What is Soil?
The definition of soil varies depending on the person considering it. To a civil engineer planning a construction site, soil is whatever unconsolidated material happens to be found at the surface. To a miner, it is just some worthless material that is in the way and must be removed. To a farmer, it is the medium that will nourish and supply water to the crops. Even soil scientists may hold differing definitions, depending on their area of study.
For the purposes of this paper, the definition of the Soil Survey Staff (1975) will be used:
Soil... is the collection of natural bodies on the earth's surface, in places modified or even made by man of earthy materials, containing living matter and supporting or capable of supporting plants out-of-doors. Its upper limit is air or shallow water. At its margins it grades to deep water or to barren areas of rock or ice. Its lower limit to the not-soil beneath is perhaps the most difficult to define. Soil includes the horizons near the surface that differ from the underlying rock material as a result of interactions, through time, of climate, living organisms, parent materials, and relief. In the few places where it contains thin cemented horizons that are impermeable to roots, soil is as deep as the deepest horizon. More commonly soil grades from at its lower margin to hard rock or to earthy materials virtually devoid of roots, animals, or marks of other biological activity. The lower limit of soil, therefore, is normally the lower limit of biological activity, which generally coincides with the common rooting depth of native perennial plants. Yet in defining mapping units for detailed soil surveys, lower layers that influence the movement and content of water and air in the soil or the root zone must also be considered.
While a nearly infinite variety of substances may be found in soils, they are categorized into four basic components: minerals, organic matter, air and water. Most introductory soil textbooks describe the ideal soil (ideal for the growth of most plants) as being composed of 45% minerals, 25% water, 25% air, and 5% organic matter. In reality, these percentages of the four components vary tremendously. Soil air and water are found in the pore spaces between the solid soil particles. The ratio of air-filled pore space to water-filled pore space often changes seasonally, weekly, and even daily, depending on water additions through precipitation, throughflow, groundwater discharge, and flooding. The volume of the pore space itself can be altered, one way or the other, by several processes. Organic matter content is usually much lower than 5% in South Carolina (typically 1% or less). Some wetland soils, however, have considerably more organic matter in them (greater than 50% of the solid portion of the soil in some cases).
Almost any mineral that exists may be found in some soil, somewhere. The broad and deep subject area of soil mineralogy can barely be touched upon here. Only some of the elementary basics shall be discussed.
The mineral portion of soil is divided into three particle-size classes: sand, silt, and clay. [Note: Sand, silt, and clay are collectively referred to as the fine earth fraction of soil. They are <2 mm in diameter. Larger soil particles are referred to as rock fragments and have their own size classes (pebbles, cobbles, and boulders). Rock fragments do not play a significant role in ACE Basin soils.] The three particle size classes are defined as follows:
2 mm-0.05 mm
0.05 mm-0.002 mm
Mineralogically, sand, and silt are just small particles of rock and are largely inert. The two important differences among them are their relative capacity to hold water that is available for uptake by plants and their effects on soil drainage.
Clay particles are mineralogically different from sand and silt. Clay minerals form at or near earth's surface, in soil or in water. Most clays belong to a class of minerals called phyllosilicates, which have formed from the breakdown products of other minerals. Like all phyllosilicates, clay minerals have a sheet-like structure, which is revealed when the crystals are observed through a scanning electron microscope. More familiar phyllosilicate minerals that are often large enough to be seen with the naked eye are the micas such as muscovite and biotite.
Due to isomorphous substitution, in which one ion is substituted for another in the crystal structure of a mineral, many clays have a net negative charge. That is, if all the protons and all the electrons that are part of the clay mineral's crystal structure were counted, there would be more electrons than protons. Another source of negative charge on clays is the ionization of hydroxyl groups at the edge of crystal (called broken edge charge).
The net negative charge of clay minerals is responsible for a property called cation exchange capacity, or CEC. When placed in a solution, clay minerals attract cations (positively charged particles) to their surfaces. The bonds between the clay mineral surface and the cations are relatively weak, and these cations can be exchanged for other cations that are dissolved in the solution.
The significance of CEC is that cations moving through a soil in solution may be held by the soil. Sometimes these cations (usually metals) are plant nutrients, like potassium, calcium, and magnesium. The loosely held nutrients can then be taken up by plant roots or by other soil organisms. This is one of the ways that soils store nutrients for future biological use. The cation exchange property is also responsible for the soil's ability to filter some environmental contaminants from water.
There are many different phyllosilicate clay minerals. Two that are commonly found in the soils of the ACE Basin study area include kaolinite and members of the smectite group of clay minerals. (Montmorillonite is one of the better known smectites.) Kaolinite does not shrink and swell when dried or wet, which makes it ideal for making bricks and pottery. It also has many other commercial uses. Kaolinite has a very low CEC. Smectites, on the other hand, have a high CEC and a very high shrink-swell capacity. Soils with a high smectite content are known to cause problems with the construction of buildings, roads, and other infrastructure. Most soils in the ACE Basin study area have only low to moderate shrink-swell potentials, however. The few soils with high shrink-swell potential also happen to be saltwater wetlands.
