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).
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.
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.
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 development—often 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.
Generalised Theory of Soil
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.
The soil profile is one of the most important concepts in soil science. It is a key to understanding the processes that have taken in soil development and is the means of determining the types of soil that occur and is the basis for their classification. The soil profile is defined as a vertical section of the soil from the ground surface downwards to where the soil meets the underlying rock. The soil profile can be as little as 10 cm thick in immature soils and as deep as several metres in tropical areas where the climate is conducive to rapid alteration of the underlying rock to form soil. In temperate areas, the soil profile is often around a metre deep and in arid areas somewhat shallower than this.
Virtually all soil profiles are composed of a number of distinctive layers, termed horizons, interpretation of which is the key to understanding how the soil has formed. Most soils will have three or more horizons. Soils that have not been cultivated will normally have L, F and H layers at the surface. These layers largely represent different degrees of decomposition of organic matter, the L layer representing the litter layer formed of recognisable plant and soil animal remains, the F layer below, the fermentation layer, usually consisting of a mixture of organic matter in different stages of decomposition, and the H layer, the humose layer, consisting largely of humified material with little or no plant structure visible. Below these, and in cultivated soils occupying the surface layer, is the A horizon composed of a more or less intimate mixture of mineral and organic matter.
The A horizon is often referred to as the ‘ploughed layer’ in cultivated soils. It is an important part of the soil because it is a source of plant nutrients and contains the majority of plant roots. The A horizon may lie directly on the B horizon or, in well developed soils, there may be an intermediate leached horizon, termed E or A2, depending on the nomenclature system used. The E/A2 horizon is usually paler in colour than the horizons above and below because it is a horizon that has been subject to leaching and loss of components compared to the A and B horizons.
The B horizon is the horizon most widely used to identify soil types. Its morphology is important in supporting the classification of soils. In some soils the B horizon results purely from the weathering of the underlying rock whereas in other soils this weathering is supplemented by the translocation of materials from overlying horizons. Thus the B horizon needs to be inspected carefully in order to understand the genesis of the soils. B horizons may have a number of different subscripts indicative of the nature of the materials that have moved into the horizon, e.g. Bh indicates the translocation of humus into the horizon, Bs, the translocation of sesquioxides. These subscripts will vary according to the nature of the soil component that has accumulated but also with the system of nomenclature of soil types.
Below the B horizon is the C horizon. This latter horizon is often consistent with the parent material and may have been little altered from the material in which the soil originally formed.
Most soils have A, B and C horizons. Some, generally weakly developed, soils may have A horizons lying directly on C horizons. When next you see a profile down through the soil, perhaps in an excavated pit or in a roadside cutting, take time to look at the profile and see if you can identify some of the different soil layers that make up the profile.
Soils vary enormously in characteristics, but the size of the particles that make up a soil defines its gardening characteristics:
- Clay: less than 0.002mm
- Silt: 0.002-0.05mm
- Sand: 0.05-2mm
- Stones: bigger than 2mm in size
- Chalky soils also contain calcium carbonate or lime
The dominating particle size gives soil its characteristics and because the tiny clay particles have a huge surface area for a given volume of clay they dominate the other particles:
- Clay soils have over 25 percent clay. Also known as heavy soils, these are potentially fertile as they hold nutrients bound to the clay minerals in the soil. But they also hold a high proportion of water due to the capillary attraction of the tiny spaces between the numerous clay particles. They drain slowly and take longer to warm up in spring than sandy soils. Clay soils are easily compacted when trodden on while wet and they bake hard in summer, often cracking noticeably. These soils often test the gardener to the limits, but when managed properly with cultivation and plant choice, can be very rewarding to work with
- Sandy soils have high proportion of sand and little clay. Also known as light soils, these soils drain quickly after rain or watering, are easy to cultivate and work. They warm up more quickly in spring than clay soils. But on the downside, they dry out quickly and are low in plant nutrients, which are quickly washed out by rain. Sandy soils are often very acidic
- Silt soils, comprised mainly of intermediate sized particles, are fertile, fairly well drained and hold more moisture than sandy soils, but are easily compacted
- Loams are comprised of a mixture of clay, sand and silt that avoid the extremes of clay or sandy soils and are fertile, well-drained and easily worked. They can be clay-loam or sandy-loam depending on their predominant composition and cultivation characteristics
- Peat soils are mainly organic matter and are usually very fertile and hold much moisture. They are seldom found in gardens
- Chalky or lime-rich soils may be light or heavy but are largely made up of calcium carbonate and are very alkaline
Where building or landscaping has mixed up different soils, it can be very difficult to tell what type of soil you have, and it may change markedly over a short distance.