Geographic information system
A geographic information system is a system designed to capture, manipulate, analyze and present spatial or geographic data. GIS applications are tools that allow users to create interactive queries, analyze spatial information, edit data in maps, present the results of all these operations. GIS sometimes refers to geographic information science, the science underlying geographic concepts and systems. GIS can refer to a number of different technologies, processes and methods, it is attached to many operations and has many applications related to engineering, management, transport/logistics, telecommunications, business. For that reason, GIS and location intelligence applications can be the foundation for many location-enabled services that rely on analysis and visualization. GIS can relate unrelated information by using location as the key index variable. Locations or extents in the Earth space–time may be recorded as dates/times of occurrence, x, y, z coordinates representing, longitude and elevation, respectively.
All Earth-based spatial–temporal location and extent references should be relatable to one another and to a "real" physical location or extent. This key characteristic of GIS has begun to open new avenues of scientific inquiry; the first known use of the term "geographic information system" was by Roger Tomlinson in the year 1968 in his paper "A Geographic Information System for Regional Planning". Tomlinson is acknowledged as the "father of GIS". One of the first applications of spatial analysis in epidemiology is the 1832 "Rapport sur la marche et les effets du choléra dans Paris et le département de la Seine"; the French geographer Charles Picquet represented the 48 districts of the city of Paris by halftone color gradient according to the number of deaths by cholera per 1,000 inhabitants. In 1854 John Snow determined the source of a cholera outbreak in London by marking points on a map depicting where the cholera victims lived, connecting the cluster that he found with a nearby water source.
This was one of the earliest successful uses of a geographic methodology in epidemiology. While the basic elements of topography and theme existed in cartography, the John Snow map was unique, using cartographic methods not only to depict but to analyze clusters of geographically dependent phenomena; the early 20th century saw the development of photozincography, which allowed maps to be split into layers, for example one layer for vegetation and another for water. This was used for printing contours – drawing these was a labour-intensive task but having them on a separate layer meant they could be worked on without the other layers to confuse the draughtsman; this work was drawn on glass plates but plastic film was introduced, with the advantages of being lighter, using less storage space and being less brittle, among others. When all the layers were finished, they were combined into one image using a large process camera. Once color printing came in, the layers idea was used for creating separate printing plates for each color.
While the use of layers much became one of the main typical features of a contemporary GIS, the photographic process just described is not considered to be a GIS in itself – as the maps were just images with no database to link them to. Two additional developments are notable in the early days of GIS: Ian McHarg's publication "Design with Nature" and its map overlay method and the introduction of a street network into the U. S. Census Bureau's DIME system. Computer hardware development spurred by nuclear weapon research led to general-purpose computer "mapping" applications by the early 1960s; the year 1960 saw the development of the world's first true operational GIS in Ottawa, Canada, by the federal Department of Forestry and Rural Development. Developed by Dr. Roger Tomlinson, it was called the Canada Geographic Information System and was used to store and manipulate data collected for the Canada Land Inventory – an effort to determine the land capability for rural Canada by mapping information about soils, recreation, waterfowl and land use at a scale of 1:50,000.
A rating classification factor was added to permit analysis. CGIS was an improvement over "computer mapping" applications as it provided capabilities for overlay and digitizing/scanning, it supported a national coordinate system that spanned the continent, coded lines as arcs having a true embedded topology and it stored the attribute and locational information in separate files. As a result of this, Tomlinson has become known as the "father of GIS" for his use of overlays in promoting the spatial analysis of convergent geographic data. CGIS built a large digital land resource database in Canada, it was developed as a mainframe-based system in support of federal and provincial resource planning and management. Its strength was continent-wide analysis of complex datasets; the CGIS was never available commercially. In 1964 Howard T. Fisher formed the Laboratory for Computer Graphics and Spatial Analysis at the Harvard Graduate School of Design, where a number of important theoretical concepts in spatial data handling were developed, which by the 1970s had distributed seminal software code and systems, such as SYMAP, GRID, ODYSSEY – that served as sources for subsequent commercial development—to universities, research centers and corporations worldwide.