The relative combination of sand, silt, and clay in a soil defines its texture. From the discussion of the properties of the soil particle sizes above, it should be obvious that soil texture is important in determining the nutrient-holding abilities of a soil. Along with soil structure (the arrangement of soil particles in aggregates), the texture of soil is also important to water-holding capacity, water movement, and the amount and movement of soil air in a given soil. All of this is important to the health and type of plants and other organisms that can exist in a particular soil.
Once the percent by weight of sand, silt, and clay are known (or, rather, any two of them), the soil texture can be plotted on the triangular graph known as the soil textural triangle. The region on the graph where the three particle size percentages meet is the soil's texture. Loam has been determined to be the texture best suited to the growth of most agricultural crops, having the optimum combination of heavy and light soil qualities.
Soil Organic Matter
Soil organic matter (SOM) is a complex mixture of substances that can be highly variable in its chemical content. It ranges from freshly deposited plant and animal parts to the residual humus stable organic compounds that are relatively resistant to further rapid decomposition.
The elemental composition of SOM includes carbon, oxygen, hydrogen, nitrogen, phosphorus, and sulfur. Nitrogen, phosphorus, and sulfur are plant nutrients that are slowly released during decomposition and are then available to plants, as well as other soil organisms. Other elemental nutrients may also be held in complex SOM. (See related section: Decomposers.)
Like the phyllosilicate clays, SOM has CEC. By weight, SOM has a much higher CEC than clays, and is, therefore, an important source of this soil property, especially in wet soils and soils low in clay minerals. SOM can also form complexes with metals and organic materials (including insecticides and herbicides), sometimes rendering them immobile and/or inert.
An important physical property of SOM is its ability to absorb and hold large quantities of water. The mass and volume of water that can be absorbed by SOM often exceed the mass and volume of the SOM itself.
The Soil Profile
There are between 15,000 and 20,000 soils in the United States, 265 of which are found in South Carolina. Of those, about 100 are found in the ACE Basin study area. These soils are differentiated from one another by the characteristics and properties of their profiles.
A soil profile is a physical and chemical description of the layers (called horizons) that make up the soil, from the surface to the depth where pedogenic (soil forming) processes are no longer evident. If a person digs a hole in the ground and looks at the wall of that hole, he is looking at the soil's profile. The horizons of the profile have formed and differentiated from the original parent material in placethey are not the result of geologic processes, although some features of a profile may be caused by geologic events (faults, lithologic discontinuities, buried soils, etc.).
A typical profile of a mature soil in South Carolina includes the following horizons:
- O horizon (O stands for organic) includes litter layer, duff, and sometimes a humus layer. These materials are differentiated by the degree of decomposition of the organic matter. This horizon is often missing in cultivated soils, manicured lawns, and severely eroded soils.
- A horizon contains a mixture of organic and mineral components. This layer most closely resembles the ideal soil; commonly referred to as topsoil.
- E horizon (E stands for elluvial) is stripped of much of its clay and sometimes staining agents, and is thus often lighter in color than the others. It is lower in organic matter than the A horizon.
- B horizon is a zone of illuviation (accumulated substancesclays, organic matter, iron and aluminum compounds) that have been leached from overlying horizons.
- C horizon is lightly weathered parent material.
Specific types of the above-mentioned horizons are denoted by a subscript letter. For example, a B horizon with a certain amount of illuviated clay in it is called a Bt horizon. Other specific horizons will be mentioned, as needed, in later sections. These horizons are also often subdivided further; a B horizon may have several parts if characteristics such as texture or color change with depth. These subhorizons are denoted by an Arabic number. For example, a sequence of B horizons found in a soil may be Bt1, Bt2, Btg.
In addition to the soil horizons mentioned in the text (O, A, E, B, C), a reader may find other horizon designationsmentioned in published soil surveys or in the USDA's Official Soil Descriptions (http://www.statlab.iastate.edu/soils/osd/) .
A number of conceptual models of soil formation have been postulated over the years. The two that have been key in our basic understanding of soils and soil formation are those of Hans Jenny (1941) and Roy W. Simonson (1959).
Five Factors of Soil Formation
Jenny (1941) addressed the question of which environmental factors are responsible for the soils we have today. Recognizing these factors is extremely useful for field scientists when looking over a landscape and predicting the soil types that are found upon it. These factors include the following:
- Parent Material - What was there before soil formation began?(Possibilities include mud deposited by a river, sand deposited by ocean, rock that weathers and breaks down, etc.);
- Organisms - usually refers to vegetation and microorganisms, but includes the complete biological community;
- Climate- on both large and small scales;
- Relief, or landscape position;
How do these factors determine the types of soils found in the ACE Basin study area?
Parent materials in the ACE Basin study area were mostly deposited by the ocean or rivers and streams. In some cases these sediments were reworked by wind. The principle to remember is that fluids with higher energy (fast-moving and/or large waves) can hold larger particles than fluids with lower energy. Muds high in silt and clay were deposited by slow-moving or still air and water, while the fluids that deposited sandy sediments were moving fast enough to retain suspended silts and clays. (Fluids, in this context, include both liquids and gases.) Sandy, non-alluvial soils in the ACE Basin study area were likely once beach and dune deposits. Finer textured soils were probably once marshes and other backwater areas that were protected from strong ocean waves and currents.
