Soil

BACKGROUND

Soil composition: Soil consists of broken rock particles that occur in nature and decaying organic matter from the surface of the earth. Thus, soil is a complex mixture of both organic and mineral components. It has a solid phase as well as a liquid phase and a gas phase near the surface. This rich mixture of organic compounds is capable of supporting an enormous variety of life, ranging from bacteria and fungi to plants and animals.

The upper boundary of soil lies between the soil and either air or shallow water. The lower boundary of soil is located at 200 centimeters; generally, soil cannot support life at the lower boundary.

The geological foundation of soil dates back to the time period currently referred to as the "Ice Age." It is theorized that land was covered with huge sheets of ice, several hundred feet thick. When these sheets of ice began to melt, they brought along with them gravel, pebbles, organic substances, and various types of refuse, which in turn affected many crops. The Ice Age left its mark on the world's plants, animals, and people. The variety of all species was reduced; most species were located around the equator and then expanded as the great ice sheets melted northward.

Scientists usually describe soil according to its compaction, color, moisture content, organic content, pH, profile, structure, temperature, and texture. There are three types of soil: sandy soil, loam soil, and clay soil. Sandy soil is comprised mainly of sand (80%-100%), silt (0%-10%), and clay (0%-10%). It tends to be light and free-draining and usually does not hold water well due to low organic content. Loam soil is composed primarily of sand (25%-50%), silt (30%-50%), and clay (10%-30%). It is heavier than sandy soil; and like sandy soil, loam does not hold water well due to low organic content. Clay soil is comprised mainly of sand (0%-45%), silt (0%-45%), and clay (50%-100%). It is typically not free draining, and water takes a longer time to penetrate it. Clay soil is heavy and tends to be hard to work with when dry.

In addition to the rock component (e.g., sand, silt, and clay), soil also contains organic matter, cations (positively charged ions), and minerals. The organic material is derived from the decay of plant and animal matter, which becomes a food source for soil organisms, such as bacteria and fungi. These organisms change the nitrogen to a form that plants can use. Cations include sodium, potassium, magnesium, and calcium. Micronutrient minerals include manganese, copper, iron, zinc, boron, and molybdenum.

Soil uses: Soil is principally used in agriculture, where it serves as a nutrient base. It is also used in mining and construction industries. Soil is a foundation in virtually all construction projects. Large quantities of soil are used in surface mining, road building, and dam construction. In addition, soil is used in earth sheltering during which soil is compacted against building walls for the purpose of creating external thermal mass. Earth sheltering is used to reduce heat loss and to maintain a constant indoor air temperature; the practice has a long history of use in human civilization. In addition, because soil is an absolute requirement for tree growth, the fiber industry is dependent on soil.

Soil degradation: Soil degradation can take the form of soil erosion, salinization, nutrient loss, or compaction. Soils can degrade, without any loss of soil particles, as a result of farming practices. When a farmer ploughs, the soil loses its organic matter and changes composition. When this happens, valuable soil organisms are lost.

Soil erosion is the displacement of soil. Soil salinization refers to an excess of salts in the soil. In addition to being associated with poor crops, salinization also increases erosion. Nutrient loss may be caused by the use of land for the purpose of producing crops; when the crops are removed, the nutrients taken from the soil in the form of food are ultimately lost. Soil compaction may occur due to the weight of livestock or machinery on the soil. Soil that is compacted is less able to absorb water from rainfall. This leads to runoff (the flow of water over the soil surface) and erosion. In addition, compacted soil makes root growth difficult.

According to secondary sources, U.S. soil erodes at 17 times the rate at which it forms. This problem can also be found worldwide: in Asia, Africa, and South America, soil is eroded at 34 times the rate at which it is formed.

Reducing soil degradation: Consumers and farmers are becoming more aware of the problem of soil erosion. As a result, a number of measures have been undertaken to attempt to reduce soil erosion, such as crop rotation. Other measures to reduce erosion include contour plowing (fields are plowed along the contours of the land to help reduce soil erosion), strip cropping (growing crops in a systematic arrangement of strips across a field), and terracing (building a series of step like benches that are supported by either sod or stone walls; each level slows the flow of water runoff, thus slowing the erosion process).

Composting is a method in which organic household products (such as foods) and yard waste (such as leaves and twigs) are heated to form a rich, soil-like residue. The product, known as compost, can be used as a natural fertilizer.

Eutrophication refers to an increase in nitrogenous-containing compounds in an ecosystem. Sources of nitrogenous-containing compounds include sewage and the natural fertilizer, manure. Eutrophication decreases the amount of available oxygen, which may lead to loss of plant life on land or in water.

