Soil

For the heavy metal band, see SOiL.

Soil is the material on the surface of a lithosphere subject to weathering, and especially the earthy portion of that material.

Loess field in Germany

Overview

On Earth, soil is generally considered to be weathered material capable of supporting plant life. Soil forms through a variety of soil formation processes, and includes weathered rock "parent material" combined with dead and living organic matter and air. Soils are vital to all life on Earth because they support the growth of plants, which supply food and oxygen and absorb carbon dioxide and nitrogen.

Introduction

Because of its position in the landscape and its dynamic physical, chemical, and biologic functions, soil is among our most important natural resources. Indeed, engineers, agronomists, chemists, geologists, geographers, biologists, microbiologists, sylviculturists, sanitarians, archaeologists, and specialists in regional planning, all depend on, and contribute to, knowledge of soils. While the general concept of soil is well established, the definition of soil varies, hinging on the perspective of the discipline or occupation employing it as a resource.

Soil occupies the pedosphere, one of Earth's spheres that the geosciences use to conceptually organise the Earth. This is the conceptual perspective of pedology and edaphology, the two main branches of soil science. Both branches apply a combination of soil physics, soil chemistry, and soil biology. Due to the numerous interactions between the biosphere, atmosphere, and hydrosphere that are hosted within the pedosphere, more integrated, less soil-centric concepts are also valuable. Many concepts essential to understanding soil come from individuals not identifiable strictly as soil scientists. This highlights the interdisciplinary nature of soil concepts.

The understanding of soil is incomplete. Despite the duration of mankind's dependence on and curiosity about soil, exploring the diversity and dynamic of this opaque resource continues to reward. New and refreshing avenues of soil research are compelled by our need to understand soil in the context of climate change , greenhouse gases , and carbon sequestration.

Vasily V. Dokuchaev, a Russian geographer, is credited with identifying soil as a resource whose distinctness and complexity deserved to be separated conceptually from geology and crop production and treated as a whole.

Previously, soil had been considered a product of physicochemical transformations of rocks, a dead substrate from which plants derive nutritious mineral elements. Soil and bedrock were in fact equated. Dokuchaev considers the soil as a natural body having its own genesis and its own history of development, a body with complex and multiform processes taking place within it. The soil is considered as different from bedrock. The latter becomes soil under the influence of a series of soil-formation factors (climate, vegetation, country, relief and age). According to him, soil should be called the "daily" or outward horizons of rocks regardless of the type; they are changed naturally by the common effect of water, air and various kinds of living and dead organisms.

What is soil?

Soil classification

Main article: soil classification

Soil classification is a contentious subject, from the structure of the classification system itself, to the definitions of classes and types, and finally in the application in the field. Every classification system starts with its own individual definition of soil. The essential problem is that soils do not reproduce or have DNA like living organisms, so no "objective" criteria can be used to choose among classifications. The most qualified specialists in the world can and do spend hours during a field trip at each soil profile, arguing about the classification. Yet, in a well-constructed system, the different names refer to similar concepts, so that interpretations do not vary widely.

The Unified Soil Classification System (or USCS) is used in engineering, geology and soil science disciplines to describe the texture, grain size, and the shear strength of a soil.

The USDA adopted its own soil texture classification system in 1938. It is used in soil survey, environmental health professionals, and for septic drain field design. The FAO used the USDA system in the FAO-UNESCO world soil map and recommended its use

The FAO developed a supra-national classification, the FAO soil classification, also called World Soil Classification, which offers useful generalizations about soils pedogenesis in relation to the interactions with the main soil-forming factors. It was first published in form of the UNESCO Soil Map of the World (1974) (scale 1 : 5 M.). Many of the names offered in that classification are known in many countries and do have similar meanings. Originally developed as a legend to the Soil Map of the World, the classification has been applied by United Nations sponsored projects. Many countries have modified this system to fit their particular needs.