Soils of alluvial origin (flood plain soils) also vary in texture, from sands to clays. When a stream of water is concentrated through a small channel, its flow rate is more rapid than when the same amount of water on the same slope is spread out over a wider area. (This is the reason sluices were constructed for old water-powered mills.) River water confined within the river's banks moves at a higher velocity than when the river floods and its waters spread over the flood plain. When a river floods and overflows onto its flood plain, its velocity immediately decreases and it starts dropping its sediment load. The larger, heavier sand particles drop out first, near the banks. In some cases, a natural sandy levee forms on either bank of the river. Finer and finer particles are dropped the farther out the floodwaters' reach. Floodwaters often create ponds on the outer margins of flood plains. Clay-sized particles settle out in these areas.
The deposition of soil parent materials on flood plains is further complicated because the stream meanders back and forth. Sandy stream channel sediments may be buried by the finer sediments of ponded backwaters and oxbow lakes. Finer sediments in the flood plain may also be buried or eroded away by a meandering channel. All these scenarios result in differences in the soils that subsequently form on these sites.
Other important parent materials in the ACE Basin study area are those high in calcium carbonates. A plethora of marine organisms leaves some sort of calcareous remains that have a profound effect on soils that form in sediments that include these materials. The presence of calcium carbonate in soil drastically changes the soil chemistry, and thereby the chemical processes that occur, and the community of organisms that colonize the soil. Dwarf palmetto (Sabal minor) is a well-known indicator species used by soil scientists to identify calcareous soils in the field, since this species requires soils with a near neutral to alkaline pH.
Organisms affect the type of organic matter that is added to the soil, the rate at which the organic matter is decomposed, the part of the soil to which the organic matter is added and translocated, and the types of chemical reactions that occur in the soil.
One of the most notable effects that soil organisms have on soils in the ACE Basin study area is on the amount of organic matter that is present. In wetland soils, SOM tends to build up because the anaerobic soil bacteria are less efficient than their aerobic cousins at decomposing it. (See related section: Decomposers.)
Climate affects soils by governing the rate at which chemical reactions can take place and the amount of percolating water that translocates materials from one part of the soil to another. The climate and its effects on soil change on a regional basis in areas of low relief like the ACE Basin study area and the rest of the Southeastern Coastal Plain. Differences in soil types from one part of the Basin to another are not attributed to climatic change. The whole area has a warm, moist climate most of the year, which is conducive to relatively high chemical reaction rates responsible for chemical weathering and biological activity. This is all conducive to relatively rapid rates of soil formation. (See related section: Climatology.)
Local relief is the environmental factor that has the greatest effect on the soils of the ACE Basin study area. Changes in elevation of only a few feet produce major changes on soil properties in this region, all attributable to the topography 's effect on soil water.
Simply stated, water runs downhill. When water drains from the soil on local topographic highs, it drains into the low areas on the landscape. Soils in low-lying areas are saturated closer to the surface for longer periods of time than soils on higher ground.
The organisms living on or in these wetter soils must have ways of adapting to the limited availability of soil air. Vegetation has hydrophytic characteristics, and soil bacteria are either anaerobes or facultative anaerobes.
On the other hand, organisms living on the topographic high points must be adapted to xeric conditions. Often, the origins of the landforms making up these topographic highs are old, sandy beach and dune ridges. Soils that form there drain quickly and retain very little water. These two different soil conditions affect both the soil chemistry and the amounts of organic matter added to the soil each year.
All pedogenic (soil forming) processes occur over time. Young soils show only minimal profile developmentoften only an A horizon overlying a C horizon. As the soil matures with time, additional subsurface horizons form.
The development of soil through time can be easily observed in the Southern Coastal Plain. The youngest landforms and soils are closest to the ocean gradually increasing in age inland. While the soils of the ACE Basin study area are all fairly young, this increase in soil development is still evident.
The original intent of Jenny's factors of soil formation model was to develop a numerical equation that used information on each factor to determine the characteristics of the resultant soil. It is unlikely that this will come to pass. Obviously, these five factors are not always independent of each other. In addition, soil is a highly complex system that is only partly understood. However, Jenny's model has proved invaluable to field soil scientists and landscape ecologists the world over.
Generalized Theory of Soil Genesis
Roy W. Simonson's conceptual model of soil genesis takes a different approach. Instead of concentrating on the external factors that influence the type of soil that forms in a given location, he considers the pedogenic processes that occurred within the soil body.
First, he divides soil formation into two steps:
- the accumulation of parent materials, and
- the differentiation of horizons in the profile.
Horizon differentiation is divided into four basic categories of changes:
Simonson (1959) uses the changes that organic matter undergoes in soil as an example. Organic matter is added to soils as plant and animal remains, often at the surface. The action of organisms removes some of this SOM as it decays, usually in gaseous forms that escape to the atmosphere. Some SOM may leach with percolating rainwater to deeper horizons. The processes of decay also transform the organic matter into different organic substances. Similar examples can be made with mineral substances.
