Soil (Forensic Science)
Soil has been used as material evidence in crime scene investigations since the 1890’s. For many years, basic microscopy and morphological analyses were the primary means of soil comparison, but increasingly sophisticated techniques have greatly enhanced forensic scientists’ ability to compare the contents of soil samples. Depending on the type of case and the other types of evidence available, physical examination of soil alone might provide the complementary information needed. Soils can be classified into different types based on their physical characteristics. Geologists, for example, classify soils according to particle size distribution, pH, color, and moisture content as well as other physical features. The analysis of soils for forensic purposes, however, often requires more detail than simple physical examination can provide. Forensic scientists look at soil not as an isolated material but as a group of materials, including any particles and any organisms that are part of a given sample.
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Chemical Analyses (Forensic Science)
The quantities of soils found at crime scenes are not necessarily abundant, and small samples often limit the techniques forensic scientists can use to perform some physical analyses. Small sample size is not an impediment to analysis of soil’s content, however. Scientists can chemically analyze soils for trace elements and metals using techniques such as mass spectrometry (MS), which establishes a relationship between the mass and the ratio of the elements in a sample. MS technology is often coupled with other, more sensitive technologies to elucidate the elemental composition of a wide array of samples, ranging from the simplest to the most complicated matrices, including, but not limited to, drugs, chemical warfare agents, and environmental samples. Some of the technologies used in combination with MS are inductively coupled plasma (ICP-MS), gas chromatography (GC-MS), liquid chromatography (LC-MS), glow discharge (GD-MS), and capillary electrophoresis (CE-MS).
Other analysis methods that do not involve mass spectrometry can provide similar results, such as inductively coupled plasma-optical emission spectrometry (ICP-OES) and atomic absorption spectroscopy (AAS). These various techniques provide different separation matrices and principles, and analysts must decide which should be used based on the type of sample being analyzed, the limit of detection, and the output resolution requirement.
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Molecular Analyses (Forensic Science)
When the amount of soil recovered at a crime scene is sufficient for both physical and chemical analyses, a more specific soil profile can be obtained, and this can help establish soil uniqueness. In some instances, however, the recovered amount of soil is too minute to allow either physical or chemical analysis. In such cases, information on soil content may be obtained through DNA (deoxyribonucleic acid) analysis of microbial, fungal, and plant genomes present in the soil. Recent studies have shown that such analysis can provide unique information about the organism or material in question. Novel molecular techniques coupled with separation technologies used for human DNA analysis have been able to provide unique soil “fingerprints” that can be compared with known samples.
Specific markers exist in the DNA of every organism. Plants have sequences repeated in tandem, as is the case with humans. Microbes and fungi contain conserved and variable regions throughout their genomes; the differences encountered in the variable regions are what give each organism its unique identity. Ribosomal ribonucleic acid (rRNA) has been the marker of choice in the analysis of microbial communities because, unlike protein markers, rRNA is ubiquitous.
Terminal restriction fragment length polymorphism (TRFLP) and amplicon length heterogeneity (ALH) have both proven successful in determining the microbial community composition of...
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Further Reading (Forensic Science)
Conklin, Alfred R. Introduction to Soil Chemistry: Analysis and Instrumentation. Hoboken, N.J.: John Wiley & Sons, 2005. Textbook describes the chemical properties of soil and the different methods that can be used to analyze soil samples.
Heath, Lorraine E., and Venetia A. Saunders. “Assessing the Potential of Bacterial DNA Profiling for Forensic Soil Comparisons.” Journal of Forensic Sciences 51, no. 5 (2006): 1062-1068. Discusses the use of microbial DNA in establishing differences and similarities in soil material.
Moreno, Lilliana I., et al. “Microbial Metagenome Profiling Using Amplicon Length Heterogeneity-Polymerase Chain Reaction Proves More Effective than Elemental Analysis in Discriminating Soil Specimens.” Journal of Forensic Sciences 51, no. 6 (2006): 1315-1322. Compares and contrasts the different methods of soil analysis to determine which is better suited for forensic comparisons.
Petraco, Nicholas, and Thomas Kubic. “A Density Gradient Technique for Use in Forensic Soil Analysis.” Journal of Forensic Sciences 45, no. 4 (2000): 872-873. Describes the preparation of soils for physical characterization based on density.
Pye, Kenneth. Geological and Soil Evidence: Forensic Applications. Boca Raton, Fla.: CRC Press, 2007. Provides guidance regarding the potential value and limitations of geological and soil evidence in forensic...