The USA Soil Taxonomy developed by United States Department of Agriculture's Natural Resources Conservation Service provides an elaborate classification of soil types according to several parameters (most commonly their properties) and in several levels: Order, Suborder, Great Group, Subgroup, Family, and Series.

Other types

In addition to the above classification systems, some specialized systems for typifying soils exist:

• Wetland soils can qualify as hydric soils, and be subject to environmental protection requirements.
• Soils with excellent crop producing potential can qualify as prime farmland soils and be subject to land use restrictions.
• Former soils which become buried below the effects of active soil forming processes are called paleosols.

Vernacular classification systems

Vernacular soil classification systems are developed by the land users. Their structure is either nominal, giving unique names to soils or landscapes, or descriptive, naming soils by their characteristics such as red, hot, fat, or sandy. Soils are distinguished by obvious characteristics, such as physical appearance (e.g., color, texture, landscape position), performance (e.g., production capability, flooding), and accompanying vegetation. A vernacular distinction familiar to many:

• Heavy soils and light soils. Light soils have lower clay content and higher organic matter than heavy soils. They often drain better and dry out sooner, giving them a lighter color. Lighter soils, with their lower moisture content and better structure, take less effort to turn and cultivate. Contrary to popular belief light soils do not weigh less than heavy soils on an air dry basis nor do they have more porosity.

Soil components

• Composition -- A typical surface soil is half solids and half voids or porosity. Water and air are thus integral components of soils. Water occupies the void space between soil particles and is held by surface tension on particle surfaces. Air occupies the remaining void space. Both water and air components of soils are important to plant growth and other life in the soil profile of a particular ecosystem
• Soil texture -- refers to soil grain size, sand = 2 to 0.02 or 0.05 mm; silt = 0.02 or 0.05 to 0.002 mm; clay = <0.002 mm. Soil texture influences available water capacity and movement, fertility, and workability or "tilth". "Loam" is a name for one of various mixtures of these three particle sizes. The ratio of these particles determines soil texture classification.
• Soil structure -- the arrangement of soil particles into aggregates, which may have various shapes, sizes and degrees of development or expression. Soil structure influences aeration, water movement, erosion resistance, and root penetration.
• Slope -- the inclination of the ground surface. Slope influences runoff of rainfall, soil erosion, stability, and machinery operation.
• Soil color -- Soil color often indicates soil moisture status and is used for determining hydric soils. Often described using general terms, such as dark brown, yellowish brown, etc., soil colors are also described more technically by using Munsell soil color charts, which separate color into components of hue (relation to red, yellow and blue), value (lightness or darkness) and chroma (paleness or strength).
• Soil chemistry -- A complex subject within soil science; the most important subjects are:

Soil pH -- The acidity or alkalinity of soils, which affects plant growth and soil fertility.

Cation Exchange Capacity -- The soil's ability to retain and exchange positively charged ions, which include major plant nutrients.

Reduction and oxidation (Redox) reactions are also important.

