Soil Essay ⋆ Environment Essay Examples ⋆ EssayEmpire

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Soils function in a myriad of ways, including supporting plant growth, serving as a major component of the hydrologic cycle, recycling nutrients, providing habitat for soil organisms, and acting as an engineering medium. The importance of these soil functions to the environment as well as society cannot be understated.

Soil’s role as a medium for plant growth includes anchoring the plant’s root system; providing air, water, and nutrients to the roots; and protecting the root system from the temperature fluctuations (and extremes) that occur at the surface. Soil’s support of plant life and role in nutrient cycling provides food and fiber for subsistence as well as livelihoods. As a component of the hydrologic cycle, soils serve as a reservoir that fills up in times of abundant precipitation and, in time, releases the stored water to transpiring plants and recharges groundwater reservoirs.

Soils serve as a recycler by hosting microbial populations that assimilate organic waste, including plant and animal detritus, and converting it to humus, mineral forms that can be used by plants and animals, and carbon dioxide (used in photosynthesis). Soil’s ability to incorporate organic wastes allows the treatment of animal, domestic, and industrial wastes, though application should be monitored with an eye to the texture and permeability properties of the soil to which these treatments are applied. Soils provide an engineering medium by either acting as the building material (e.g., gravel, sand, clay, fill, bricks) or providing the foundation on which structures are built. Soil properties including bearing strength, compressibility, shear strength, and stability are, therefore, very important. Knowledge of soils as an engineering medium prevents problems with roadway construction and maintenance, slope failures initiated by inappropriate roadcuts, or structural failures of buildings.

Environmental Factors

V.V. Dokuchaev and his colleagues in Russia were the first to conceive ideas concerning the environmental factors associated with soil formation; these ideas were later formalized by H. Jenny. These “soil forming factors” include time, parent materials, climate, topography, and biota. The concept of time in soil formation is related to the process of weathering, as, over time, weathering processes produce the raw materials necessary for soil formation. Physical weathering breaks down rocks into individual mineral grains. Physical weathering is aided by the abrasive properties of water, wind, and ice, as well as the stress induced by changes in temperature.

Chemical weathering processes make use of water, oxygen, and acids in the breakdown of soil minerals to soluble nutrients useable by plants. Water molecules are integral to chemical weathering processes including hydration, hydrolysis, and dissolution, while oxygen and other electron donors are involved in oxidation-reduction reactions. Acids (e.g., carbonic acid formed by carbon dioxide dissolving in water) accelerate weathering by increasing the activity of hydrogen ions in water. These chemical weathering processes occur simultaneously and are interdependent. Some minerals, like quartz, are resistant to weathering, while other minerals are altered, decomposed, or recombined to produce new minerals. Those minerals resistant to weathering are called primary minerals, and the new minerals generated by weathering are called secondary minerals. Weathering also produces soluble materials.

Parent materials may have formed in place or been transported by wind, water, ice, or gravity. Climate, in particular precipitation and temperature, greatly influences the nature of weathering as well as its intensity. Arid regions experience predominately physical weathering, while chemical weathering predominates in humid tropical climates. Warm temperatures combined with abundant precipitation produce the most highly weathered soils. Climate’s role in soil formation, and climate changes as they are related to human influence, should, therefore, be noted. Soils experience the effects of climate change through alterations in temperature and water status that affect organic matter decomposition, nutrient cycling rates, and weathering.

Other soil forming factors include biota, topography, and time. Biota (e.g., vegetation, microorganisms, soil animals) influence processes including organic matter accumulation and profile mixing. More organic matter accumulates in grassland areas than under forest cover because the roots of the grasses introduce organic matter into the soil, while forests depend primarily upon leaf litter to add organic matter. Profile mixing, or pedoturbation, occurs when animals such as gophers, moles, and prairie dogs, as well as earthworms and termites, bring about soil mixing through burrowing activities.