Simonson (1959) further postulates that all the changes that occur in our many different soils occur in ALL soils, only at different rates. The rate of these changes is controlled by environmental factors, such as those outlined by Jenny (1941). The ultimate result of the pedogenic changes is the soil that exists today, and the differences among soils are due to the varying rates of all these processes.
Stages of Soil Formation
All soil formation begins with the accumulation of parent material. The next step is the buildup of organic materials at the surface. Pioneer species (most often grasses and alga in this area) live and die, and organic matter begins to build up on the surface of the material and also beneath the surface in the rooting zone.
The A horizon starts to form once enough organic matter has been transformed by soil biota into humic materials. The humic materials coat the soil particles, coloring them brown and black. The formation of a recognizable A horizon takes decades or, in some cases, centuries.
The B horizon begins to form as dissolved and suspended materials are carried downward to greater depths with percolating rainwater. These materials include humic substances, suspended clays, salts, and metals, including iron and aluminum. It is likely that the largely insoluble iron and aluminum cations and oxides move in complex with dissolved organic material (chelation), and also in complex with suspended clay minerals.
The A horizon continues to increase in thickness, and the B horizon continues to develop. The A horizon will increase in thickness and SOM content, until it reaches a steady state in which the rate of fresh organic matter additions equals the losses by decay, illuviation, and erosion. This steady state is affected by certain environmental changes, including climatic change and vegetational succession (or cultivation). The B horizon will continue to receive illuviated material as it is formed in the A horizon, or sometimes as it is deposited on the surface (especially wind-blown clays).
The E horizon forms as the top of B horizon moves deeper into the soil. In some forested areas, such as the Southeast region of the United States, the movement of illuvial materials occurs at a faster rate than the illuvial materials are formed (largely clays and organic matter). This results in a gap between the A horizon and the B horizon. The E horizon is usually the same texture as the A horizon, and the soil particles are largely stripped of staining agents, such as organic matter and metal oxides. These materials have elluviated from the E into the B horizon.
Minerals continue to weather. Clays in B horizon weather to less active minerals (kaolinite).
"Bases" are leached from soil. Certain cations are referred to as acids or bases in soil science, even though they do not fit any chemical definition of the term. The acidic cations, including aluminum and iron cations, are so called because their presence in the soil tends to decrease pH. (The reactions responsible for this will not be explained here.) The presence of the basic cations in large amounts usually coincides with neutral to high pH soil systems. These bases are often plant macro-nutrients, like calcium, potassium, and magnesium. The loss of basic cations results in low fertility soils.
Silicate clay minerals completely break down into iron and aluminum oxides. Soil is extremely infertile. This occurs in tropical climates. While some of these metal oxide clays exist in South Carolina soils, they do not dominate.
Soils and the ACE Basin/Low Country Landscape
The general soil mapof the ACE Basin study area depicts the different kinds of landscapes of the area, and the soil associations found on them. Before describing these soils, some explanation is in order.
A soil association is a group of soils that are geographically related and found in a characteristic repeating pattern across a landscape. They are grouped together in a single map unit on the general soil map. The soils for which the association is named are rarely, if ever, the only soils that exist in the soil association. Sometimes as much as half the area of a map unit includes what are referred to as soils of minor extent. The soil that covers the most land area in an association is named first, the next most common soil second, and so on. A soil association map gives soil scientists an idea as to what the landscape is like. It should not be used for detailed land use planning. (See related section: Soil Survey.)
Dune Ridge and Trough Landscapes
These landscapes are found just inland from beaches or land that was near a beach during late Pleistocene or Holocene times. They consist of long sand dunes with low-lying troughs between them, paralleling the coastline, or former coastline. The dominant soil orders found in this type of landscape are the Entisols, Inceptisols, and Spodosols. Entisols exhibit the least amount of profile development, followed by Inceptisols. Spodosols can form quickly, given the right conditions. (See related section: Soil Taxonomy.)
These soil associations have moderate and severe limitations for urban use, but since they are found in prime oceanfront locations, homes are often built on them.
Fripp-Baratari: This association is found on Hunting Island and the northeastern part of Helena Island. Both are sandy throughout their profiles. Fripp series soils (Typic Quartzipsamments) are found on the dunes and have no subsurface horizon development, just a gray-brown A horizon that is a few inches thick. They are excessively drained. Baratari series soils (Aeric Alaquods) are found in the low troughs and may be occasionally flooded. Baratari soils, being of the Spodosol order, have a dark Bh horizon that is stained with illuviated organic material. This horizon exists at the level of the water table. The soil is poorly drained.
