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CHAPTER 4

RESIDUAL TROPICAL SOIL DEVELOPMENT

Soil Development

The simplest definition of a soil is an unconsolidated layer of weathered rock which lies

upon bedrock and is a medium for plant growth (Bates and Jackson, 1984). Soil

scientists, such as Birkeland (1984), defines a soil as

“ . . . a natural body consisting of layers or horizons of mineral and/or organicconstituents of variable thickness which differ from the parent material in theirmorphological, physical, chemical, and mineralogical properties and theirbiological characteristics".

Soil Classification and Taxonomy

Soil scientists use physical features such as color, texture, and mineral composition to help

classify soils (Duchaufour, 1982; Birkeland, 1984; Jenny, 1994; Buol et al., 1997; Soil

Survey Staff, 1999). There are several soil classification systems in use today. Many of

these identifying features are not preserved as the soil lithifies into a paleosol. Geologists

such as Mack and James (1994) and Retallack (1988, 1994, 1997, 2001) have modified

the soil classification systems to classify paleosols based upon features preserved in the

lithified soils. Most paleosols, for example, are identified by features such as root traces,

soil horizons and soil structures (Retallack, 1988). Some soils and paleosols, such as

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residual soils, often lack fossils, root traces and other identifying features, and are

classified instead based upon mineral compositions.

Soils are classified based upon physical and chemical characteristics. There are two main

soils classification systems used: The Soil Survey Staff (1993, 1999) and FAO -

UNESCO (Food and Agriculture Organization of the United Nations) (1988). The soil

classification system used by the United States Soil Survey Staff is described in greater

detail below.

Duchaufour (1982) recognized that these classification systems do not take into account

the conditions necessary for soil formation, especially in regards to tropical / residual soils.

His system, while not widely used, is helpful in the classification of tropical soils such as

laterites. Table 4-1, below, shows the approximate relationship between Duchaufour,

FAO-UNESCO and USA-SSS’s classifications.

Soils are classified into orders, suborders, great groups, subgroups, and families based

upon the type of soil (i.e., mineral, organic, or both), the properties and characteristics of

specific horizons, color, and other physical characteristics (Soil Survey Staff, 1999). Each

soil order has a specific letter designation, as do the suborders, great groups, subgroups

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Duchaufour (1982)

FAO - UNESCO(Food and Agriculture

Organization of the UnitedNations, 1988)

USA(Soil Survey Staff, 1975, 1992)

fersiallitic soilscambisols, calcisols, luvisols,alisols

alfisols inceptisols

andosols andosols inceptisols

ferruginous soilsluvisols, alisols, lixisols,plinthosols

alfisols, ultisols

ferrisolsnitisols, acrisols, lixisols,luvisols, plinthosols

ultisols, oxisols

ferrallitic soils ferralsols, plinthosols oxisols

vertisols vertisols vertisols

podzols podzols spodosols

Table 4-1: World Soil Classification Systems

and families. These letter designations are not the same as those used for soil horizons.

Soil Horizons

Soils commonly contain layers, commonly called horizons by soil scientists. The Untied

States Department of Agriculture’s Soil Survey Division Staff Soil Survey Staff, 1993)

defines a soil horizon as follows:

“A soil horizon is a layer, approximately parallel to the surface of the soil,distinguishable from adjacent layers by a distinctive set of properties produced by

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the soil-forming processes. The term layer, rather than horizon, is used if all of theproperties are believed to be inherited from the parent material or no judgment ismade as to whether the layer is genetic.”

Each soil has it’s own “vertical distribution pattern” (Jenny, 1994), but nearly all have the

following basic profile:

• Horizon A: The Eluvial or leached horizon, from which most minerals and other

substances have been removed (Duchaufour, 1982; Jenny, 1994). Corresponds

with master soil horizon A (described below).

• Horizon B: The Illuvial horizon, in which the minerals and other substances

leached from horizon A accumulates (Duchaufour, 1982; Jenny, 1994).

