Results

Overview

The twenty soil series discussed in this text represent eight of the twelve soil orders (Table 1). Samples were taken from 10 states and Puerto Rico from central Texas and Oklahoma in the west to New Jersey in the North and Puerto Rico in the south. The soils thus represent a wide range of soil characteristics and climates. A variety of minerals were present and their properties are presented. The first section is devoted to general features, organic matter morphology, soil aggregates, clay coatings, lithorelicts and quartz morphology. In the second section these features are examined on a soil by soil basis.

Features

Two of the most interesting parts of the SEM examination of the sands and silts from the water dispersed samples were the organic matter and carbonates. Normally these fractions of a soil are not seen because they are destroyed by pretreatments. Organic material appeared in several morphologies. Many soils contained organic material showing some features of the plants from which it was formed (Fig. 1). The remains of cell walls are visible in these photographs. Other organic material appeared as thin sheets or filaments (Fig. 2). Filamentous organic matter was not common but was very visible when present (Fig. 2a, b). It is probably fungal material (D. A. Zuberer, 1996, personal communication). The sheet-like organic matter is probably more weathered. It was rarely observed as flat sheets (Fig. 2c) but was commonly present as long rolled up sheets (Fig. 2d, arrow). Several samples contained pollen grains. The organic material did not exclusively appear as a part of aggregates and in most cases where it did appear mixed with mineral grains, organic matter did not appear to be holding the aggregate together. This is contrast with what many researchers have stated. The Heiden soil from Oklahoma contained many interesting forms of CaCO3 (Fig. 3) including snail shells with recrystallized calcite on the surface, coccoliths, rhombohedrons, bladed varieties, botryoidal forms, and apparent limestone lithorelicts.

Many of the soils had aggregates of clay-sized material. In most cases, the aggregates appeared in a compact form much like a piece of whole soil suggesting that the dispersion process had failed to initiate complete dispersion of the samples (Fig. 4). The Hayesville soil had aggregates which were very high in Fe and Al, probably a mixture of Fe oxide and gibbsite with very low Si (Fig. 4a). The rounded outer surface is probably the result of rounding during the water dispersion process or during the separation and drying of the sands. Another aggregate from the same soil is dominated by Fe oxides with quartz grains intermixed (Fig. 4b). Note the dendritic pattern formed by Fe oxide growing on the upper right portion of the aggregate (arrow). Other aggregates from the same soil were similar to the aggregate in Fig. 4a due to rounding but with voids and a larger particle size in some areas (Fig. 4c, d). One area of larger particle size is near the opening to the aggregate interior (Fig. 4d). Notice how the grains appear to interlock and the apparently large pore in the interior. Other samples had aggregates which had a more open fabric sometimes nearly monomineralic suggesting that dispersion of the mineral was not sustained (Fig. 5). An example from the Kirkland soil (Fig. 5a) was an unusually large aggregate of sand grains which appears to be held together by thin sheets of organic material. The apparent role of organic material as an aggregating agent is not unique to this sample but is rare compared to those which are cemented by Fe or Al oxides. The grain near the center on top of the aggregate is K feldspar (arrow). Some aggregates from the Memphis soil (Fig. 5b) are dominated by Fe oxides and show cracks which may have formed during drying. An aggregate from the Maury soil is similar to the earlier aggregates in being cemented by Fe oxides with organic matter. The organic matter in some aggregates from the Sharkey soil had a morphology that resembles vermiforms (Fig. 5d). Note the filamentous nature of the organic matter on the right side. Other clay material stayed in the sand fraction as coatings of sand particles (Fig. 6). Thick Fe oxide coatings on grains from the Marlton soil (Fig. 6a, b) prevented identification of the grain in the interior. A closeup of the grain surface (Fig. 6b) shows the Fe oxide to have a platy morphology and therefore probably hematite or lepidocrocite. The surface of this grain from the Tennessee soil is coated with undispersed clay (Fig. 6c, d). Note the face-to-face character of the clay on the surface.

Small parent rock particles are sometimes present as lithorelicts in a soil. When present, these particles may contain some clay minerals (Fig. 7). Glauconite grains in the Marlton samples (Fig. 7a, b) are granules of Fe rich clay which form due to biological activity in oceanic sediments. The closeup (Fig. 7b) shows the platy morphology of the material making up the grain. Notice the lack of a consistent long-range ordering of the material. Serpentinite, a rock formed predominately of a Mg rich clay, was found in the Nipe soil (Fig. 7c). The preservation of such a weatherable mineral in a lateritic soil would be appear unlikely. The serpentine was probably preserved by an Fe oxide coating. Schist relicts (Fig. 7d) were also found in the Cecil samples as lithorelicts.