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Background (Encyclopedia of Global Resources)
Typical soil is about 45 percent minerals and about 5 percent organic matter. The other 50 percent of soil consists of pores that hold either water or air. The liquid portion of soil contains dissolved minerals and organic compounds, produced by plants and microorganisms. The gases found in soil often are the same as those found in the air above it. Soil can support plant life if climate and moisture are suitable. It is a changing and dynamic body, adjusting to conditions of climate, topography, and vegetation. In turn, soil influences plant and root growth, available moisture, and the nutrients available to plants. While “the soil” is a collective term for all soils, “a soil” means one individual soil body with a particular length, depth, and breadth.
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Soil Profile (Encyclopedia of Global Resources)
In a typical soil, the top layer is usually dark with decomposing organic matter; the layers below are sand, silt, clay, or some combination of the three. Soil scientists classify soils on the basis of soil profile and soil formation.
Typically the top soil layer is called the O horizon, or organic matter horizon. It has rotten logs, leaf litter, and other recognizable bits of plants and animals. Underneath the O horizon is the A horizon. It is characterized by thoroughly decomposed organic matter. Water passing through the A horizon carries clay particles and organic acids through it into the B horizon. Clay or organic substances passing into the B horizon glue soil particles together, forming soil aggregates. Soil aggregates—granular, columnar, and so on—are indicators of a mature, healthy soil. The lowest level of the soil profile is the C horizon. It contains bedrock or soil parent material that shows little or no evidence of plant growth or soil formation.
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Soil Formation (Encyclopedia of Global Resources)
Soil formation takes hundreds, even thousands, of years. Parent material, climate, organisms, topography, and time all contribute. Sources of parent material include igneous, sedimentary, and metamorphic rocks (fragments of which may be deposited by water, wind, and ice), and plant and animal deposits.
Soil formation is the result of the physical, chemical, and biochemical breakdown of parent material. It also reflects the processes of weathering and change within the soil mass. Many substances are added to soil—rain, water from irrigation, nitrogen from bacteria, sediment, salts, organic residues, and a variety of substances created by humans. However, many substances are also removed from the soil—water-soluble minerals, clay, plants, carbon dioxide, and nitrogen. Other transformations also are occurring: Organic matter is decomposing, and minerals are solubilizing and changing chemical form. Clays and soluble salts that move along with the soil water cause color and chemical changes in the soil.
Parent material is a primary determinant of soil type or soil classification. All soils at the lowest category of soil classification are distinct if the parent material differs. The differences in parent materials—weathering rates, the plant nutrient content, and soil texture resulting from parent material breakdown—contribute to the formation of distinctive soils. For example, sandstone yields sandy soil with low...
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Effects of Climate (Encyclopedia of Global Resources)
Soils slowly change color and density as a result of wetting and drying, warming and cooling, and freezing and thawing. During weathering—the rubbing, grinding, and moving of rocks by water, wind, and gravity—rocks are split into smaller and smaller fragments. Soil is composed of fragments 2 millimeters or less in diameter.
The expansion force of water as it freezes is sufficient to split minerals. However, water also is involved in chemical weathering—solution, hydrolysis, carbonation, reduction, oxidation, and hydration. A simple example of solution, the dissolving of minerals in liquid, is the dissolving of salt in water. The salts then move along with the liquid. In hot arid climates, salts can move to the surface as water evaporates, creating salt flats. In wetter climates, salts can move through the soil, depleting it of necessary plant nutrients and contaminating groundwater.
Hydrolysis is the splitting of a water molecule to form hydroxides and soluble hydroxide compounds, such as sodium hydroxide. Hydration is the addition of water to minerals in rock. When a mineral such as hematite (an oxide of iron) hydrates, it expands, softens, and changes color. Carbonation is the reaction of a compound with carbonic acid, a weak acid produced when carbon dioxide dissolves in water. Water often contains carbonic acid and other organic acids produced by organic matter decomposition. These acids increase the power...
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Biological Weathering (Encyclopedia of Global Resources)
Biological weathering is a combination of physical and chemical disintegration of rocks to produce soil. The roots of plants can crack rocks and break them apart. Plant roots also produce carbon dioxide, which combines with water to produce carbonic acid. Carbonic acid dissolves certain minerals, speeding the breakdown of parent material and chemically changing the soil.
Plants and animals also add humus (organic matter) to soil, increasing its fertility and water-holding capacity and speeding rock weathering. Animals such as earthworms, ants, prairie dogs, gophers, and moles also contribute to soil aeration and fertility by mixing the soil. In areas where animal populations are large, they can influence both the formation and destruction of soil.