TECHNIQUE

Planting: There are several types of soil used for planting crops. Light soil requires less power to till (mixing soil and/or turning it over). Less energy is also required for seedlings to penetrate these soils than heavy soils. In sandy soil, the surface dries out too quickly, making deeper planting necessary in order to place seeds in moist soil. Shallow plantings are required in cold, wet soil because the oxygen concentration and temperature at greater depths may be too low for germination.

Soil testing: In order to assess the health of soil, soil testing is used. In addition, historical soil output and visual inspection are important guides for farmers. A soil test measures soil fertility and is the only precise way to determine whether the soil is acidic, neutral, or alkaline. Soil testing is inexpensive and can be used to maintain good plant health and maximum crop productivity. Additionally, because only a small percentage of nutrients in the soil are available to plants, soil tests can be very useful. Nutrient content varies with soil depth; therefore, soil should be tested at the depth where the roots for a given plant species are expected to grow.

Plant-available nitrogen is in the form of nitrate or ammonium. Excessive irrigation or rainfall will leach nitrate from the soil. In addition, pH can also affect nitrogen. For instance, the ammonia in soil exists as ammonium ion at a pH of about 7.0; however, at a pH greater than 8.0, the ammonia exists in the gas form and is subject to vaporization. Fertilizers may be useful for treating a deficiency in nitrogenous compounds in soil; they may replete the lost nitrogen, which is essential for plant growth.

The availability of micronutrients, such as iron, zinc, copper, manganese, and boron, is largely pH-dependent. Specifically, the availability of micronutrients decreases as pH increases, except for molybendum, which is more available at a lower pH. The most common micronutrient deficiencies are that of boron and zinc. A pH between 6.0 and 7.5 is suitable for most crops. The target pH is primarily determined by the crop.

Lime increases the soil's pH. A soil pH test indicates if liming is needed. The SMP (Shoemaker, MacLean, and Pratt) lime requirement test is used to determine how much lime is needed to increase the pH of six inches of soil.

Some plants, including rhododendron, azaleas, cranberries, and blueberries, require acidic soil. In order to maintain acidic soil, fertilizers such as ammonium sulfate are used. Excess soluble salts are measured via electrical conductivity (EC). Cation exchange capacity (CEC) is used to measure the capacity of a soil to retain and release potassium, sodium, magnesium, and calcium.

Many mineral nutrients are also removed from soil as a result of plant growth and the harvesting of crops. Thus, soil testing can be used to determine if and when liming, fertilizers, and other agents should be used. In this manner, soil economy is promoted, and agents such as fertilizers are not used unnecessarily.

Limiting soil erosion: Soil erosion is measured visually, physically, chemically, and biologically. Visual methods include comparing aerial photographs taken at different times. Physical methods include measuring ruts or channels in the soil. Chemical methods include measuring the loss of organic matter and any alterations in the cation exchange capacity. Biological changes include loss of soil microbes or decay of plant material.

Soil erosion can be decreased by a number of agricultural methods. For instance, erosion can be limited by practicing contour farming, also known as contour plowing. During this type of farming, fields are plowed along the natural slope of the land. Specifically, ruts are made such that the furrows are level relative to one another. Runoff, if it occurs, will not go down the slope, but rather, around the slope. This practice is used to both reduce erosion and increase water infiltration.

Terracing can also help prevent soil erosion. During terracing, a slope is transformed into a set of shorter steps, each of which is more level than the original slope. Treated wood is a common material used for building terraces. Rain will penetrate into the soil, rather than being a source of runoff.

Another method to limit soil erosion is called strip cropping. This farming technique involves growing particular crops in rows in a regular arrangement equal width apart. This agricultural method will reduce erosion caused by wind. Suitable strip crops are grasses or legumes, which can provide wind shelter to other crops. Strips are oriented to the prevailing wind direction, so as to limit erosion caused by wind.

In addition, growing coverage crops (to decrease exposed soil) or winter crops helps control erosion. For example, vetiver grass (Chrysopogon zizanioides) is a non-invasive grass that provides a hardy and robust ground cover. It has been promoted as an agent to not only reduce soil erosion, but also to improve soil health. In addition, vetiver grass has been used to stabilize potential landslide sites.

Interseeding, or sowing a coverage crop into a cash crop, may also be used to decrease erosion. Building diversions may also help.