Wando-Seabrook-Seewee: This association is found on Helena, Lady, and Port Royal Islands. (NOTE: This association does not always have dune and trough topography,) All three soils are sandy throughout their profiles. Wando series soils (Typic Udipsamments) are excessively drained and are found on the tops of the dune ridges. Seabrook series soils (Aquic Udipsamments) are moderately well drained and are found at intermediate levels and on the top of lower ridges. Neither soil has any subsurface profile development. The somewhat poorly drained Seewee series soils (Aeric Alaquods) are at the lower parts of the landscape and have the characteristic dark Bh horizon beneath the surface, at about the level of the water table.
Kiawah-Seabrook-Dawhoo: This association is found on Edisto Island and in other parts of Charleston County to the east. Seabrook and Dawhoo soils are sandy throughout, and Kiawah soils have just enough silt and clay in them to make the texture a loamy sand. Seabrook series soils (Aquic Udipsamments) are moderately well drained and occupy the highest parts of the landscape. Kiawah series soils (Aeric Ochraqualfs) are somewhat poorly drained and are found at intermediate elevations. There is some horizon development in these soils. Dawhoo series soils (Typic Humaquepts) are poorly drained. Due to their wetness, the A horizons have accumulated fairly high amounts of organic matter.
Flood Plain/Salt Marsh Landscapes
The extensive and contiguous flood plain/salt marsh landscape is the heart of the ACE Basin study area. No soils are more productive and have a greater impact on the ecosystem, the economy, and the history of South Carolina than those found on this landscape. The sediments and organic materials in which these soils formed are all of Holocene age. These landscapes are almost perfectly level. They are often flooded, and these flood waters deposit new sediments regularly. The dominant soil orders found here include Entisols, Inceptisols, and Histosols. As previously noted, the Entisols and Inceptisols are young soils. Histosols contain large amounts of organic matter, as might be expected in wet soils. The soils of this landscape would present severe limitations for any urban use. (See related section: Wetland Soils.)
Torhunta-Osier-Pickney: This association is found on the valleys of the upper Combahee River and the Salkehatchie River. It typifies the sort of sediments and soils found on flood plains throughout the world. Osier series soils (Typic Psammaquents) are poorly drained, very limited in profile development, and sandy throughout. They are found in former stream channels and right alongside streams, wherever swiftly moving water has deposited sediment. Torhunta series soils (Typic Humaquepts) are extremely poorly drained and contain high amounts of organic matter in the A horizon. They are found in broad drainage ways that are frequently flooded for short durations with slowly moving water. Pickney series soils (Cumulic Humaquepts) are very poorly drained and contain high amounts of organic matter in a thick A horizon. They are often flooded by slowly moving water and are usually saturated. They are often found along the outer margins of the flood plain.
Santee-Argent-Cape Fear: This association is found in the valley of the upper Ashepoo River and its tributaries. The whole association is often flooded, and all three soils have a loamy surface and clay subsoil. The Santee series (Typic Argiaquolls) is very poorly drained and is one of South Carolina's few Mollisols. Mollisols are more typical of the prairies and high plains of the Midwest. They are quite fertile and high in bases. They are most abundant nearer to the sea in this map unit, rather than inland, and are underlain by a layer of marl. The Argent series (Typic Endoaqualfs) are poorly to very poorly drained. They, too, are more abundant near the sea than inland. They may have calcium carbonate accumulations in the subsoil. Some areas covered by this soil were drained and diked for use as rice fields prior to 1893. Cape Fear series soils (Typic Umbraquults) are very poorly drained and are more abundant inland than near the sea. They have a surface horizon high in organic matter. These soils are quite fertile but cannot be used for typical agricultural crops due to flooding and saturated conditions. Drainage, if it were legal, would rarely be possible since there would be no place to drain the water. Most of the area is in mixed hardwoods.
Pungo-Levy: This association is found in a large area between the Edisto and Ashepoo Rivers, and up the Ashepoo valley, beyond the town of the same name. This association is adjacent to areas flooded with salt water. (However, this association is not.) The Pungo series soils (Typic Medisaprists) are very poorly drained and occasionally flooded. These are rich soils with mucky organic surface horizons several feet thick. The very poorly drained Levy series soils (Typic Hydraquents) are high in organic matter in this area, but not enough to call them organic soils (Histosols). Levy series soils are continuously wet. Although flood waters are often affected by tides, the waters flooding Levy soils are fresh. Most of the area covered by this association was once diked and drained for rice fields prior to 1893. Some of the area is now maintained as waterfowl habitat. Today, most of the association is covered by marsh grasses and water-tolerant shrubs. That probably developed under forest vegetation.
