most soils contain more than one color. Therefore, the matrix or dominant background color and mottles or colors different from the background must be described. While the matrix is simply described by a Munsell number, the mottles must be described by their abundance, size, and contrast to the background.
Abundance is the relative amount of mottling. It is described by three classes. Mottles that occupy less than 2 percent of the exposed horizon are classified as few; 2 to 20 percent as common; and more than 20 percent as many. Size is a measure of the estimated average diameter of individual mottles along their greatest dimension. Mottles less than 5 mm in diameter are classified as fine; 5 to 15 mm as medium; and greater than 15 mm as coarse. Contrast is an indication of the relative difference in color between the matrix and mottles. If the contrast in color is only recognizable after close examination, it is classified as faint. A distinct pattern is readily seen although not striking. It may vary one or two hues or several value or chroma units. Mottles are considered prominent when they are the outstanding feature of the horizon. The colors of the matrix and mottles are separated by several units of hue, value, and chroma. Texture is the relative percentages of sand-, silt-, and clay-sized particles in a soil. It is a soil's single most influential physical property. Texture influences soil permeability, water infiltration rate, porosity, and fertility. Soil particles are classified into one of three groups based on size (diameter): clay (<0.002 mm); silt (0.002 to 0.05 mm); and sand (>0.05 mm) (Figure 3). In addition, larger objects may be described as pebbles (2 to 75 mm); cobbles (75 to 250 mm); stones (250 to 600 mm); and boulders (>600 mm). These soil particle size boundaries are not totally arbitrary, as they roughly match changes in properties associated with the differing size fractions. Chemically, sand- and silt-sized particles are relatively inert. They differ in that sand is large enough to resist erosion by wind. Sand-sized particles are predominantly quartz (SiO2) with small amounts of silicate-based primary minerals. Feldspars, hornblende, and micas may total up to 20 percent of the sand fraction in soil. Sand tends to have angular rough surfaces, whereas silt is spherical and more polished. Silt also is predominantly quartz with slightly larger amounts of primary minerals and iron and aluminum oxides. Wind easily erodes the smaller silt grains. Clay particles are chemically active and stick together in aggregates that resist wind erosion and increase soil porosity. The clay fraction in most temperate region soils is dominated by layer alumino-silicate minerals. In the humid tropics, where weathering is more intense, iron and aluminum oxides and hydrous oxides are the dominant minerals present. The USDA has specified twelve different textural classes of soil based on particle-size distribution. The textural class can be determined with any two particle size groupings. For example, using the triangle illustrated in Figure 4, the classification of a soil with 30 percent clay and 10 percent silt would be determined in the following way:
The triangle in Figure 5 illustrates the relationship of texture and size, which is further explained in the following paragraphs. Sand Sand is the largest textural class. Sandy soils are dominated by the properties of sand: weak structure, rapid infiltration rate, slight erosion potential, loose consistence, and low fertility. When the soil is moist and molded into a ball, it will easily crumble when touched (Figure 6). Sands contain 85 to 100 percent sand, 0 to 15 percent silt, and 0 to 10 percent clay. Sand is further divided into the following four categories.
More than 25 percent of sand particles are 0.50 mm diameter in size or larger, and less than 50 percent are between 0.05 and 0.50 mm. Twenty-five percent of the particles are larger than 0.25 mm. Less than 50 percent measure between 0.25 and 0.05 mm. More than 50 percent of the particles are between 0.10 and 0.25 mm or less than 25 percent are greater than 0.25 mm and less than 50 percent range between 0.05 and 0.10 mm. More than 50 percent of the particles are between 0.10 and 0.05 mm. This category contains 70 to 85 percent sand, 0 to 30 percent silt, and 10 to 15 percent clay. Because loamy sand contains more clay than does sand, it is slightly cohesive and can be molded into a ball that will maintain its form under gentle pressure. Soil squeezed between the thumb and forefinger, however, will not form a ribbon (Figure 6). Silt Silts are highly erodible, relatively infertile soils. They contain 80 to 100 percent silt, 0 to 20 percent sand, and 12 percent or less clay. They can be molded into a ball that keeps its shape under gentle pressure. The low percentage of clay precludes the formation of a ribbon. Silts are distinguished from loamy sands by placing a small amount of excessively wet material in the palm of your hand and rubbing the wet soil. Silt feels floury, whereas loamy sand feels gritty (Figure 6). Clay Clayey soils have a very slow infiltration rate, drain slowly, are very sticky and plastic when wet, and form hard clods when dry (Figure 6).
These soils contain 40 to 100 percent clay, 0 to 45 percent sand, and 0 to 40 percent silt. The high clay content makes these soils extremely sticky and plastic. They are readily shaped and, when molded, resist deformation if squeezed with moderate pressure. Pressure between the thumb and forefinger will create a ribbon longer than 5 cm. Clay feels non-gritty but not very slippery when excessively wet. Silty clays are similar to clays. They contain 40 to 60 percent clay, 0 to 20 percent sand, and 40 to 60 percent silt. They form a ribbon greater than 5 cm in length and are very smooth when excessively wet. This category contains 35 to 55 percent clay, 45 to 65 percent sand, and 0 to 20 percent silt. Like the other clayey soils, sandy clays form long ribbons. When excessively wet, however, the higher sand content gives them a gritty feel. Loamy soils have characteristics intermediate between those of sandy and clayey soils. These soils can be molded, and, as clay content increases, the mold becomes firm and resists deformation under moderate to strong hand pressure. Also, as the clay content increases, the infiltration rate slows and the soil forms hard clods when dry (Figure 6).