By the late 1970s two public domain GIS systems were in development, by the early 1980s, M&S Computing (late
Topsoil is the upper, outermost layer of soil the top 5 inches to 10 inches. It has the highest concentration of organic matter and microorganisms and is where most of the Earth's biological soil activity occurs. Topsoil is composed of mineral particles, organic matter and air. Organic matter varies in quantity on different soils; the strength of soil structure decreases with the presence of organic matter, creating weak bearing capacities. Organic matter condenses and settles in different ways under certain conditions, such as roadbeds and foundations; the structure becomes affected. The soil's volume decreases, it suffers wind erosion. Plants concentrate their roots in and obtain most of their vital nutrients from this layer. Actual depth of the topsoil layer can be measured as the depth from the surface to the first densely packed soil layer known as subsoil. In soil classification systems, topsoil is known as the "O Horizon or A Horizon," therefore, it is the top layer. Commercially available topsoil in the United Kingdom should be classified to British Standard BS 3882 with the current version dated 2015.
The standard has several classifications of topsoil with the final classification requiring material to meet certain threshold criteria such as Nutrient Content, Extractable Phytotoxic Elements, Particle Size Distribution, Organic Matter Content, Carbon:Nitrogen ratio, Electrical Conductivity, Loss on Ignition, pH, Chemical and Physical Contamination. The topsoil should be sampled in accordance with the British Standard and European Norm BS EN 12579:2013 Soil improvers and growing media - Sampling. During construction of garden areas for housing plots the topsoil should be underlain by a layer of suitably certified subsoil that conforms to the British Standard BS 8601:2013 Specification for subsoil and requirements for use, it is always recommended that for construction projects that topsoil is placed in accordance with the DEFRA report Construction Code of Practice for the Sustainable Use of Soils on Construction Sites When starting a gardening project, it is crucial to check whether or not the soil is satisfactory.
Different types of plants vary in their nutrient needs and preferred soil conditions, many are adapted to particular conditions. However, some general guidelines for "desired levels of Topsoil nutrients" have been made, broadly suitable for many plants; the two common types of Topsoil are Bagged Topsoil. The following table illustrates major differences between the two. Alternatively the British Standard relates to other working values: This is for a multipurpose grade and certain levels can alter with regard to soil pH. Other uses specified in the standard that allows for a variety of uses in different and specific scenarios includes: Acidic, Low Fertility, Low Fertility Acidic and Low Fertility Calcareous; these uses are limited to specific site scenarios and acceptance should be on a case by case basis for construction projects. Topsoil is the primary resource for plants to grow and crops to thrive and the main two parameters for this are Carbon and Nitrogen; the Carbon provides energy and Nitrogen is a tissue builder and plants require them in a range of ratios to enable suitable growth.
An optimum figure for Topsoil in the UK is a ratio of less than 20:1. This ensures that the soil has a suitable energy reserve as well as tissue building material to enable the plants to thrive. A sawdust has a carbonaceous base and this a high C:N ratio while an Alfalfa Hay has a low carbonaceous content and can have a C:N ratio in the order of 12:1. A variety of soil mixtures are sold commercially as topsoil for use in improving gardens and lawns, e.g. container gardens, potting soil and peat. Another important yet not known use for topsoil is for proper surface grading near residential buildings such as homes. "The ground around the home should slope down six inches for the first ten feet away from the home. This can be done by adding topsoil." A major environmental concern known as topsoil erosion occurs when the topsoil layer is blown or washed away. Without topsoil, little plant life is possible; the estimated annual costs of public and environmental health losses related to soil erosion exceed $45 billion.