Mineral Properties

Quartz was found in a variety of morphologies (Fig. 8-10). Quartz from residual soils (Fig. 8) was observed as single crystals (Fig. 8a), compound crystals (Fig. 8b), or grains with fairly sharp edges some surfaces of which may be crystal faces (Fig. 8c, d). Quartz from soils formed from sedimentary sources (Fig. 9) appeared less angular ranging from nearly indistinguishable from that of residual soils (Fig. 9a) to those showing conchoidal fracturing from wind transport (Fig. 9b). Other quartz grains are very rounded due to longer water transport (Fig. 9c, d). Some forms of SiO2 are not so easily identifiable (Fig. 10). Some soils had parts of quartz overgrowths which apparently broke off the original grain during fractionation (Fig. 10a). The grain on the right in Fig. 10b is some form of crystalline SiO2 according to the EDS data and the presence of apparent crystal faces. The quartz grain in Fig 10c has a surface morphology which is not easily explained. The form of SiO2 shown in Fig. 10d is unknown. It resembles volcanic ash but is too unweathered. Grains identified as plant opal (phytoliths) range from relatively unweathered (Fig. 11a) to the more common extremely etched (Fig 11b-d). Some minerals are found in soils which are not formed there but persist because they are not easily weathered (Fig. 12). The Cecil soil has a high concentration of Al2SiO5 (sillimanite or kyanite) (Fig. 12a). The mineral was probably not more than a few percent of the sand but was very visible because of the distinctive morphology. Ilmenite such as the grain shown in (b) was found in several soils. The presence of other minerals give insight into the degree of weathering of the soil. Gibbsite is a mineral found in highly weathered soils. An example from the Hayesville soil (Fig. 12c) exhibits a morphology suggesting that it may have grown outward into an open space. Research has shown that the gibbsite is more probably formed deep in the C horizon of the soil and persists into the soil (Calvert et al., 1980). A TiO2 grain from the Alachua Bt sample had a partial coating of Fe oxide and has an unusual set of linear pores apparently resulting from crystallographic weathering of some type (Fig. 12d). This mineral may or may not have been formed in the soil but is normally considered unweatherable so finding the mineral so heavily etched is a sign of a high degree of weathering.

Sometimes weathering can be an aid to the identification of a mineral. The Wayah soil contained many weathered, zoned plagioclase grains (Fig. 13). The exact nature of the zoning is unclear as the EDS patterns did not appear to support a compositional difference but the effects of the zoning on grain features is evident especially in Fig. 13b along the right edge of the grain. The grain shown in Figures 13c and 13d is especially weathered with the crystallographic controls on weathering especially noted in the closeup (d). The crystallographically controlled weathering morphology of feldspars greatly aids their identification.

Grain morphology may suggest the solution composition relative to the solubility of a mineral by weathering or the lack thereof. Several soils contained apatite, Ca5(PO4)3(OH) (Fig. 14). The grain from the Maury Bt (Figure 14a) is nearly unweathered. The persistence of the crystal morphology implies that the soil solution may have been saturated with respect to apatite. This is in contrast to the grain from the Heiden C (Fig. 14b) which is etched suggesting an undersaturated soil solution with respect to apatite. Minerals are not always found in expected morphologies or in the expected size fraction. Iron oxides are almost always found as very fine particles but the Maury Bt had sand-sized crystals of Fe oxide, probably hematite (Fig. 14c, d). In other cases, grain morphologies can reveal information about the source materials for a soil. The quartz morphologies in the Decatur soil had 2 very different morphological types (Figure 15). Much of the sand was slightly altered, angular quartz (Fig. 15a, b) while many of the remaining grains (Fig. 15c, d) were highly etched almost to the point of falling apart. The strongly contrasting morphologies suggest two parent materials, probably fresh material from the Piedmont and heavily preweathered sediments (possibly chert).

Several soils had sand size phyllosilicates (Fig. 16). The Humatas soil had kaolinite vermiforms (a and b) as did the Alachua Bh (Fig. 17 a, b). Several soils contained muscovite. The grain shown in Fig. 16c from the Wayah soil appears fairly fresh except near the edges where the onset of weathering is shown by the frayed edges especially at the top and right. The mica grains shown in Fig. 16d are biotite grains from the Hayesville soil.

