Soil profile and soil properties
What is soil?
The development of a soil from inorganic and organic materials is a complex process. Intimate interactions of the rock and hydrologic cycles produce the weathered rock materials that are basic ingredients of soils. Weathering is the physical and chemical breakdown of rocks and the first step in soil development. Weathered rock is further modified by the activity of soil organisms into soil, which is called either residual soil or transported soil, depending on where and when it has been modified. The more insoluble weathered material may remain essentially in place and be modified to form a residual soil, such as the red soils of the Piedmont in the south-eastern United States. If weathered material is transported by water, wind, or glaciers and then modified in its new location, it forms a transported soil, such as the fertile soils formed from glacial deposits in the American Midwest. A soil can be considered an open system that interacts with other components of the geologic cycle. The characteristics of a particular soil are a function of climate, topography, parent material(the rock or alluvium from which the soil is formed), time (age of the soil), and organic processes(activity of soil organisms). Many of the differences we see in soils are effects of climate and topography, but the type of parent rock, the organic processes, and the length of time the soil forming processes have operated are also important.
Soil profile or Soil Horizons
Vertical and horizontal movements of the materials in a soil system create a distinct layering, parallel to the surface, collectively called a soil profile. The layers are called zones or soil horizons. Our discussion of soil profiles will mention only the horizons most commonly present in soils. The O horizon and A horizon contain highly concentrated organic material; the differences between these two layers reflect the amount of organic material present in each. Generally, the O horizon consists entirely of plant litter and other organic material, while the underlying A horizon contains a good deal of both organic and mineral material. Below the O or A horizon, some soils have an E horizon, or zone of leaching, a light-coloured layer that is leached of iron-bearing components. This horizon is light in colour because it contains less organic material than the O and A horizons and little inorganic colouring material such as iron oxides. The B horizon, or zone of accumulation, underlies the O, A, or E horizon and consists of a variety of materials trans-located downward from overlying horizons. Several types of B horizon have been recognized. Probably the most important type is the argillic B, or horizon. A horizon is enriched in clay minerals that have been trans-located downward by soil-forming processes. Another type of B horizon of interest to environmental geologists is the horizon, characterized by the accumulation of calcium carbonate. The carbonate coats individual soil particles in the soils and may fill some pore spaces (the spaces between soil particles), but it does not dominate the morphology (structure) of the horizon. A soil horizon that is so impregnated with calcium carbonate that its morphology is dominated by the carbonate is designated a K horizon. Carbonate completely fills the pore spaces in K horizons, and the carbonate often forms in layers parallel to the surface. The term caliche is often used for irregular accumulation or layers of calcium carbonate in soils. The C horizon lies directly over the unaltered parent material and consists of parent material partially altered by weathering processes. The R horizon, or unaltered parent material, is the consolidated bedrock that underlies the soil. However, some of the fractures and other pore spaces in the bedrock may contain clay that has been trans-located downward. The term hardpan is often used in the literature on soils. A hardpan soil horizon is defined as a hard (compacted) soil horizon. Hardpan is often composed of compacted and/or cemented clay with calcium carbonate, iron oxide, or silica. Hardpan horizons are nearly impermeable and, thus, restrict the downward movement of soil water.
Soil Color
One of the first things we notice about a soil is its colour, or the colours of its horizons. The O and A horizons tend to be dark because of their abundant organic material. The E horizon, if present, may be almost white, owing to the leaching of iron and aluminium oxides. The B horizon shows the most dramatic differences in colour, varying from yellow-brown to light red-brown to dark red, depending upon the presence of clay minerals and iron oxides. The horizons may be light coloured due to their carbonates, but they are sometimes reddish as a result of iron oxide accumulation. If a true K horizon has developed, it may be almost white because of its great abundance of calcium carbonate. Although soil colour can be an important diagnostic tool for analysing a soil profile, one must be cautious about calling a red layer a B horizon. The original parent material, if rich in iron, may produce a very red soil even when there has been relatively little soil profile development. Soil colour may be an important indicator of how well drained a soil is. Well-drained soils are well aerated (oxidizing conditions), and iron oxidises to a red colour. Poorly drained soils are wet, and iron is reduced rather than oxidised. The colour of such a soil is often yellow. This distinction is important because poorly drained soils are associated with environmental problems such as lower slope stability and inability to be utilised as a disposal medium for household sewage systems (septic tank and leach field).