Conventional agriculture encourages the depletion of topsoil because the soil must be plowed and replanted each year. Sustainable techniques attempt to slow erosion through the use of cover crops in order to build organic matter in the soil; the United States alone loses 3 tons of topsoil per acre per year. This is of great ecological concern as one inch of topsoil can take between 500 and 1,000 years to form naturally. On current trends, the world has about 60 years of topsoil left; because of its use in commercial application and due to the environmental concerns regarding erosion, it is important for consumers to determine how much topsoil they need for a given project. Topsoil is sold by the cubic yard in the United States. To calculate the amount of topsoil you will need, "simply take your length multiplied by your width multiplied by your depth divide the total by 27; this will give you your cubic yards needed for the project. Ex: 10L x 10W x = 41.66/27 = 1.54 cubic yards of topsoil needed.".
Further reading USDA Electronic Field Office Technical Guide - Detailed soil conservation guides tailored to individual states/counties
Time is the indefinite continued progress of existence and events that occur in irreversible succession through the past, in the present, the future. Time is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, to quantify rates of change of quantities in material reality or in the conscious experience. Time is referred to as a fourth dimension, along with three spatial dimensions. Time has long been an important subject of study in religion and science, but defining it in a manner applicable to all fields without circularity has eluded scholars. Diverse fields such as business, sports, the sciences, the performing arts all incorporate some notion of time into their respective measuring systems. Time in physics is unambiguously operationally defined as "what a clock reads". See Units of Time. Time is one of the seven fundamental physical quantities in both the International System of Units and International System of Quantities.
Time is used to define other quantities – such as velocity – so defining time in terms of such quantities would result in circularity of definition. An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event constitutes one standard unit such as the second, is useful in the conduct of both advanced experiments and everyday affairs of life; the operational definition leaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measured. Investigations of a single continuum called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy. Temporal measurement has occupied scientists and technologists, was a prime motivation in navigation and astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, the beat of a heart.
The international unit of time, the second, is defined by measuring the electronic transition frequency of caesium atoms. Time is of significant social importance, having economic value as well as personal value, due to an awareness of the limited time in each day and in human life spans. Speaking, methods of temporal measurement, or chronometry, take two distinct forms: the calendar, a mathematical tool for organising intervals of time, the clock, a physical mechanism that counts the passage of time. In day-to-day life, the clock is consulted for periods less than a day whereas the calendar is consulted for periods longer than a day. Personal electronic devices display both calendars and clocks simultaneously; the number that marks the occurrence of a specified event as to hour or date is obtained by counting from a fiducial epoch – a central reference point. Artifacts from the Paleolithic suggest that the moon was used to reckon time as early as 6,000 years ago. Lunar calendars were among the first to appear, with years of either 13 lunar months.
Without intercalation to add days or months to some years, seasons drift in a calendar based on twelve lunar months. Lunisolar calendars have a thirteenth month added to some years to make up for the difference between a full year and a year of just twelve lunar months; the numbers twelve and thirteen came to feature prominently in many cultures, at least due to this relationship of months to years. Other early forms of calendars originated in Mesoamerica in ancient Mayan civilization; these calendars were religiously and astronomically based, with 18 months in a year and 20 days in a month, plus five epagomenal days at the end of the year. The reforms of Julius Caesar in 45 BC put the Roman world on a solar calendar; this Julian calendar was faulty in that its intercalation still allowed the astronomical solstices and equinoxes to advance against it by about 11 minutes per year. Pope Gregory XIII introduced a correction in 1582. During the French Revolution, a new clock and calendar were invented in attempt to de-Christianize time and create a more rational system in order to replace the Gregorian calendar.
The French Republican Calendar's days consisted of ten hours of a hundred minutes of a hundred seconds, which marked a deviation from the 12-based duodecimal system used in many other devices by many cultures. The system was abolished in 1806. A large variety of devices have been invented to measure time; the study of these devices is called horology. An Egyptian device that dates to c. 1500 BC, similar in shape to a bent T-square, measured the passage of time from the shadow cast by its crossbar on a nonlinear rule. The T was oriented eastward in the mornings. At noon, the device was turned around so. A sundial uses a gnomon to cast a shadow on a set of markings calibrated to the hour; the position of the shadow marks the hour in local time. The idea to separate the day into smaller parts is credited to Egyptians because of their sundials, which operated on a duodecimal system; the importance of the number 12 is due to the number of lunar cycles in a year and the number of stars used to count the passage of night.