Characteristics of Individual Soils

Alachua Bh - This soil sample is from the spodic horizon of a Pomona sand from Florida. The Pomona soil is very deep, poorly to very poorly drained, forming in sandy and loamy marine sediments on broad low ridges of the Lower Coastal Plain. The soil is classified as a sandy, siliceous, hyperthermic Ultic Haplaquod. Both the silt and sand fractions are dominated by quartz grains which show rounding suggesting multiple sedimentary cycles (Figure 8c). Very few other minerals are present in the sand. The silt also contains other minerals which have survived in intensive leaching (Table 2). These minerals include kaolinite (Fig 17a, b), TiO2 (Fig. 17c), and kyanite (Fig. 17d). About half of the grains counted were identified as kaolinite in the silt and were present as vermiforms while the remainder appeared to be larger plates with rough edges from transport and may have actually been hydroxy-interlayered vermiculite (HIV). One interesting composite grain (Fig. 17b) appears to suggest that kaolinite vermiforms are actively forming in the soil. This grain is made up of the remains of a muscovite plate which is deficient in K but appearing intact on the left side. The right side of the grain is a kaolinite vermiform with the center showing the effects of delamination. Apparently, the kaolinite vermiform is using the delaminated muscovite as a template for crystal growth. Such a process has been suggested by Pevear (Pevear et al, 1991) but morphological evidence for the reaction is quite unusual.

Alachua Btg - The Alachua Btg sample is from an arenic taxajunct to the Millhopper series, a loamy, siliceous, hyperthermic Arenic Paleudult. This soil is very deep, moderately well drained, and moderately permeable forming in thick sandy and loamy marine sediments in Central and South Florida. The sand fraction of this sample is dominated by round quartz grains similar to those in Fig. 8c with a few other minerals such as TiO2. The silt fraction is similar to the sand and includes plant opals (Fig. 18a), and kyanite (Fig. 18b) in addition to quartz.

Boonville - The Boonville soil is a very deep, poorly drained, very slowly permeable Texas soil which is classified as a fine, montmorillonitic, thermic Ruptic-Vertic Alba-qualf. It is formed on upland positions in colluvial and alluvial sediments of the Yegua formation. Some of the quartz in the Ap sample have very thick silica overgrowths (Fig. 9d) while other grains are severely etched. The silt fraction of the Ap horizon is dominated by quartz with feldspars quite common (Table 2). Some of the feldspars are heavily weathered (Fig. 19a) and others are not visibly weathered (Fig. 19b). Opal phytoliths are also common in this soil (Fig. 19b, arrow).

Cecil - The Cecil soil is a very deep, well drained, moderately permeable upland soil formed from deeply weathered residuum from felsic igneous and metamorphic rocks in the Piedmont region of the Southeastern US. It is classified as a clayey, kaolinitic, thermic Typic Kanhapludult. This Ap sample from Georgia has relatively angular, unweathered quartz grains and other relatively unweatherable minerals such as kyanite (Fig. 12a). The sand fraction of the Bt horizon had rounded quartz crystals (Fig. 8a) and some schist lithorelicts (Fig. 7d).

Consumo - The Consumo soil is a moderately deep, moderately permeable soil which is classified as a clayey, mixed, isohyperthermic Typic Haplohumult. This Puerto Rican soil formed from basic volcanic rocks. Opal phytoliths were quite common in the sand of the Ap horizon with some composite quartz grains and unusual SiO2 forms (Fig. 10b). Pollen grains and many aggregates were also observed. The sand fraction of the B horizon of the Consumo soil was almost entirely composed of aggregates although a few quartz grains, possibly cristobalite (Fig. 10b) and phytoliths (Fig. 11c, d) were also observed.

Decatur - The Decatur soil is deep, well drained, moderately permeable soil classified as a clayey, kaolinitic, thermic Rhodic Paleudult. It is formed in upland positions from weathered residuum from limestone. The sand fraction from the Ap horizon contained many floccules and aggregates (Fig. 20). The sand also had two contrasting quartz morphologies, one angular relatively fresh grain surface (Fig. 15a, b) and the other very etched and weathered (Fig. 15c, d). The silt fraction of the Decatur Ap is dominated by quartz with some feldspars (Table 2). Also present are opal phytoliths, TiO2, and a few weatherable minerals. The dual nature of quartz varieties was also found in the sand fraction from the Bt. Some grains had such thick silica overgrowths that the exterior was broken off during the particle size fractionation (Fig. 9a).