These loams contain 85 to 43 percent sand, 0 to 50 percent silt, and 0 to 20 percent clay. They are slightly cohesive and can form ribbons less than 2.5 cm in length. When wet, they have a very gritty feel. Sandy loams are further divided into the following categories: This group contains more than 25 percent sand-sized particles greater than 0.50 mm in diameter and less than 50 percent between 0.05 and 0.50 mm. More than 30 percent of this group is made of particles greater than 0.25 mm in diameter; less than 25 percent measures between 1 and 2 mm; and less than 30 percent falls between 0.05 and 0.25 mm. More than 30 percent of the fine sandy loams have particles that range in size between 0.05 and 0.10 mm; 15 to 30 percent are greater than 0.25 mm. More than 30 percent of these loam particles range between 0.05 and 0.10 mm in diameter or more than 40 percent range between 0.05 and 0.25 mm (half of which are less than 0.10 mm) and less than 15 percent are greater than 0.25 mm. Silt loams contain 0 to 50 percent sand, 50 to 88 percent silt, and 0 to 27 percent clay. They are slightly cohesive when wet and form soft clods when dry. Silt loams feel smooth when wet and can form a ribbon less than 2.5 cm in length. Loams contain 23 to 52 percent sand, 28 to 50 percent silt, and 7 to 27 percent clay. Slightly cohesive, they form ribbons less than 2.5 cm long, and feel moderately smooth when wet. Containing 45 to 80 percent sand, 0 to 28 percent silt, and 20 to 35 percent clay, these loams are moderately cohesive, forming ribbons between 2.5 and 5.0 cm in length. When wet, they have a gritty feel. This group contains 0 to 20 percent sand, 60 to 73 percent silt, and 27 to 40 percent clay. Ribbons 2.5 to 5.0 cm long can be formed. When wet, the soil has a moderately gritty feel. Clay loams contain 20 to 45 percent sand, 15 to 53 percent silt, and 27 to 40 percent clay. These soils are sticky and plastic when wet and hard when dry. They form ribbons 2.5 to 5.0 cm in length and are moderately gritty when wet. Soil structure is the aggregation of primary particles into secondary shapes or forms called peds. Shrink/swell, freeze/thaw, and other forces in soil bring particles into close proximity, where they can be cemented together. Organic matter forms a weak cementing agent that may eventually give way to stronger bonding by humus. Silica, metal oxides, and carbonates also cement peds. Structure is described by grade, class, and type. Grade Grade represents the stability or distinctiveness of the ped. Because it is moisture dependent, the grade is normally described when the soil is slightly moist. Structural grades are classified as follows:
Peds can be seen in place with careful observation, however, they cannot be removed intact. Peds can be readily seen in place and, once removed, will remain intact with gentle handling. Peds are distinctive in place and will withstand considerable handling. Class refers to the size of the ped. Since some structural types are inherently larger than others, a size range for each structural type has been determined, as illustrated in Table 2. The class designations are: very fine or very thin, fine or thin, medium, coarse or thick, and very coarse or very thick. These range, respectively, from the smallest to the largest ped size for each type. Type Type refers to the shape of an individual ped (Table 2). Structural types are classified as follows:
Individual soil particles do not form aggregates; soil tends to have a sandy texture very low in organic matter. These spheroids or polyhedrons are of roughly equal size in all dimensions and have plane or curved surfaces with slight or no accommodation to the faces of surrounding peds. Nonporous peds are generally found in sandy, low-organic-matter soils. These soil particles are similar to the granular class, however, the peds are porous. These particles are much longer and wider than tall. The flat peds are arranged around a horizontal plane. Angular blocky peds are of roughly equal size in all dimensions; blocks or polyhedrons have plane or curved surfaces that are casts of the molds formed by the faces of the surrounding peds. Faces are flattened, and most vertices are sharply angular. These particles tend to occur in B horizons or where moderate amounts of clay are present. Basically the same as the angular blocky particles, the subangular blocky faces are mixed, rounded, and flattened with many rounded vertices. These particles, with two horizontal dimensions, are smaller than the vertical and taller than long or wide. They are arranged around a vertical line with vertical faces well defined and angular vertices without rounded caps. They are generally found in arid regions below the surface in horizons with moderate to high clay content. Columnar particles are like the prismatic particles but with rounded caps. The shape of these particles cannot be determined; they cling together in huge masses with no definite arrangement along lines of weakness. They are normally very hard. Bulk density is a measure of a soil's compactness, defined as a soil's oven-dry mass divided by its volume including the pore space. Soil is sampled by driving a metal cylinder of known volume into the soil. The cylinder is removed with a soil core intact. With a straight edge, the soil is leveled to the edges of the cylinder. In the lab, the intact core is oven dried at 100 ºC until there is no more weight change with additional drying. The oven-dry weight is determined and divided by the cylinder's volume. The bulk density of soil in good physical condition ranges from 0.8
to 1.6 g cm-3. Roots tend to proliferate more in soil with low
bulk density. Soil with high organic matter content tends to have good
structure and lower bulk density than similar soil with low organic matter.
Cultivation destroys structure, reducing organic matter and increasing
bulk density.
|