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Topography (Encyclopedia of Global Resources)
The shape of the land is referred to as its topography. Each landform—valleys, plains, hills, and mountains—is covered with a crazy quilt of different soil types. For example, the steep sides of the Sandia Mountains near Albuquerque, New Mexico, which are severely eroded by wind and summer rains, contain a variety of soil types—forest soils, sandy soils, and rocky soils. Sand, silt, and clay eroded from the mountains and nearby extinct volcanoes combine in the moist and fertile Rio Grande Valley. The valley has deep sandy soils, layered sand and clay soils, and soils eroded by flash floods.
Soils located in similar climates that develop from similar parent material on steep hillsides usually have thin A and B horizons because less water moves through the soil. Similar materials on shallower slopes allow more water to pass through them. Topography and climate work together either to allow or to prohibit plant growth and organic matter deposition. Without moisture, plants cannot grow to impede soil erosion, and soil development is slow. With moisture, plants can grow, hold the soil in place, add organic matter to the soil, and speed soil development.
The age of a soil may be reckoned in tens, hundreds, or thousands of years. Under ideal conditions, a soil profile may develop in two hundred years; however, under less favorable conditions soil development may take several thousand years.
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Soil Classification (Encyclopedia of Global Resources)
Scientists have identified and classified soils for hundreds of years. Soils can be grouped according to agronomic use, color, organic matter content, texture, moisture condition, and other characteristics. Each of these groupings serves a particular purpose. U.S. soil scientists adopted a system of soil classification on January 1, 1965, that was based on the knowledge they had about soil genesis, morphology, and classification. The U.S. system is divided into six categories: order, suborder, great group, subgroup, family, and series. (Soil taxonomy is patterned after the worldwide system of plant and animal taxonomy, which contains phylum, class, order, family, genus, and species.)
Changes to the system have proceeded through a number of major revisions or approximations. The system can be used to classify soils anywhere in the world. Soil classification is based on similar physical, chemical, and mineralogical properties. The minimum volume of soil that scientists consider when they classify a soil is the pedon, which can range from 1 to 10 meters square and is as deep as roots extend into a soil.
The U.S. soil classification system recognizes twelve soil orders. The differences among orders reflect the dominant soil-forming processes and the degree of soil formation. Each order is identified by a word ending in “-sol.” Each order is divided into suborders, primarily on the basis of properties that influence...
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Soil Texture, Structure, and Consistency (Encyclopedia of Global Resources)
Soil texture is determined by the percent of sand, silt, and clay in a soil sample. Most fertile or productive soils have a loam texture, or about equal amounts of sand, silt, and clay, and a high organic matter content (about 5 to 10 percent). Soil texture determines the water-holding and nutrient-holding capacity of a soil. Thus, clay soils have a high nutrient-holding capacity, but they waterlog easily. Sandy soils have a lower nutrient-holding capacity but dry out easily. Farmers base their plans of how to fertilize and irrigate their crops partly on the texture of the soil.
Soil structure refers to how soil particles are glued together to form aggregates. During soil formation, soil particles are glued together with clay, dead microorganisms, earthworm slime, and plant roots, and they form air and water channels. Plants need these channels so they can absorb nutrients, water, and air. Soil structure may be destroyed when farmers cultivate wet or waterlogged soils with heavy farm machinery; destroying soil structure makes a soil unsuitable for plant growth.
Soil consistency is the “feel” of a soil and the ease with which a lump can be crushed in one’s fingers. Common soil consistencies are loose, friable, firm, plastic, sticky, hard, and soft. Clay soils, for example, are sticky or plastic when they are wet, but they become hard or harsh when they are dry. The best time to work a clay...
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Soil Aeration and Soil Moisture (Encyclopedia of Global Resources)
Soil aeration relates to the exchange of soil air with atmospheric air. Growing roots need oxygen and are constantly expiring carbon dioxide. Unless there is a continuous flow of oxygen into soil and carbon dioxide out of the soil, oxygen becomes depleted. When their oxygen supply is cut off, the roots will die.
Soil moisture refers to water held in soil pores. A plant draws water from soil the same way a child draws water from a cup with a straw. When the cup is full, it is easy for the child to draw up the water, but as the cup empties, the child must work harder to get water. Similarly, plants draw water from soil easily when the soil has plenty of water. As the soil dries, however, plants must work harder to pull water out of the soil until they reach a wilting point.
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Soil Fertility (Encyclopedia of Global Resources)
Plants absorb many of the nutrients they need from soil, including phosphorus, potassium, calcium, magnesium, sulfur, boron, chlorine, cobalt, copper, iron, manganese, molybdenum, and zinc. They may obtain carbon, hydrogen, and nitrogen from the air and water.