Dyke - The Dyke soil is a very deep, well drained soil classified as clayey, mixed, mesic Typic Rhodudult. It typically forms from colluvium. This Ap sample from Georgia has a sand fraction which contains complex quartz crystals which sometimes appear to have a complex history of overgrowths (data not shown).

Foley - The Foley soil is a very deep, poorly drained, very slowly permeable soil formed in silty material high in Na on stream terraces in the lower Mississippi River valley. It is classified as a finesilty, mixed, thermic Albic Glossic Natraqualf. The sand fraction from the Ap horizon from Louisiana contains many aggregates (Fig. 21a). Some feldspar grains in this fraction are rounded from transport but otherwise are fresh (Fig. 21b, arrow). A few grains of non-crystalline silica are also present. The sand fraction from the Btg2 horizon has many quartz grains, many coated with Fe oxides and a few aggregates.

Hayesville - The Hayesville soil is a very deep, moderately permeable, well drained soil formed on slopes in the Southern Appalachia mountains. The soil is classified as a clayey, kaolinitic, mesic Typic Kanhapludult. The soil formed in residuum from metamorphic rocks in the Piedmont region of the Southeastern United States. The sand fraction from the Ap horizon of this North Carolina sample features undispersed soil aggregates (Figure 4b -d) and quartz sand grains (Fig. 8c, 8d, 22a). The argillic horizon sand fraction contains quartz, mica (Fig. 22b) and some aggregates (Fig. 4a). The sand fraction from the C horizon contains quartz and gibbsite (Fig. 12c).

Heiden - The Heiden soil is a well drained, very slowly permeable soil formed from deeply weathered shale. The soil is classified as a fine, montmorillonitic, thermic Udic Haplustert. This Oklahoma sample from the C2 horizon is quite calcareous. The sand contains a few aggregates, apatite, and calcite (Fig. 3a - d). The silt fraction contains significant concentrations of quartz and calcite with common feldspars and muscovite (Table 2). The calcite appears as surface coatings (Fig. 3e) and as a variety of fossils (Fig. 3f, arrows).

Humatas - The Humatas soil is a very deep, well drained soil formed in weathered volcanic rocks in Puerto Rico. The soil is classified as a clayey mixed, isohyperthermic Typic Haplohumult. The sand fraction from the Ap horizon contains many aggregates, quartz and some organic material (Fig. 1c and 2a). The sand fraction from the B horizon of this soil contains many kaolinite vermiforms (Fig. 16a and b) along with aggregates and ilmenite (Fig. 12b).

Kirkland - The Kirkland soil is very deep, well drained, very slowly permeable soil formed in mixed sediments over redbeds. It is classified as a fine, mixed, thermic Udertic Paleustoll. The sand fraction from the Bt horizon of this Oklahoma soil contains organic matter (Fig. 1a), feldspar (Fig. 23a), quartz, and aggregates (Fig. 5a). The silt for this soil is dominated by quartz with high concentrations of feldspars (Table 2). Some opal phytoliths were noted in this fraction. This is somewhat surprising as the sample came from a subsurface horizon. Some feldspar grains in this sample are rounded but still show traces of cleavage (Fig. 23b). The quartz appears in some cases to have a complex overgrowth history (Fig. 23c). Titanium oxide grains also were observed (Fig. 23d) as were rare weatherable minerals.

Marlton - The Marlton soil is a clayey, glauconitic, mesic Typic Hapludult. It is deep, fine textured, well to moderately well drained and formed in the US Coastal Plains from glauconitic sediments. The sand fraction of the Ap horizon of this New Jersey sample was dominated by quartz (Fig. 24a). Many sand grains were coated with Fe oxides having a platy morphology (Fig. 6a, b). In some grains the coating is so thick that they bridge grain contacts (Fig. 24b, arrow). The sand fraction of the Marlton B2 contained many glauconite grains (Fig. 7a, b). Most sand grains of this soil had adhered grains (Fig. 24a).

Maury - The Maury soil is a fine, mixed, mesic Typic Hapludult formed in upland positions from silty material and weathered limestone or old alluvium. The soil is deep and well drained. The sand fraction of the Ap horizon of this Kentucky soil contained many aggregates (Fig. 5c) and common apatite (Fig. 25a). The silt fraction is dominated by quartz with common K feldspars and some plagioclase and other minerals (Table 2). The quartz in the silt commonly had a flat crystal morphology. Organic remains (Fig. 25b) and anatase (Fig. 25c) crystals were also observed. The sand fraction of the Bt contained quartz (Fig. 25d), apatite (Fig. 14a) and large well formed Fe oxide crystals which may be pseudomorphs (Fig. 14c, d). The silt fraction from the Bt horizon of this soil is similar in composition to the A horizon sample but has less K feldspars and more muscovite (Table 2). Some quartz grains in the silt were very etched.