Bohicket-Capers-Handsboro: All the land bordering the St. Helena Sound, as well as much of the land surrounding the lower portions of the three rivers and the sea islands, is within this soil association. All three major soils in this association are very poorly drained. Bohicket series soils (Typic Sulfaquents) are flooded by salt water twice daily with the tides and may be covered with between 15 and 91 cm (6 and 36 inches) of water. They are usually highly dissected by tidal streams. New sediments are periodically added to this soil. Bohicket series soils are found at the lowest points on this landscape. Capers series soils (Typic Sulfaquents) are very similar to Bohicket series soils, with the difference between the two beyond the scope of this work. Capers soils are found at slightly higher elevations than Bohicket soils (a matter of inches) and are not greatly dissected by tidal streams. Some areas covered by Capers soils are flooded twice daily by tides, while other areas are only flooded by extremely high tides. One of the big differences between the two, from an agricultural standpoint, is that Capers soils can bear the weight of cattle. Handsboro series soils (Typic Sulfhemists) are found in areas between lands that are flooded by salt water and lands that are flooded by fresh water. They are composed of thick layers of organic material interspersed with horizons of mineral material. They are usually flooded twice daily. A few areas were once diked and drained for rice cultivation. The vegetation of this entire association is dominated by Spartina grass. Some small areas that have been used to deposit dredge material form small wooded "islands" within the marsh. As the taxonomic names of these soils suggest, these soils contain abundant amounts of reduced sulfur, produced during the respiration of anaerobic soil organisms. Hydrogen sulfide gas (H2S), or marsh gas, is responsible for the characteristic odor of these areas. The association is unsuitable for any urban and most agricultural uses. Its most important uses are as wildlife habitat, nurseries for marine organisms, and buffers between the ocean and the land.
High Sandy Ridges
These landscapes are found both near the present day Atlantic coast and further inland, on older marine terraces. Since they are relatively well drained, land use capabilities are significantly different from those of the rest of the ACE Basin study area. Only slight to moderate limitations for many urban uses exist on many of these soils. Generally speaking, the degree of profile development in these soils increases with distance from the present-day coastline.
Echaw-Blanton-Chipley: This association is found on a wide ridge that stretches across Colleton County, between the Combahee and Edisto Rivers. Most of Interstate 95 follows this ridge across the county. The association is also found above the flood plain of the Salkehatchie River. Of the two associations labeled High Sandy Ridges, this one is on an older landscape. Echaw series soils (Oxyaquic Alorthods) are moderately well drained and are found at slightly lower elevations than the other soils. Like all Spodosols, they have a dark B horizon, stained with organic matter. Blanton series soils (Grossarenic Paleudults) are excessively drained to well-drained soils that occupy the highest ridges on this landscape. They have extremely thick sandy horizons (>100 cm or 40 inches) over sandy loam B horizons. The parent material is likely partially aeolian in origin (sand dunes). Chipley series soils (Aquic Quartzipsamments) are moderately well drained and occupy the intermediate elevations in this association. They are sandy throughout and have little horizon development. Subsurface horizons are stained with varying amounts of iron oxides. Plinthite nodules (semi-hard iron concretions ) are commonly found at the depth of the fluctuation water table. Nearly half of the area in this association is covered by soils of minor extent.
Chipley-Eddings-Lakeland: This association is found between the Ashepoo and Combahee Rivers on high ridges bordering the salt marshes of the Bohicket-Capers-Handsboro association. It is also found in the interior of Edisto Island and on a few long ridges in Charleston County. This association's proximity to the coast indicates that it is on younger landscapes than the association described above. Chipley soils, described in the preceding association, are found at intermediate elevations. Eddings series soils (Grossarenic Paleudults) are well drained and occupy the higher elevations in this association. Similar to the Blanton series, they have thick, sandy surface horizons (>100 cm or 40 inches) over sandy loam subsoils. Lakeland series soils (Typic Quartzipsamments) are excessively drained. They are at similar elevations to Eddings soils, but are most commonly found adjacent to tidal streams. Like all Entisols, they have little horizon development, aside from the A horizon.
Low Relief Uplands
The soil associations grouped in this category have many similarities to each other, as well as similarities to soil associations throughout the Southern Coastal Plain. The main similarity is that most of them represent toposequences, which are groups of soils that have different landscape positions, but the other four environmental factors influencing their development are the same. That is, they formed in the same parent material, exist in the same climate, are approximately the same age, and are influenced by the same organisms. Landscape position is the reason for differences found among members of a toposequence. The key here is the landscape position's influence on soil drainage. (Low-lying soils are wet; elevated soils are dryer.) This is a good way to think about the landscape of the Southern Coastal Plain in general; however, the five factors of soil formation are not completely independent of each other. For example, landscape position and drainage have a powerful influence on the types of organisms that can live in these soils, especially in the case of microorganisms. The different populations of microorganisms cause many of the chemical differences among these soils. [Note: Several of the other associations in the other landscape groups are also toposequences.] Only two representative soil associations will be described in this section. Only relative landscape position and drainage will be mentioned for the other associations.
Ogeechee-Yemassee-Yauhannah: This association is found in several places in Colleton County, between Walterboro and the coast. The towns of Ritter and Hendersonville are found in this association. The landscape is nearly level, and an untrained eye might find it difficult to discern the ridges from the depressions. The difference in elevation among these three soils is often a matter of inches. Ogeechee series soils (Typic Endoaquults) are poorly drained and are found in upland depressions and in poorly defined drainage ways. The surface horizon is dark gray loam, underlain by gray, sandy clay loam B horizons. This gray coloration is typical of wetland soils. Yemassee series soils (Aeric Endoaquults), somewhat poorly drained, are found at the tops of low ridges and at intermediate elevations on higher ridges. The surface horizon is dark gray loamy sand, underlain by a mottled-gray sandy-clay-loam subsoil. Yauhannah series soils (Aquic Hapludults) are moderately well drained and are found on the higher ridges in this association. The subsoil is a brown or yellow sandy clay loam. Large portions of this association are covered by soils of minor extent. The colors of the subsoils are mentioned here because they allow field soil scientist to differentiate between them.