• The soil profile -- A vertical cut that exposes soil layering or soil horizons. Horizons are formed by combined biological, chemical and physical alterations. A, B, and C symbols are used to describe the topsoil, subsoil and substratum, respectively.
• Permeability -- The ability of a soil to transmit water or air. Faster or greater permeability often occurs in sandy or gravelly soils due to large pore spaces. Slower permeability typically occurs in finer textured clay soils, or compacted soils with little structure.
• Drainage -- The rate in which water is removed from a soil. Drainage influences most uses of soils, whether for agriculture, silviculture or urban. Classes of soil drainage are those found in soil survey reports, such as well drained, moderately well drained, somewhat poorly drained, poorly drained, and very poorly drained. Soil color patterns (such as mottle patterns or redoximorphic features) often indicate soil drainage class. The most productive agricultural soils are typically well drained or moderately well drained, as are the soils easiest to develop for urban use. By contrast, hydric soils are poorly or very poorly drained. A soil's natural drainage rate can be significantly increased by subsurface "tile" drainage.
• Soil biology -- Soils serve as habitats for soil organisms varying in size from microorganisms to small animals. The character of soils is intricately tied to bioturbation and the biochemical functions performed by soil organisms. The soil is home to a large proportion of the world's genetic diversity, which changes by soil type: desert soils (like aridisols) generally have more complex soil biotic communities and genetic diversity than tropical soils (like oxisols).
• Soil smell -- earthy odors are normally the product of microbial activity. Actinomycetes is responsible for the familiar smell associated with fresh turned garden soil and finished compost. At the other end of the pleasant/not-so-much memories scale is the rotton egg smell that deeply anaerobic microbial processes produce in wetland soils.
• Soil tilth -- Where soil mineral particles are both separated and bridged by organic matter breakdown products, and soil biota exudates, it makes the soil easy to work. Cultivation, earthworms, frost action and rodents mix the soil and decreases the size of the peds. This structure allows for good porosity and easy movement of air and water. This combination of ease in tillage, good moisture and air handling capabilities, and good structure for planting and germination, are definitive of the phrase good tilth

Soils vary widely in composition and structure from place to place. Soils are formed through weathering of minerals and organic matter. Weathering is the action of wind, rain, ice, sunlight and biological processes, which breaks parent material down into smaller particles and alters their chemistry. Soils form from weathering in place although many soils are comprised entirely of transported weathered material which arrives as flood sediments, eolian deposits, or dust fall. The proportions and types of minerals and organic matter help determine the characteristics of a particular soil.

Soils are also affected by human habitation. People can alter soils to make them more suitable for plant growth through the addition of organic materials and natural or synthetic fertilizer, and by improving their drainage or water-retaining capacity. Human actions also can degrade soils through the depletion of nutrients, pollution, soil contamination, and soil compaction, and by increasing the rate of erosion, which is the relocation of soil through the movement of water or wind.

Natural soil development

Main article: Pedogenesis

An example of soil development from bare rock occurs on recent lava flows in warm regions under heavy and very frequent rainfall. In such climates plants become established very quickly on basaltic lava, even though there is very little organic material. The plants are supported by the porous rock becoming filled with nutrient bearing water, for example carrying dissolved bird droppings or guano. The developing plant roots themselves gradually breaks up the porous lava and organic matter soon accumulates but, even before it does, the predominantly porous broken lava in which the plant roots grow can be considered a soil.

Chemical processes in soils

Main article: Soil chemistry

Weathering releases ions such as Potassium (K⁺) and Magnesium (Mg²⁺) into the soil solution. Some of these elements (as ions) are taken up by bacteria, fungi and plants. The remaining portion can form secondary minerals, be chelated into organic complexes or be adsorbed into ion exchange complexes. Anion exchange complexes affect negatively charged ions (phosphate) and compounds. Anion exchange surfaces occur most typically in humus. Cation exchange complexes affect positively charged ions. Cation exchange surfaces are typically clay minerals such as montmorillonite and organic materials such as humus. When the level of ions is relatively low in the soil solution, equilibrium processes convey ions into solution, where they satisfy demand for nutrients by plants, bacteria and fungi.

The pH level in soils affects the activity and availability of ionic nutrients (examples are Ca²⁺, Mg²⁺, K⁺, Na⁺) and non-nutrients (H⁺, Al³⁺). Nutrient uptake is highest in a neutral pH range of 5.5 to 8.2. At pH levels below 5.0, increased aluminum activity can have a toxic affect, exacerbating reduced nutrient availability. Additionally, Ca²⁺, Mg²⁺, K⁺, Na⁺ can be displaced by H⁺ and Al³⁺. Subsequent leaching can result in lower soil fertility and productivity. At elevated soil pH levels nutrient availability is limited, especially for zinc and phosphorus. Additionally, differential removal of cations can result in elevated Na⁺ relative to Ca²⁺ and Mg²⁺ with a deleterious affect on soil structure, permeability and tilth. Contributors to soil acidification include "acidic" parent material (granite), plant root exudates, decomposition of certain types of organic residue (pine needles), chemical changes that occur when perennially wet sediments are dried, acidifying fertilizers (anhydrous ammonia, ammonium sulfate), and natural rain as well as acid rain phenomena. Sources of alkalinity include "basic" parent material (serpantine, limestone) and airborne soil particulates from alkaline areas. To raise a soil's pH, farmers can apply alkaline materials such as lime. To lower a soil's pH, farmers can apply acid-forming materials such as elemental sulfur. To increase calcium content in an alkaline soil, farmers can apply gypsum.