Memphis - The Memphis soil is a finesilty, mixed, thermic Typic Hapludalf which formed in loess. The soil is very deep, well drained, and moderately permeable forming in an upland or terrace position of the Mississippi River Coastal Plains. The sand fraction of the Ap horizon of this Kentucky sample contains many quartz particles (Fig. 9a, b) which have angular features typical of loess sediments. Large aggregates were also observed. The sand fraction of the Memphis Bt contained many aggregates (Fig. 5b) and Fe oxide coated grains. Iron oxide appeared to cement many aggregates. The silt fraction of this sample is dominated by quartz with a relatively high concentration of K feldspars and some plagioclase, muscovite and rare opal phytoliths, TiO2, and Fe2O3 (Table 2). The quartz in the silt commonly had conchoidal fracture marks (Fig. 26a) as would be expected for quartz deposited as loess. K feldspar was quite common and rare weatherable ferromagnesium minerals (Fig. 26b) were also observed.

Nipe - The Nipe soil from Puerto Rico is a fine, sesquic, isohyperthermic Anionic Acrudox. It is very deep, well drained with moderate to rapid permeability formed in extremely weathered Ferich residuum from serpentinite in stable, mesalike upland positions. The sand fraction is made up mostly of aggregates with some quartz, organic matter (Fig. 1d), opal phytoliths (Fig. 11a) and a few lithorelicts of serpentinite (Fig. 7c) apparently preserved by being coated by Fe oxides.

Ocala NF - This Florida soil formed from deep, excessively drained, rapidly permeable aeolian and marine sands on uplands. The soil series designation for this soil is the Astatula series which is classified as a hyperthermic, uncoated Typic Quartzipsamment. The sand fraction of the C horizon of this soil contains no floccules or aggregates and is predominately quartz grains which have been well rounded by sedimentary transport (Fig. 9c). The silt fraction of this sample is about 83% quartz with a small amount of muscovite and kaolinite and very small amounts of rutile, feldspar and other minerals (Table 2). Anatase (TiO2) was observed as silt size tabular crystals (Fig. 27a) as were pollen grains which according to the EDS had been replaced by SiO2 (Fig. 27b).

Orangeburg - The Orangeburg soil is a very deep, well drained, moderately permeable soil of the Coastal Plain. The soil formed in Coastal Plains sediments and is classified as a fineloamy, siliceous, thermic Typic Kandiudult. The sand of this Georgia Ap sample is dominated by quartz (Fig. 28a) with some aggregates (Fig. 28b). The silt of this sample is mostly quartz with K feldspar, muscovite, and kaolinite along with other minerals and opal phytoliths (Table 2). Silt size filamentous organic material was common (Fig. 2b).

Sharkey - The Sharkey soil from Louisiana is a very-fine, montmorillonitic, nonacid, thermic Vertic Haplaquept. The soil formed in clayey Mississippi River alluvium on the lower part of levees or in backswamps and is very deep, poorly drained, and very slowly permeable. The sand from the Ap horizon contains many aggregates (Fig. 5d) and discrete organic material (Fig. 1b). The silt of this sample is mostly quartz with plagioclase feldspar, muscovite and other minerals (Table 2). Long hexagonal tubes of calcite were observed in the silt (Fig. 29a) as were other weatherable minerals (Fig. 29b).

Wayah - The Wayah soil is a very deep, well drained, moderately permeable soil which forms from the residuum of felsic and mafic igneous rocks on ridges and sideslopes in the Southern Appalachian mountains. The soil is classified as a coarseloamy, mixed, frigid Typic Haplumbrepts. The sand from the Ap horizon of this sample from North Carolina contains good examples of weathered feldspar grains and fresh quartz and mica (Fig. 16c). Muscovite in the sample appears fresh with some fraying at the edges (Fig. 16c). Plagioclases appear visibly zoned with obvious structurally controlled weathering (Figure 13). Compound quartz crystals are not uncommon (Fig. 8b). The sand fraction from the Bw2 horizon of this soil contains relatively unweathered muscovite and etched plagioclase grains.