The most precise timekeeping device of the ancient
Soil science is the study of soil as a natural resource on the surface of the Earth including soil formation and mapping. Sometimes terms which refer to branches of soil science, such as pedology and edaphology, are used as if synonymous with soil science; the diversity of names associated with this discipline is related to the various associations concerned. Indeed, agronomists, geologists, physical geographers, biologists, silviculturists, sanitarians and specialists in regional planning, all contribute to further knowledge of soils and the advancement of the soil sciences. Soil scientists have raised concerns about how to preserve soil and arable land in a world with a growing population, possible future water crisis, increasing per capita food consumption, land degradation. Soil occupies the pedosphere, one of Earth's spheres that the geosciences use to organize the Earth conceptually; this is the conceptual perspective of pedology and edaphology, the two main branches of soil science. Pedology is the study of soil in its natural setting.
Edaphology is the study of soil in relation to soil-dependent uses. Both branches apply a combination of soil physics, soil chemistry, soil biology. Due to the numerous interactions between the biosphere and hydrosphere that are hosted within the pedosphere, more integrated, less soil-centric concepts are valuable. Many concepts essential to understanding soil come from individuals not identifiable as soil scientists; this highlights the interdisciplinary nature of soil concepts. Dependence on and curiosity about soil, exploring the diversity and dynamics of this resource continues to yield fresh discoveries and insights. New avenues of soil research are compelled by a need to understand soil in the context of climate change, greenhouse gases, carbon sequestration. Interest in maintaining the planet's biodiversity and in exploring past cultures has stimulated renewed interest in achieving a more refined understanding of soil. Most empirical knowledge of soil in nature comes from soil survey efforts.
Soil survey, or soil mapping, is the process of determining the soil types or other properties of the soil cover over a landscape, mapping them for others to understand and use. It relies on distinguishing the individual influences of the five classic soil forming factors; this effort draws upon geomorphology, physical geography, analysis of vegetation and land-use patterns. Primary data for the soil survey are supported by remote sensing; as of 2006, the World Reference Base for Soil Resources, via its Land & Water Development division, is the pre-eminent soil classification system. It replaces the previous FAO soil classification; the WRB borrows from modern soil classification concepts, including USDA soil taxonomy. The classification is based on soil morphology as an expression pedogenesis. A major difference with USDA soil taxonomy is that soil climate is not part of the system, except insofar as climate influences soil profile characteristics. Many other classification schemes exist, including vernacular systems.
The structure in vernacular systems are either nominal, giving unique names to soils or landscapes, or descriptive, naming soils by their characteristics such as red, fat, or sandy. Soils are distinguished by obvious characteristics, such as physical appearance and accompanying vegetation. A vernacular distinction familiar to many is classifying texture as light. Light soil content and better structure, take less effort to 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. Contemporaries Friedrich Albert Fallou, the German founder of modern soil science, Vasily Dokuchaev, the Russian founder of modern soil science, are both credited with being among the first to identify soil as a resource whose distinctness and complexity deserved to be separated conceptually from geology and crop production and treated as a whole; as a founding father of soil science Fallou has primacy in time. Fallou was working on the origins of soil before Dokuchaev was born, however Dokuchaev's work was more extensive and is considered to be the more significant to modern soil theory than Fallou's.