Corresponds with master soil horizon B (described below).

• Horizon C: The parent rock. Corresponds with master soil horizons C and R

(described below).

There are two basic types of soil horizons: a genetic horizon and a diagnostic horizon. A

genetic horizon is used to “express a qualitative judgement about the kinds of changes that

are believed to have taken place in a soil” (Soil Survey Staff, 1999). There are 7 master

soil horizons and layers, represented by the capital letters O, A, E, B, C, R, and W. These

letters are combined with one or more of the 27 suffix symbols, represented by lower case

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letters, to describe a genetic soil horizon. Table 4-2 lists the master horizons and suffix

symbols used by soil scientists. These letter designations are not the same as, nor are they

interchangeable with, the letter designations used with the diagnostic soil horizons.

During the course of this field work, I found it convenient to give each of the paleosol

horizons a letter designation of A, B, C, and D. This was done for convenience, and does

not correspond with the master soil horizons and layers used by the Soil Survey Staff.

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Letter Description

O Horizons or layers composed primarily of organic matter. May or may not have beensaturated.

A Mineral horizons or layers formed at the surface or below an O horizon/layer. Nearlyall of the original rock structure has been destroyed.May exhibit either 1) and accumulation of humus mixed in with the mineral matter anddoes not have the characteristics of E or B horizons and/or 2) evidence of man-madeactivities such as cultivation, etc.

E A mineral horizon where the dominant feature is the absence of silicate clay, iron,aluminum or some combination of all of these minerals, resulting in a concentration ofsand and silt sized particles. Nearly all of the original rock structure has beendestroyed.

B Horizons formed below an A, E, or O horizon. Nearly all of the orginial rock structurehas been destroyed. Dominated by one or more of the following characteristics:1 Illuvial concentration of silicate clay, iron, aluminum, humus, carbonates, gypsum

and or silica2 Evidence of the removal or addition of carbonates3 Residual concentrations of oxides4 Sesquioxide coating which lowers the color value, raises the chroma, or becomes

redder in hue without the apparent illuviation of iron5 Alteration that either forms a silicate clay or liberates oxides. A granular, blocky or

prismatic structure is created if changes in volume occur as a result of a change inmoisture content.

6 Brittleness7 Strong gleying

C Sediment, saprolite or bedrock.

R Strongly cemented to indurated bedrock

W Water

Table 4-2: Master Soil Horizon and Layer Designations

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Residual Tropical Soils

Based on the information presented in the previous section, the paleosols of the Silverado

Formation are likely residual soils. Consequently, a discussion of residual soils is in order.

Formation of tropical residual soil requires the following: physical and chemical

weathering, the leaching of insoluble materials and the accumulation of insoluble residues,

and the movement of fine particles downward (lessivage). Physiochemical mechanism is a

type of hydrolysis that occurs only in tropical soils (Duchaufour, 1982). This type of

weathering occurs as a function of neutral to slightly acidic hydrolysis, and is generally not

influenced by surface organic matter (Duchaufour, 1982; Fookes, 1997). Kaolinites,

bauxites and laterites all are examples of residual soils.

Kaolinites

The word “kaolinite” has two basic meanings. First, it refers to a high-aluminum clay

mineral belonging to the kaolin group. It is typically a secondary mineral formed via

weathering or hydrothermal alteration of aluminum silicates (Klein and Hurlbut, 1985).

The term can also refer to a type of residual soil formed in tropical environments and

nearly constant rainfall. The lack of a dry season prohibits the development of iron oxides,

leaving a residual soil of kaolinite and possibly quartz.