Topographical impacts on soil formation are related to slope, aspect, and landscape position. For example, steeper slopes will experience less water infiltration, thus generating less vegetative cover and organic matter accumulation, while simultaneously being more likely to experience erosion, all of which contribute to soils that are, as a result, shallow and poorly developed. Other landscape positions, such as swales or the toeslopes of hills, tend to accumulate soil materials eroded from other parts of the landscape and thus result in a deeper profile. Time is required for any of these soil forming processes to produce results. Understanding the processes of soil formation and differences among the soils produced by various soil forming factors is relevant to society in terms of managing soils effectively as well as using soils as a record from which to reconstruct past environmental, social, and cultural history.

Soils and Crop Prodvction

Effective soil management is extremely important for crop production. Over the last 60 years, the world has seen a dramatic rise in agricultural production, primarily in cereal grains such as wheat, corn, and rice. These yield increases came about in response to the post-World War II population increase. Yield increases were made possible not only by intensifying agricultural land use, through irrigation and application of fertilizers and pesticides, but also by expanding agricultural lands. While agricultural intensification has benefits, such as maintaining necessary soil macronutrients (through fertilizer application) and soil organic matter levels (by increasing crop residue), it also has environmental drawbacks. One of the drawbacks associated with agricultural intensification is that the focus on crop monocultures has reduced biodiversity and made crops vulnerable to disease, thus encouraging pesticide use.

Salinization is another drawback that may be brought about by irrigation, particularly in arid or semiarid areas in which salts have accumulated in the soils previously exposed to long dry periods during which evapotranspiration processes have drawn water, along with soluble minerals, to the surface. If drainage to leach salts is not properly provided for when irrigating, salts are left behind when plants uptake water, eventually resulting in their accumulation in the soil. According to P. Rengasamy, salinization currently affects 2.05 billion acres (831 million hectares) globally. Excess nutrient application is another potential drawback to intensifying agricultural land use.

Nutrient and Chemical Application

Nutrient management in soils is important not only for agricultural production, but for protection of groundwater and surface water resources as well. Soil nutrients may be managed by recycling nutrients at a particular site (e.g., applying the manure generated by animals fed the products of a field back onto that field), recycling nutrients more generally (e.g., through land application of sewage effluent), or by tracking the balance between the nutrients inputs to the system and its outputs. Nutrient management is particularly important in light of water quality issues, as nutrients in runoff from agricultural landscapes, particularly nitrogen and phosphorus, have been known to increase nutrient levels in the waters draining these lands. One means by which nutrient runoff may be reduced is by limiting excess nutrient application and timing fertilizer application to coincide with periods of rapid nutrient uptake. Vegetation buffer strips along riparian corridors prevent most of the sediment and nutrients in runoff from reaching streams, and cover crops stabilize soil as well as augment soil quality. Crop rotation is also noted as a means by which yields may be maintained and even increased, while simultaneously decreasing fertilizer requirements.

In addition to nutrients and their impacts on our soil and water resources, we must also be aware of the risks associated with organic and inorganic chemicals applied to soils. Chemicals move through the soil environment taking a variety of routes-absorption, breakdown by microbes, uptake by plants or animals, or loss through volatilization, leaching, or runoff. The soil-water interface is of great importance because chemicals that enter water sources may do so at levels hazardous to human health. It should be noted that some soils (e.g., sandy soils in an area of abundant rainfall) pose a greater potential risk to surface or groundwater contamination because of their high permeability.

Methods for remediating soils with chemical contamination include modifying agricultural practices to limit or eliminate pesticide use, use of less toxic or mobile compounds, or use of chemicals that degrade more rapidly. Other methods for remediating soils apply physical and chemical treatments to the soil or enlist plants and microorganisms. Physical treatment of the soil uses temperature (i.e., incineration) to hasten chemical decomposition. Chemical treatments used in soil remediation include soil washing to leach pollutants or the use of surfactants to bind soluble organic contaminants until they are degraded.