Soil had been considered a product of chemical transformations of rocks, a dead substrate from which plants derive nutritious 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. According to him, soil should be called the "daily" or outward horizons of rocks regardless of the type. A 1914 encyclopedic definition: "the different forms of earth on the surface of the rocks, formed by the breaking down or weathering of rocks". Serves to illustrate the historic view of soil. Dokuchaev's late 19th century soil concept developed in the 20th century to one of soil as earthy material, altered by living processes. A corollary conc
Soil salinity is the salt content in the soil. Salts occur within soils and water. Salination can be caused by natural processes such as mineral weathering or by the gradual withdrawal of an ocean, it can come about through artificial processes such as irrigation and road salt. Salts are a natural component in soils and water; the ions responsible for salination are: Na+, K+, Ca2+, Mg2+ and Cl−. As the Na+ predominates, soils can become sodic. Sodic soils present particular challenges because they tend to have poor structure which limits or prevents water infiltration and drainage. Over long periods of time, as soil minerals weather and release salts, these salts are flushed or leached out of the soil by drainage water in areas with sufficient precipitation. In addition to mineral weathering, salts are deposited via dust and precipitation. In dry regions salts may accumulate, leading to saline soils; this is the case, for example, in large parts of Australia. Human practices can increase the salinity of soils by the addition of salts in irrigation water.
Proper irrigation management can prevent salt accumulation by providing adequate drainage water to leach added salts from the soil. Disrupting drainage patterns that provide leaching can result in salt accumulations. An example of this occurred in Egypt in 1970; the change in the level of ground water before the construction had enabled soil erosion, which led to high concentration of salts in the water table. After the construction, the continuous high level of the water table led to the salination of the arable land. Salinity in drylands can occur when the water table is between two and three metres from the surface of the soil; the salts from the groundwater are raised by capillary action to the surface of the soil. This occurs when groundwater is saline, is favored by land use practices allowing more rainwater to enter the aquifer than it could accommodate. For example, the clearing of trees for agriculture is a major reason for dryland salinity in some areas, since deep rooting of trees has been replaced by shallow rooting of annual crops.
Salinity from irrigation can occur over time wherever irrigation occurs, since all water contains some dissolved salts. When the plants use the water, the salts are left behind in the soil and begin to accumulate. Since soil salinity makes it more difficult for plants to absorb soil moisture, these salts must be leached out of the plant root zone by applying additional water; this water in excess of plant needs is called the leaching fraction. Salination from irrigation water is greatly increased by poor drainage and use of saline water for irrigating agricultural crops. Salinity in urban areas results from the combination of irrigation and groundwater processes. Irrigation is now common in cities; the consequences of salinity are Detrimental effects on plant growth and yield Damage to infrastructure Reduction of water quality for users, sedimentation problems, increased leaching of metals copper, cadmium and zinc. Soil erosion when crops are too affected by the amounts of salts. More energy required to desalinateSalinity is an important land degradation problem.
Soil salinity can be reduced by leaching soluble salts out of soil with excess irrigation water. Soil salinity control involves watertable control and flushing in combination with tile drainage or another form of subsurface drainage. A comprehensive treatment of soil salinity is available from the United Nations Food and Agriculture Organization. High levels of soil salinity can be tolerated. Sensitive crops lose their vigor in saline soils, most crops are negatively affected by saline soils, only salinity-resistant crops thrive in saline soils; the University of Wyoming and the Government of Alberta report data on the salt tolerance of plants. Field data in irrigated lands, under farmers' conditions, are scarce in developing countries. However, some on-farm surveys have been made in Egypt and Pakistan; some examples are shown in the following gallery, with crops arranged from sensitive to tolerant. Graphs of crop yield and soil salinity in farmers' fields ordered by increasing salt tolerance. From the FAO/UNESCO Soil Map of the World the following salinised areas can be derived.
Arabidopsis thaliana responses to salinity Salt tolerance of crops Desalination Environmental impacts of deicing salt Water softening U. S. Salinity Laboratory Salinity in Australia Article on water and salt balances in the soil Download leaching model for saline soils Salt of the Earth Documentary produced by Prairie Public Television
A mnemonic device, or memory device, is any learning technique that aids information retention or retrieval in the human memory. Mnemonics make use of elaborative encoding, retrieval cues, imagery as specific tools to encode any given information in a way that allows for efficient storage and retrieval. Mnemonics aid original information in becoming associated with something more accessible or meaningful—which, in turn, provides better retention of the information. Encountered mnemonics are used for lists and in auditory form, such as short poems, acronyms, or memorable phrases, but mnemonics can be used for other types of information and in visual or kinesthetic forms, their use is based on the observation that the human mind more remembers spatial, surprising, sexual, humorous, or otherwise "relatable" information, rather than more abstract or impersonal forms of information. The word "mnemonic" is derived from the Ancient Greek word μνημονικός, meaning "of memory, or relating to memory" and is related to Mnemosyne, the name of the goddess of memory in Greek mythology.