Composting is an ancient method of attempting to restore soil health by adding lost nutrients back to the soil. In addition to being produced naturally, compost can be made by the consumer. Specifically, compost is made by heating organic waste products, such as food waste, manure, and dead leaves. Bulking agents, such as wood chips, are added to speed up the decomposition of the mixture. Composting can be very simple and may be done indoors or in a backyard. Compost is made of a three part mixture: greens, browns, and water. Greens include vegetable waste, grass clippings, and coffee grounds. Browns include twigs, branches, and dead leaves. Water is added, as moisture facilitates the breakdown of the organic material. Initially, the compost is acidic, due to the production of organic acids from the process of decomposition; as compost matures, the pH is between 6.0 and 8.0.

Soil erosion conservation and awareness is a growing concern, both economically and socially. Farmers and a number of organizations, such as the European Society for Soil Conservation (ESSC), the International Soil Conservation Organization (ISCO), the National Soil Erosion Research Laboratory, and the World Association of Soil and Water Conservation (WASWC), have gained momentum in recent years. In addition, the Internet has made it possible for individuals to have a sounding board to voice their opinions on soil degradation.

THEORY/EVIDENCE

General: Soil degradation is an umbrella term that encompasses soil erosion, salinization, nutrient loss, or compaction. Soil degradation may be the result of deforestation, overgrazing, agriculture, and industrialization. Current agricultural practices have resulted in a global problem of soil degradation. To a large extent, nutrient loss is considered by some to be inevitable when the population is increasing at a rate faster than agricultural resources can support. Soil conservation refers to a number of strategies used to limit the amount of soil degradation. Composting is an ancient method of attempting to restore soil health by adding lost nutrients back to the soil. Crop rotation is used to minimize soil nutrient loss. Crop coverage is used to reduce erosion.

The nitrogen cycle: Organic nitrogen is present in manure, sewage, compost, and decomposing roots or leaves. These products are converted into an organic soil material known as humus, which is plentiful in organic nitrogen. However, plants cannot use organic nitrogen. Soil microbes transform organic nitrogen into inorganic nitrogen, which plants can use. There are several different types of organic nitrogen. Ammonium (NH4 +) is stored in the soil. In contrast, nitrate (NO3 -) and nitrite (NO2 -) are not held by the soil particles; these compounds can leach out of the soil and enter the groundwater. Alternatively, nitrate and nitrite can be converted into nitrogen gases (N2, NO, or N2O) or ammonia gas (NH3). These gases escape from the soil and enter the atmosphere.

Soil contamination: Soil contamination results when hazardous solid or liquid substances, such as chemical solvents, fuel, or radioactive waste, are either spilled or dumped directly on soil. Generally, contaminants in the soil are physically or chemically attached to soil particles; occasionally, contaminants are trapped in the small spaces between soil particles. Soil contamination can occur, for example, from the use of pesticides, the dumping of oil, fuel, or industrial waste, or the leaching of materials from landfills. Contamination can impact the health of animals and humans through ingestion, inhalation, or physical contact with the contaminated soil.

Landslides: Landslides are a source of soil erosion. According to the National Landslide Hazards Program (NLHP), landslides are responsible for $1-2 billion in damage every year in the United States. Landslides occur when enormous masses of rock and earth move down a slope; they are caused by a loss of the natural stability of a slope. They may be associated with other natural disasters such as earthquakes, volcanic eruptions, floods, and wildfires. Urban expansion and recreation in hillsides has increased, resulting in more people at risk of harm, should a landslide occur.

Deforestation: Forests can be lost due to climate change, fires, or hurricanes. However, most recent deforestation is from humans. Specifically, deforestation may be a consequence of industrialization. For instance, wood is needed for fuel and industrial uses (e.g., paper). According to the FAO (the Food and Agriculture Organization), in 2005, deforestation occurred worldwide at a rate of 34.5 million acres (14 million hectares) per year. Furthermore, the FAO estimates that a quarter of the greenhouse gases released into the atmosphere are the direct result of deforestation. Thus, deforestation is considered to be a significant contributor to global warming. In addition, deforestation changes the ecosystem and therefore, may affect local biodiversity. Deforestation (logging) can lead to erosion because trees provide protection to the soil.

Soil erosion: Soil erosion occurs by downward movement in response to gravity or by force via living organisms. It occurs mainly from the action of water and wind and has been present for hundreds of millions of years. Soil erosion may occur due to heavy rains on weak soil, vegetation depleted by drought, steep slopes, sudden climate change, intensive farming, housing development, and road construction.

Generally, in background erosion, soil is slowly repleted at the same slow rate at which it is lost. However, soil is currently eroding at an accelerated rate, in which the loss of soil exceeds the rate of soil formation. Accelerated soil erosion is a consequence of overutilization of land, including deforestation, overgrazing, and road-building. Moreover, fallen leaves can absorb rain water; the absence of leaves removes another layer of protection to the soil from the eroding effects of rainfall. In addition, logging can cause soil compaction, which reduces regrowth of vegetation. Similarly, overgrazing can reduce vegetation, and barren lands are more susceptible to erosion.