Bladen-Argent-Wahee: This association covers large areas of Colleton County, between the major river flood plains of the ACE Basin study area. The town of Jacksonboro lies in this association. The landscape is nearly level, and like the one above, many might find it difficult to discern the ridges from the depressions. Bladen series soils (Typic Albaquults) are poorly drained and occupy the intermediate elevations of this association. The surface is a black loam, underlain by gray clay subsoil. Argent series soils (Typic Endoaqualfs) are very poorly drained soils that occupy the lowest elevations on this landscape. These wetland soils have clayey subsoil that sometimes contains calcium carbonate concretions at depth. They are higher in bases than the other two soils in this association, perhaps as a result of the translocation of these materials from the higher elevation soils. Wahee series soils (Aeric Endoaquults) are somewhat poorly drained and occupy the highest elevations in this landscape. The surface horizon is somewhat sandier than those of Bladen and Argent soils, possibly due to aeolian deposits of sand. The subsoil is mottled gray clay.
Goldsboro-Lynchburg-Rains: Goldsboro series soils (Aquic Paleudults) are moderately well drained and are found at the highest elevations on this landscape. Lynchburg series soils (Aeric Paleaquults) are somewhat poorly drained and are found at intermediate elevations on this landscape. Rains series soils (Typic Paleaquults) are poorly drained and are found in depressions and drainage ways.
Lynchburg-Rains-Paxville: Lynchburg series soils (Aeric Paleaquults) are somewhat poorly drained and are found on top of low ridges. Rains series soils (Typic Paleaquults) are poorly drained and are found at intermediate elevations on broad, low areas and depressions. Paxville series soils (Typic Umbraquults) are very poorly drained and are found in drainage ways at the lowest elevations.
Coosaw-Williman: Coosaw series soils (Aquic Arenic Hapludults) are moderately well drained and occupy higher elevations. Williman series soils (Arenic Endoaquults) are poorly drained and occupy the lower elevations.
Coosaw-Williman-Ridgeland: Coosaw series soils (Aquic Arenic Hapludults) are moderately well drained, and occupy the higher elevations. Williman series soils (Arenic Endoaquults) are poorly drained, and occupy the lower elevations. Ridgeland series soils (Oxyaquic Alorthods) are somewhat poorly drained, and occupy intermediate elevations.
Bladen-Coosaw-Wahee: Bladen series soils (Typic Albaquults) are poorly drained and occupy the lowest elevations. Coosaw series soils (Aquic Arenic Hapludults) are moderately well drained and occupy higher elevations. Wahee series soils (Aeric Endoaquults) are somewhat poorly drained and occupy the intermediate elevations.
Mouzon-Brookman-Wahee: Mouzon series soils (Typic Albaqualfs) are poorly drained and occupy the intermediate elevations of this landscape. Brookman series soils (Typic Umbraqualfs) are very poorly drained and occupy the lowest elevations of this landscape. Wahee series soils (Aeric Endoaquults) are somewhat poorly drained and occupy the intermediate elevations of this landscape.
Wadmalaw-Yonges-Meggett: Wadmalaw series soils (Umbric Endoaqualfs) are poorly drained and occupy the lowest elevations on this landscape. They are commonly covered with water. Yonges series soils (Typic Endoaqualfs) are poorly drained and are on the higher elevations on this landscape. Meggett series soils (Typic Albaqualfs) are also poorly drained and are found on intermediate elevations on this landscape.
Yonges-Eulonia-Edisto: Yonges series soils (Typic Endoaqualfs) are poorly drained and are found at the lowest elevations on this landscape. Eulonia series soils (Aquic Hapludults) are moderately well drained and are found at the higher elevations. Edisto series soils (Glossaquic Hapludalfs) are somewhat poorly drained and are found at intermediate elevations on this landscape.
R. Scharf, SCDNR Land, Water, and Conservation Division
Jenny, H. 1941. Factors of soil formation. McGraw-Hill, New York, NY.
Simonson, R.W. 1959. Outline of a generalized theory of soil formation. Soil Science Society of America Proceedings 23:152-156.
Soil Survey Staff. 1975. Soil taxonomy: a basic system of soil classification for making and interpreting soil surveys. US Department of Agriculture, Soil Conservation Service. U.S. Government Printing Office, Washington, DC.
Soils form in response to interactions between soil-forming factors such as climate, parent material, biotic inputs, top-og-raphy and time. Outcomes from this ‘pedogenesis’ fall into four broad groups, entisols, oxisols, vertisols and alﬁsols (Table 16.1, Figure 16.1). Evolution of natural ecosystems and agri-cultural options vary accordingly, with more productive communities based on sites that are richer in plant-available nutrients.