Bioremediation is the method by which plants and animals are used to degrade contaminants. Microbes may be added to the soil to augment natural populations, or existing microbial populations may be stimulated by satisfying their nutrient needs, thus boosting their metabolism and ability to break down chemicals. Plants assist remediation efforts by metabolizing contaminants, accumulating them in their tissues (which can then be harvested), or exuding compounds that stimulate local bacterial growth and, thereby, degradation of the contaminant. Bioremediation has been used to deal with a variety of contamination scenarios, from leaking underground storage tanks (LUST) to mine tailings.

Erosion

Soil erosion, another pressing problem in soil management, occurs when soil is left exposed to rain or wind. Erosion not only impacts soil quality, in terms of organic matter, nutrient losses, and degraded soil structure, but also incurs social and economic costs associated with air and water pollution stemming from the sediment and dust produced. Conservation tillage is a means of stemming soil erosion and increasing soil quality. This process leaves vegetative cover on the soil surface; conservation tillage is, therefore, associated with a variety of terms from stubble mulching to no-till depending on the amount of vegetative cover left.

Connection to Climate

Soils also play a role in global climate processes, as they serve as a carbon sink as well as a source of greenhouse gases (i.e., carbon dioxide, methane, and nitrous oxide). Soils act as a sink by sequestering carbon, removing it from the atmosphere during photosynthesis and storing it as humus. Carbon sequestration can be promoted by restoring degraded soils, as well as maintaining the quality of prime agricultural soils. Degraded soils may be improved by management practices that include erosion control, conservation tillage, and organic matter accumulation through increased crop production and use of cover crops. In addition, wetlands and marshlands also serve as carbon sinks, making restoration and reclamation of these lands another way to promote carbon storage.

Major avenues through which soils serve as sources of carbon include land conversions to agriculture, deforestation, and excess tillage. Land conversions to agriculture remove large portions of the plant material that prior to cultivation would have been returned to the soil, while tillage breaks up plant residues and makes them more available to microbial decomposers. Researchers have pointed out that, without regard to the real or perceived risk to global warming, soil carbon sequestration is important enough to be pursued for its own merits, particularly in light of its relation to soil aggregation, aeration, erosion, and nutrient cycling.

Maintaining Soil Quality

These various issues affecting soils, from nutrient management to soil erosion, point to the importance of maintaining soil quality, defined by the Natural Resource Conservation Service (NRCS) as a soil’s ability to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation. General practices for maintaining soil quality include adding organic matter, avoiding excessive tillage, managing fertilizer and pesticide use, and increasing ground cover as well as plant diversity. Maintaining soil quality is important to the environment as well as society, in terms of food security as well as in support of the ecosystems services provided by soils.

Bibliography:

C. Brady and R.R. Weil, The Nature and Properties of Soils (Prentice Hall, 2002);
N.K. Fageria, V.C. Baligar, and B.A. Bailey, “Role of Cover Crops in Improving Soil and Row Crop Productivity,” Communications in Soil Science and Plant Analysis (v.36/19-20, 2005);
S. Fennessy and J.K. Cronk, “The Effectiveness and Restoration Potential of Riparian Ecotones for the Management of Nonpoint Source Pollution, Particularly Nitrate,” Critical Reviews in Environmental Science and Technology (v.27/4, 1997);
Jenny, Factors of Soil Formation: A System of Quantitative Pedology (McGraw-Hill, 1941);
A. Natarajan, S. Subramanian, and J.J. Braun, “Environmental Impact of Metal Mining: Biotechnological Aspects of Water Pollution and Remediation: An Indian Experience,” Journal of Geochemical Exploration (v.88/1-3, 2006);
Naylor, W. Falcon, and E. Zavaleta, “Variability and Growth in Grain Yields, 1950-94: Does the Record Point to Greater Instability?” Population and Development Review (v.23/1, 1997);
P. Rengasamy, “World Salinization with Emphasis on Australia,” Journal of Experimental Botany (v.57/5, 2006);
Rosenzweig and D. Hillel, “Soils and Global Climate Change: Challenges and Opportunities,” Soil Science (v.165/1, 2000).