Soil erosion is a major issue according to a number of organizations, such as the European Society for Soil Conservation (ESSC), and it will likely become an even greater issue in the future as population growth continues to expand and land resources are more intensively used, often to a point of destruction. Loss of land, greater rainfall runoff, reduced soil fertility, and increased soil erosion may result in higher contaminants in diminishing water supplies, lowered quality of drinking water, increased flooding, and diminished economic benefits and increased hardships to both rural and urban populations. However, natural soil erosion does have its benefits and is, in fact, healthy for the ecosystem. For example, natural erosion aids in water moving large gravel downstream.

Nutrient loss: Farming itself results in the loss of nutrients from the soil, as the nutrient-rich crop is removed from the soil. In addition, surface runoff also causes nutrient loss.

Nitrogen runoff: The combination of erosion and excess nitrogen from either livestock or fertilizer may lead to eutrophication (over-enrichment of nutrients) in land and water. In addition, heavy rainfall may leech out soluble nitrogen from the soil, which can lead to nutrient loss of the soil of origin and potential eutrophication of water or soil (if the destination soil already contains sufficient nitrogen).

Produce: Some people believe that soil changes the taste of produce, either making it taste better or worse, but there are many discrepancies and conflicting opinions on the subject. The evidence is currently unclear regarding what effect the type of soil used has on the nutrient content of produce.

Composting: Through decomposition, composting transforms organic material (plant matter) into a soil-like material called compost. Invertebrates (insects and earthworms) and microorganisms (bacteria and fungi) help transform the material into compost. Composting is a natural form of recycling, which continually occurs in nature. In addition to being an economical and environmentally friendly fertilizer, compost reduces the volume of household waste.

Natural vs. synthetic fertilizers: Nitrogen-based fertilizers can be natural or synthetic. Natural fertilizers include manure and compost, both of which are used extensively in organic farming. In contrast, synthetic fertilizers are created from inorganic nitrogen. Consequently, the production of synthetic fertilizers requires a large amount of fossil fuel. Nitrate is an example of a synthetic fertilizer. One major problem with nitrate is that it can leach out into groundwater.

Organic farming vs. conventional farming: Organic farming is an agricultural method that uses crop rotation, composting, and green manure (a type of cover crop). Organic farming is built upon the principle of sustainable agriculture, which involves producing food without depleting natural resources or polluting the environment. In order to be considered organic, crops must be grown and harvested in accordance with certain standards. In the United States, these standards are set by the U.S. Department of Agriculture (USDA). These include, but are not limited to, the use of natural fertilizers (such as manure or compost), cover crops and mulches, reduction of tillage, controlling pests without pesticides, creating an environment for beneficial soil organisms, agricultural methods that limit runoff and soil erosion, and crop rotation. For example, where a conventional farmer might use pesticides, an organic farmer might try insect traps or populating the area with particular birds that eat a given insect.

There is a growing concern among consumers regarding the pesticide residue in conventionally grown foods. Some research suggests that the amount of pesticides on food products is not enough to pose a significant health risk; however, some people think that any amount of pesticides in foods is too much, which is one of the many reasons people cite for "going organic."

Organic foods are the fastest growing segment in the food market. However, organic food is considerably more expensive than non-organic food, as it generates fewer crops per acre of land. Furthermore, organic food has a shorter shelf life, as there are no additives or preservatives to increase shelf life, such as the non-organic practice of waxing apples. The USDA certifies organic food but it makes no claims regarding safety and nutrition. There is currently a lack of available evidence that the nutritive content differs between conventional and organic foods. Additionally, the safety of organic food versus conventional food is a very controversial topic and is unclear at this time.

Dairy farming: Dairy farming may have a detrimental effect on soil for several reasons. Grazing results in soil compaction and depletion of vegetation, which increases the potential for soil erosion. In addition, the deposition of massive quantities of urine and feces causes soil damage. Urine can burn vegetation and is also toxic to plant roots. Urine may also lead to loss of calcium and magnesium from soil. Although feces are the basis of natural fertilizer, too much nitrogenous waste in one area can be harmful.

The nitrogen that is present in urine and feces can undergo several fates. Specifically, the nitrogen can form ammonia, dinitrogen (N2), nitrous oxide (N2O), or nitrate. Furthermore, in an area where there is more nitrogen than carbon, there is the potential for eutrophication, which can lead to loss of plant life. In addition, N2O is a potent greenhouse gas and has deleterious effects on the atmosphere.