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Laterites

There is no consensus as to what defines a laterite, nor is there agreement as to how they

are formed (Duchaufour, 1982; Jenny, 1994; Retallack, 1997). The U.S. Soil Survey Staff

(1993) defines a laterite as a plinthite, an hematite-rich mottled red/yellow and white clay

zone; others consider only the hard, hematite- and clay-rich surface crust (also called a

cuirasse) a laterite. Jenny (1994) bases his definition of a laterite profile on that of

Harrassowitz, which is comprised of the following: humus soil (may not be present); iron

crust; accumulation zone (sesquioxides - kaolinite, etc.); decomposition zone (saprolite);

and finally, fresh rock. Retallack (1997) prefers to define a laterite as a rock or part of a

soil, not a true soil. His laterite profile, from top to bottom, is similar to that described by

Jenny (1994): a red soil "cap"; the laterite, a red, hematite-rich clay zone lacking humus,

bases and silica; a "pallid zone" or a white clay zone that may or may not contain mottles;

a "white china clay"; the saprolite; the parent rock (Retallack, 1997). Duchaufour (1982)

and Fookes (1997) avoid the term "laterite" altogether, preferring Duchaufour's system of

classification for tropical soils.

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Retallack (1997) describes four basic theories on laterite development, included below:

Residuum

Laterites which develop in altered bedrock left after an long period of weathering.

The major problem with this theory is that it requires a large amount of rock to

produce enough iron for hematite or goethite to form.

Soil Horizon

In this theory, laterites develop as a precipitate either at the edge of capillary rise or

just above a fluctuating water table. This theory may account for the development of

some laterites, but it does not account for the development of laterites up to several

meters thick.

Deposit

Laterites as a depositional feature are considered to be the origin of lateritic breccias

and loose, pea-sized pisolitic aggregates. While this does appear to be valid for these

types of laterites, the theory does not explain massive laterites that grade into pallid or

kaolinitic zones and then into a saprolite.

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Groundwater Precipitate

Acidic groundwater from marshes and swamps leaches the bedrock well below the

influence of surface interactions. This theory accounts for the development of

kaolinite and hematite-rich zones, and can be used to explain the variable thicknesses

seen in some deposits, but it does not satisfactorily explain the origin of all laterites.

Both Duchaufour (1982) and Fookes (1997) favor this residual soil origin for laterites,

which appears to be similar to this theory, although they do not require a marsh or

swamp environment to begin the leaching process.

Duchaufour (1982) has determined that there are three phases of the weathering cycle for

tropical environments: fersiallitisation, where 2:1 clays (micas, chlorite) are dominant;

ferrugination, characterized by kaolinite and 2:1 clays (smectite, illite); and finally

ferrallitisation, dominated by kaolinite and gibbsite. These phases are characterized by an

increase in weathering of the primary minerals, and increase in the loss of combined silica,

and an increase in the amount of clays formed from the alteration of the primary minerals

(Duchaufour, 1982).

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Fersiallitisation

Weathering of primary minerals produces primarily 2:1 clays. There is also

considerable amounts of free iron oxides formed and the soils becomes rubified. An

argillic (Bt) horizon can develop as well. Fersiallitic soils typically occur in subtropical

or Mediterranean climates with a dry season.

Ferrugination

Weathering increases, although primary minerals such as orthoclase and muscovite can

still be present. There is an increase in the amount of newly formed 1:1 clays such as

kaolinite, especially in relation to 2:1 clays. Rubification may or may not occur.

These soils commonly form in humid subtropical environments or humid tropical

regions with a dry season.

Ferrallitisation

All minerals, except for quartz, are completely weathered and altered to kaolinite. The

entire profile has been reduced to quartz, kaolinite or gibbsite, and hematite or

goethite. No true argillic horizon forms. Hot, humid tropical regions typically

produce these types of soils, although they can be found in drier climates such as semi-

rainforests and in savannas.

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Ferrallitic soils typically form a profile comprised of three zones: The upper zone, the

middle zone, and a zone of deep weathering (saprolite) (Duchaufour, 1982).

Upper Zone

Comprised of A and B horizons. The A horizon contains a great deal of plant debris

which is rapidly decomposed, and is highly depleted in clay and iron. In contrast, the

B horizon is typically enriched in iron, producing either an ochreous (gibbsite), red

(hematite), or mottled red and white color in the soil.