Although the elements nitrogen, potassium and phosphorus, which are necessary for plant growth, may be abundant in soils, only a fraction of these elements may be in a chemical form which plants can use.

Processes such as the nitrogen cycle and carbon cycle continually exchange nitrogen and carbon nutrients between soils and the atmosphere. The raw products are initially present as gases in the atmosphere. In nitrogen fixation, atmospheric nitrogen is converted to plant available forms. In nitrogen mineralization, proteins and other organic forms are converted into mineral, plant available forms: NH₄⁺ and NO₃^-. In nitrification, NH₄⁺ is converted into the more usable NO₃^-. While NH₄⁺ is especially important to young plants and early in the growing season, NO₃^- is the dominant form of nitrogen taken up by plants. NO₃^- moves to plants by mass transport and needs transpiration to drive uptake.

The organic component of soils originate in plant debris (such as fallen leaves), animal excreta, and other decomposing organic materials. These materials, when broken down, form humus, a dark, nutrient-rich material. Chemically, humus is composed of very large molecules including esters of carboxylic acid, phenolic compounds, and derivatives of benzene. Organic materials in soils provide nutrients necessary for plant growth. Organic material also contributes to water retention, drainage ability, and oxygenation of soils.

If oxygen enters a wet soil, because of lowered ground water table, organic matter in the soil will be broken down further by oxidation, which can lead to subsidence. An example of this can be seen in soils in the Everglades region of Florida, which have been drained by canals for agriculture, primarily sugar production. Originally very high in organic content, oxygenation and compaction have led to breakdown of the soil structure and nutrient content, and degradation of the soil's ability to support continued high crop yields.

Biological processes in soil

Main article: Soil biology

Wetland soil processes

The diffusion of dissolved oxygen in saturated soils is slower than in unsaturated soils. Wetland (also referred to as hydric) soils form due to soil microbial cellular respiration in excess of soil oxygen supply, resulting in oxygen depletion. Anaerobic soil chemistry results, which creates a reducing environment. This eliminates plants and creatures not adapted for life in saturated soil conditions.

Biological soil crusts

Biological soil crusts are formed by living organisms and their by-products, creating a surface crust of soil particles bound together by organic materials.

References

• Soil Survey Staff. (1975) Soil Taxonomy: A basic system of soil classification for making and interpreting soil surveys. USDA-SCS Agric. Handb. 436. U.S. Gov. Print. Office. Washington, DC.
• Soil Survey Division Staff. (1993) Soil survey manual. Soil Conservation Service. U.S. Department of Agriculture Handbook 18.
• Logan, W. B., Dirt: The ecstatic skin of the earth. 1995 ISBN 1573220043
• Faulkner, William. Plowman's Folly. New York, Grosset & Dunlap. 1943. ISBN 0933280513
• Jenny, Hans, Factors of Soil Formation: A System of Quantitative Pedology 1941
• Why Study Soils?
• Soil notes
• Soil articles

See also

• Compost
• Derelict soil
• Humus
• Manure
• Mud
• Nitrogen cycle
• Soil mechanics
• Soils retrogression and degradation

External links

• Oregon State University's Soils (wiki)
• OpenAg.info's Soil Science Encyclopedia (wiki)