Nutrients for plant growth are derived from minerals in parent material by weathering, and four main processes are involved: oxidation/reduction, hydrolysis, solution and hydration. Minerals such as kaolinite and iron oxides tend to accumulate in soils because of their resistance to weathering but they are low in plant nutrients.
Elements released from parent material migrate during pedogenesis according to their mobility and solubility which in turn are functions of the pH and oxidation/reduction (redox) potential of the soil system. Basic cations such as Ca2+ and Mg2+ and elements such as P, S and Cl do not form insoluble hydroxides and are readily leached, whereas Al, Fe and Mn form insoluble hydroxides and accumulate.
Compared to New Zealand and much of the northern hemisphere, parent materials of Australian soils are old and weathered. Where the northern hemisphere was swept clean by glaciation during the Pleistocene, parent materials have been weathering for less than 10 000 years. By contrast, soils of inland Australia have formed on parent materials that have experienced many cycles of weathering, erosion and deposition. Some of these weathered materials have been either deposited or exposed (by denudation of the landscape) only recently. Soils formed on parent materials such as these unconsolidated sediments or sedimentary rocks are young in terms of pedogenesis but nevertheless low in nutrients, a feature usually associated with soils that have been developing for tens of thousands of years. In other areas such as central Queensland, red and yellow earths (oxisols) have been developing in excess of one million years. Texture-contrast soils (alﬁsols and ultisols), so common around the eastern seaboard and used for dryland grain cropping, have taken in excess of 25 000 years to form. By contrast, soils of New Zealand are young inasmuch as they have formed on recently deposited or recently exposed materials, but are nutrient rich compared to Australia. For example, a large area of the North Island is mantled by pumice with resulting soils <4000 years old. Volcanic loamy clay soils in the same region can be >50 000 years old.
Because so many soils have been developing for such a long time, soil characteristics cannot be attributed to recent climate, and as a corollary present-day distribution of soils cannot be explained by climatic variables alone (largely rainfall and temperature). Nevertheless, abrupt boundaries between a number of major soil zones in southeastern Australia are attributable to major changes in chemical and/or physical properties of parent materials.
Soil nutrient status is often closely correlated with parent material. For example, in eastern New South Wales and southern Queensland, chocolate soils (mollisols) and black earths (vertisols) occur only on basic parent rocks such as basalts whereas the red-brown earths (alﬁsols) occur only on granites and acidic sediments. These contrasting parent materials produce sharp differences in nutrient resources of resulting soils. Average P content (surface horizon) of a black earth is 870 ppm compared with a red-brown earth where P is usually <200 ppm (Table 16.1).
Parent material exerts a strong influence on pedogenesis in drier climates and especially at the beginning of soil devel-opment. In particular, chemical characteristics of parent rock or sediment are largely responsible for chemical fertility of resulting soils. Soils developed over rocks and sediments high in ferromagnesian minerals such as olivine and augite have high levels of basic cations and trace elements necessary for plant growth. They are also the least resistant to chemical and physical breakdown. Quartz, muscovite, zircon, tourmaline and iron oxides such as goethite and hematite are the most stable and also contain only minor amounts of the elements needed for plant growth.
Physical and chemical characteristics of parent rocks can also have a marked influence on physical fertility of soils. Resistance to erosive agents (wind and water) is largely a function of aggregate stability, a property which is dependent on organic matter, divalent cations and clay type. Formation of water-stable aggregates is also greatly enhanced by pro-liferation of plant roots and their associated ﬁlamentous micro-organisms. Regularly cropped soils thus beneﬁt greatly from a pasture (especially grass) phase.
Development of agriculture and horticulture in Australia has been markedly influenced by two soil characteristics related to the nature of the parent material, namely salinity and nutrient status. Large areas of mainland Australia are mantled by saline and/or sodic soils which have developed from sedimentary materials in which salts have accumulated over millions of years. In many areas where surface soils were not
initially saline or sodic, secondary salinisation has followed land clearing. Their potential for salinisation and sodiﬁcation is high because of a saline water table close to the surface (Section 17.1).
By world standards, large areas of Australia are deﬁcient in major nutrients (P, N, K and S) as well as trace elements (Mo, Cu, Zn and Mn). To underline this contrast, the overall P average of Australian soils is around 300 ppm whereas average P in the USA is >500 ppm and in England about 650 ppm. Low nutrient status, especially P and trace elements, is largely a consequence of time and parent material. Weathering of primary minerals and leaching of products in Late Tertiary periods have left many proﬁles low in basic cations, trace elements and P. These deﬁciencies have exercised a strong selection pressure in nature, and call for major nutrient additions in agriculture. An appreciation of soil–plant interactions ensures that such additions are used to best effect.
Figure 16.1 Soils in Australia comprise four major groups: (a) entisols (deep sands), (b) vertisols (cracking clays), c) oxisols (massive sequioxidic soils) and (d) alfisols (texture contrast soils, and in this case with differential development of the A horizon). (Based on Hubble et al. 1983, a source with a much more comprehensive account of soil types in Australia)