Middle Zone

This zone is characterized by large, irregularly shaped red or ochreous mottles and is

typically several meters thick. If Horizon B contains mottles, they extend down into

the Middle Zone, although there is less contrast between the mottles and the overall

color is lighter.

Deeply Weathered Soil / Saprolite

Saprolites are the transition zone between the residual soil and the parent rock.

Weathering is often irregular, with minerals in various phases of alteration.

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Classification of Paleosol 1

Laterites and other residual soils are difficult to classify using traditional classification

systems (Duchaufour, 1982), as they lack horizons as defined by the Soil Survey Staff

(1999) and other classification systems. Therefore, Duchaufour’s classification system

(1982), having been created for the express purpose of classifying residual soils, best suits

classifying Paleosol 1.

High precipitation rates found in humid tropical environments provide a nearly constant

influx of water, and all minerals except quartz are leached out. The remaining Al and Si

ions precipitate out as aluminum hydroxides, and further alter to kaolinite/gibbsite (Garrels

and Christ, 1965; James et al., 1981; Duchaufour, 1982; Mack, et al., 1993; Jenny, 1994;

Buol, et al., 1997; Fookes, 1997). Kaolinite will form from gibbsite when there is an

excess of Si(OH)4 in solution and in acid conditions, especially in granitic or quartz-rich

sedimentary rocks (Duchaufour, 1982; Jenny, 1994;Buol et al., 1997). Iron and

aluminum oxides tend to remain in situ; hematite forms where the soil is subject to

seasonal dry periods (Buol et al., 1997). Poor drainage within this kaolinitic horizon

produces red and white mottling. Fluctuation of watertable levels promotes the

development of a ferricrete crust (Duchaufour, 1982; Fookes, 1997).

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Figure 4-1 graphically compares a “typical” laterite profile as described by Jenny (1994)

and Retallack (1997) with Paleosol 1, Horizon A in particular. Horizon A is comprised of

an iron crust (Retallack’s laterite, Fookes’ (1994) ferricrete, and Duchaufour’s Upper

Zone), and a mottled zone and a pallid zone (Retallack’s pallid zone and Duchaufour’s

Middle Zone). Thus, Horizon A could be classified as a ferrallitic soil using

Duchaufour’s system based upon the presence of quartz, kaolinite and hematite and the

lack of other minerals.

Horizon B, similar in composition to the mottled zone of Horizon A, may be a portion of a

laterite as described by Duchaufour (1982), Jenny (1994) and Retallack (1997). The red

and white mottling of the kaolinite in addition to the etched quartz fits the description of

Retallack’s pallid zone and Duchaufour’s Middle Zone. The lack of the iron

crust/ferricrete capping the mottled zone may be because the development of the laterite

was incomplete, or, as Duchaufour (1982) and Fookes (1997) suggest, water-table levels

during formation of the horizon did not fluctuate enough to produce the ferricrete crust.

A kaolinite residual soil forms under similar conditions as does a laterite: intense

weathering and leaching due to high precipitation rates alters all minerals (except quartz)

to kaolinite (Garrels and Christ, 1965; James et al., 1981; Duchaufour, 1982; Mack et al.,

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Figure 4-1: Comparison between a laterite and Paleosol 1. Laterite characteristics andgeneralized profile from Retallack (1997).

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1993; Jenny, 1994; Buol, et al., 1997; Fookes, 1997). Horizons C and D resemble this

description of a kaolinitic residual soil, being comprised of kaolinite and quartz.

In summary, Paleosol 1 contains several characteristics of a residual soil: kaolinite and

quartz composition, etching and dissolution of quartz grains, and FeO mottling, to name a

few. As such, the paleosols of the Silverado Formation probably formed under subtropical

to tropical conditions with high precipitation rates and possibly a short, hot, dry season.