Soil Texture
The texture of a soil depends upon the relative proportions of sand-, silt-, and clay-sized particles. Clay particles have a diameter of less than 0.004 mm, silt particles have diameters ranging from 0.004 to 0.074 mm, and sand particles are 0.074 to 2.0 mm in diameter. Earth materials with particles larger than 2.0 mm in diameter are called gravel, cobbles, or boulders, depending on the particle size. Note that the sizes of particles given here are for engineering classification and are slightly different from those used by the U.S. Department of Agriculture for soil classification. Soil texture is commonly identified in the field by estimation, then refined in the laboratory by separating the sand, silt, and clay and determining their proportions. A useful field technique for estimating the size of sand-sized or smaller soil particles is as follows: It is sand if you can see individual grains; silt if you can see the grains with a 10¥ hand lens; and clay if you cannot see grains with such a hand lens. Another method is to feel the soil: Sand is gritty (crunches between the teeth), silt feels like baking flour, and clay is cohesive. When mixed with water, smeared on the back of the hand, and allowed to dry, clay cannot be dusted off easily, whereas silt or sand can.
Soil Structure
Soil particles often cling together in aggregates, called peds, that are classified according to shape into several types. The type of structure present is related to soil-forming processes, but some of these processes are poorly understood. For example, granular structure is fairly common in A horizons, whereas blocky and prismatic structures are most likely to be found in B horizons. Soil structure is an important diagnostic tool in helping to evaluate the development and approximate age of soil profiles. In general, as the profile develops with time, structure becomes more complex and may go from granular to blocky to prismatic as the clay content in the B horizons increases.
Relative Profile Development
Most environmental geologists will not have occasion to make detailed soil descriptions and analyses of soil data. However, it is important for geologists to recognise differences among weakly developed, moderately developed, and well-developed soils; that is, to recognise their relative profile development. These distinctions are useful in the preliminary evaluation of soil properties and help determine whether the opinion of a soil scientist is necessary in a particular project:
- A weakly developed soil profile is generally characterised by an A horizon directly over a C horizon (there is no B horizon or it is very weakly developed). The C horizon may be oxidized. Such soils tend to be only a few hundred years old in most areas, but may be several thousand years old.
- A moderately developed soil profile may consist of an A horizon overlying an argillic horizon that overlies the C horizon. A carbonate horizon may also be present but is not necessary for a soil to be considered moderately developed. These soils have a B horizon with trans-located changes, a better-developed texture, and redder colours than those that are weakly developed. Moderately developed soils often date from at least the Pleistocene (more than 10,000 years old).
- A well-developed soil profile is characterized by redder colours in the horizon, more trans-location of clay to the horizon, and stronger structure. A K horizon may also be present but is not necessary for a soil to be considered strongly developed. Well-developed soils vary widely in age, with typical ranges between 40,000 and several hundred thousand years and older.
Soil Chronosequences
A soil chronosequence is a series of soils arranged from youngest to oldest on the basis of their relative profile development. Such a sequence is valuable in hazards work, because it provides information about the recent history of a landscape, allowing us to evaluate site stability when locating such critical facilities as a waste disposal operation or a large power plant. A chronosequence combined with numerical dating (applying a variety of dating techniques, such as radiocarbon 14C, to obtain a date in years before the present time of the soil) may provide the data necessary to make such inferential statements as, There is no evidence of ground rupture due to earthquakes in the last 1000 years, or The last mud flow was at least 30,000 years ago. It takes a lot of work to establish a chronosequence in soils in a particular area. However, once such a chronosequence is developed and dated, it may be applied to a specific problem. Consider, for example, an offset alluvial fan along the San Andreas fault in the Indio Hills of southern California. The fan is offset about 0.6 km (60,000 cm). Soil pits excavated in the alluvial fan suggest that it is at least 20,000 years old, but younger than 45,000 years.
The age was estimated on the basis of correlation with a soil chronosequence in the nearby Mojave Desert, where numerical dates for similar soils are available. Soil development on the offset alluvial fan allowed the age of the fan to be estimated. This allowed the slip rate (the amount of offset of the fan divided by the age of the fan that is, for this part of the San Andreas fault to be estimated at about 3 cm annually. More recent work, using a numerical dating technique known as exposure dating, suggests the age of years and a total displacement of about 570 m. This new and more accurate estimation of age provides a maximum slip rate of about 1.7 cm/yr. Thus, the earlier soil date has been improved by more recent technology. As numerical dating has improved, the use of soil development as a dating tool has decreased. The slip rate for this segment of the fault was not previously known. The rate is significant because it is a necessary ingredient in the eventual estimation of the probability and the recurrence interval of large, damaging earthquakes.