Soil testing services give farmers specific fertilizer and lime recommendations based on soil texture and chemical analysis. Farmers use soil tests to determine if their soil has enough essential nutrients for a crop to grow. The absence of one essential nutrient can limit overall crop growth. Nitrogen, phosphorus, and potassium are commonly applied to the soil as commercial fertilizer and manure. Calcium and magnesium are applied as lime, which is also used to reduce the acidity of soil and to increase the solubility of some minerals. Manure and other organic matter added to soils increase water-holding and nutrient-holding capacity and therefore boost crop yields.
Agricultural extension services offer guidelines for the maximum amounts of manure, sewage sludge, fertilizer, and other chemicals that farmers should apply to soils. Farmers are encouraged to apply nitrogen fertilizer in small applications at times when plants are growing rapidly. This soil management practice decreases deep percolation losses that could pollute groundwater.
With an understanding of soil characteristics, farmers and gardeners can learn to manage a wide variety of soils. Some...
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Further Reading (Encyclopedia of Global Resources)
Ashman, M. R., and G. Puri. Essential Soil Science: A Clear and Concise Introduction to Soil Science. Malden, Mass.: Blackwell, 2002.
Brady, Nyle C., and Ray R. Weil. The Nature and Properties of Soils. 14th ed. Upper Saddle River, N.J.: Prentice Hall, 2008.
Davies, Bryan, David Eagle, and Bryan Finney. Resource Management: Soil. Brighton, Ont.: Diamond Farm Book, 2001.
Donahue, Roy L., Roy Hunter Follett, and Rodney W. Tulloch. Our Soils and Their Management: Increasing Production Through Environmental Soil and Water Conservation and Fertility Management. 6th ed. Danville, Ill.: Interstate, 1990.
Esch, Neal S., et al. Soil Science Simplified. 5th ed. Illustrated by Mary C. Bratz. Ames, Iowa: Blackwell, 2008.
Miller, Raymond W., and Roy L. Donahue. Soils: An Introduction to Soils and Plant Growth. 6th ed. Englewood Cliffs, N.J.: Prentice-Hall, 1990.
Paul, Eldor A., ed. Soil Microbiology, Ecology, and Biochemistry. 3d ed. Boston: Academic Press, 2007.
Plaster, Edward J. Soil Science and Management. 5th ed. Clifton Park, N.Y.: Delmar Cengage Learning, 2009.
Sposito, Garrison. The Chemistry of Soils. 2d ed. New York: Oxford University Press, 2008.
White, Robert E. Principles and Practice of Soil Science: The Soil as a Natural Resource. 4th ed. Malden, Mass.: Blackwell,...
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Soil (Encyclopedia of Science)
Soil, which covers most of the land surface of Earth, is a complex mixture of weathered rock debris and partially decayed organic (plant and animal) matter. Soil not only supports a huge number of organisms below its surfaceacteria, fungi, worms, insects, and small mammalsut it is essential to all life on the planet. Soil provides a medium in which plants can grow, supporting their roots and providing them with water, oxygen, and other nutrients for growth.
Soil now covers Earth in depths from a few inches to several feet. Soils began to form billions of years ago as rain washed minerals out of the molten rocks that were cooling on the planet's surface. The rains leached or dissolved potassium, calcium, and magnesiuminerals essential for plant growthrom the rocks onto the surface. This loose mineral matter or parent material was then scattered over Earth by wind, water, or glacial ice, creating the conditions in which very simple plants could evolve. Plant life eventually spread and flourished.
As these early plants died, they left behind organic residues. Animals, bacteria, and fungi fed on this organic matter, breaking it down further and enriching the parent material with nutrients and energy for more complex plant growth. Over time, more and more organic matter mixed with the parent material, a process that continues to this day.
Soil is generally...
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Soil (Science Experiments)
Life in the dirt lane
Soil pH: Does the pH of soil affect plant growth?
Commonly called dirt, soil is a central ingredient for life on Earth. Soil is the thin, outer layer of material on the surface of Earth, ranging from a fraction of an inch to several feet thick.
Plants depend on soil for their nutrients and growth. These plants are then consumed and used by animals, including people. Soils provide shelter and a home for insects and small animals. Microscopic organisms flourish in soil, breaking down dead matter, which returns nutrients into the soil for new life. People use soils directly as a material to build on and grow crops in. Soils also reveal a historical record of an area's past life and geography. Understanding the properties of a soil is a key to determining how the soil will function for a particular use.
The specific makeup of soil depends on its location, yet all soils share the same basic composition: minerals, water, air, and organicMade of, or coming from, living matter. matter, meaning matter that contains carbon and comes from living organisms. Minerals are naturally occurring inorganic or nonliving...
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