HEALTH IMPACT/SAFETY

Soil erosion: Erosion removes topsoil, which is the upper surface of the Earth's crust that is usually no deeper than about eight inches. This process reduces the levels of organic matter present in the soil, creating a less favorable environment for plant growth. No longer available to support plant growth, nutrients removed by erosion accumulate in water and may lead to algal bloom and lake eutrophication (over-enrichment of nutrients). However, eutrophication is not limited to water. It can adversely affect terrestrial (land) ecosystems as well. For instance, species that have adapted to low nutrient levels may be overtaken by other plants that grow well in the presence of high nutrient levels. Thus, a crop selected to be grown in a given area may be displaced by an unwanted crop that grows better in eutrophic conditions.

Landslides: Landslides frequently destroy everything in their path and may result in a number of other natural disasters, such as triggered waves and tsunamis (an unusually large sea wave produced by a seaquake or undersea volcanic eruption). A slope that becomes drenched with water results in a mudflow.

Soil contamination: Soil contamination results when solid or liquid hazardous materials are either spilled or dumped directly on soil. Generally, contaminants in the soil are physically or chemically attached to soil particles, although they can also be trapped in the small spaces between soil particles. Contamination can impact the health of animals and humans through the ingestion, inhalation, or touching of contaminated soil. Soil that is contaminated by such agents as organic solvents and industrial waste is potentially cancer-causing (carcinogenic). In addition, agricultural workers have an increased exposure to pesticides over their lifetimes and it is well documented that such occupational exposure increases the risk of certain types of cancer.

Dust: Small particles of soil, known as dust, can adversely affect health. Specifically, dust particles that are 2.5 microns or smaller can penetrate the lungs. The smaller the particles, the greater the lung penetration. Dust can cause breathing problems, as well as skin and eye irritation. Furthermore, dust can transport fungal spores, pollen, and bacteria. Fungal spores can transmit diseases, including histoplasmosis (infectious disease that primarily affects the lungs) and valley fever.

Mining and excavation: Soil pits and trench excavations are a potential safety hazard. According to the U.S. Department of Agriculture (USDA) National Resource Conservation Service, excavation cave-ins are responsible for 1,000 work-related injuries every year. In addition, these cave-ins kill more than a dozen children in the United States every year. The reason for this localized collapse of earth is the enormous weight of soil (3,000 pounds per cubic yard) and the resultant force on these excavation structures.

Soil bacteria: A recent study in mice reported that exposure to non-pathogenic soil bacteria, such as Mycobacterium vaccae, reduced signs of depression. Researchers from the University of Bristol in the UK found that certain soil bacteria may improve mood by boosting the immune system. In the study, mice were exposed to a harmless soil microbe called Mycobacterium vaccae and then performed a behavioral task commonly used to test the efficacy of antidepressant drugs. The researchers found that the bacteria-exposed mice continued paddling around much longer than the control mice. Mice given antidepressant drugs also appeared more determined to escape, noted researchers. These results are comparable to a medical trial conducted a few years ago in which human cancer patients treated with the bacteria reported significant increases in their quality of life. Researchers explained that while M. vaccae is no longer being pursued as a treatment for cancer because it was not shown to prolong life, patients did report increases in vitality and cognitive function and decreases in pain. The researchers suggested that the microbes affect the brain indirectly by causing immune cells to release chemicals called cytokines that may activate the nerves that relay signals from the body to the brain. The stimulated nerves cause certain neurons in the brain to release a chemical called serotonin into the prefrontal cortex, an area of the brain known to be involved in mood regulation, among other things. A lack of serotonin in the brain is thought to cause depression. The researchers concluded that the new findings take this idea, called the "hygiene hypothesis," a step further suggesting bacteria-exposure not only boosts the immune system, but also alters vulnerability to conditions such as depression as well.

FUTURE RESEARCH OR APPLICATIONS

Soil science is now an academic discipline in universities. Soil provides the foundation for food and landscape. The principles of physics, chemistry, and biology can be applied to understanding the nature of soil and the economics of soil use as well as possible ways to optimize agriculture. Land and water conservation are continued concerns, and soil science may help reduce this burden. With severely degraded areas growing each day, ecological restoration, one of the fastest growing scientific subjects today, will become more important, a discipline where soil science plays a major role. Human pressure on soil and water resources will likely continue to increase as the world population continues to grow. The threats are numerous and well-documented and include loss of organic matter and fertility, erosion, pollution, losses to urban development, and losses of soil functions and services such as water storage and nutrient cycling.

AUTHOR INFORMATION

This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).

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