Both of these words are derived from μνήμη, "remembrance, memory". Mnemonics in antiquity were most considered in the context of what is today known as the art of memory. Ancient Greeks and Romans distinguished between two types of memory: the "natural" memory and the "artificial" memory; the former is inborn, is the one that everyone uses instinctively. The latter in contrast has to be trained and developed through the learning and practice of a variety of mnemonic techniques. Mnemonic systems are strategies consciously used to improve memory, they help use information stored in long-term memory to make memorisation an easier task. The general name of mnemonics, or memoria technica, was the name applied to devices for aiding the memory, to enable the mind to reproduce a unfamiliar idea, a series of dissociated ideas, by connecting it, or them, in some artificial whole, the parts of which are mutually suggestive. Mnemonic devices were much cultivated by Greek sophists and philosophers and are referred to by Plato and Aristotle.
In times the poet Simonides was credited for development of these techniques for no reason other than that the power of his memory was famous. Cicero, who attaches considerable importance to the art, but more to the principle of order as the best help to memory, speaks of Carneades of Athens and Metrodorus of Scepsis as distinguished examples of people who used well-ordered images to aid the memory; the Romans valued. The Greek and the Roman system of mnemonics was founded on the use of mental places and signs or pictures, known as "topical" mnemonics; the most usual method was to choose a large house, of which the apartments, windows, furniture, etc. were each associated with certain names, events or ideas, by means of symbolic pictures. To recall these, an individual had only to search over the apartments of the house until discovering the places where images had been placed by the imagination. In accordance with said system, if it were desired to fix a historic date in memory, it was localised in an imaginary town divided into a certain number of districts, each of with ten houses, each house with ten rooms, each room with a hundred quadrates or memory-places on the floor on the four walls on the roof.
Therefore, if it were desired to fix in the memory the date of the invention of printing, an imaginary book, or some other symbol of printing, would be placed in the thirty-sixth quadrate or memory-place of the fourth room of the first house of the historic district of the town. Except that the rules of mnemonics are referred to by Martianus Capella, nothing further is known regarding the practice until the 13th century. Among the voluminous writings of Roger Bacon is a tractate De arte memorativa. Ramon Llull devoted special attention to mnemonics in connection with his ars generalis; the first important modification of the method of the Romans was that invented by the German poet Konrad Celtes, who, in his Epitoma in utramque Ciceronis rhetoricam cum arte memorativa nova, used letters of the alphabet for associations, rather than places. About the end of the 15th century, Petrus de Ravenna provoked such astonishment in Italy by his mnemonic feats that he was believed by many to be a necromancer.
His Phoenix artis memoriae went through as many as nine editions, the seventh being published at Cologne in 1608. About the end of the 16th century, Lambert Schenkel, who taught mnemonics in France and Germany surprised people with his memory, he was denounced as a sorcerer by the University of Louvain, but in 1593 he published his tractate De memoria at Douai with the sanction of that celebrated theological faculty. The most complete account of his system is given in two works by his pupil Martin Sommer, published in Venice in 1619. In 1618 John Willis published Mnemonica. Giordano Bruno included a memoria technica in his treatise De umbris idearum, as part of his study of the ars generalis of Llull. Other writers of this period are the Florentine Publicius. Porta, Ars reminiscendi. In 1648 Stanislaus Mink von Wennsshein revealed what he called the "most fertile secret" in mnemonics — using consonants for figures, thus expressing numbers by words, i
Soil retrogression and degradation
Soil retrogression and degradation are two regressive evolution processes associated with the loss of equilibrium of a stable soil. Retrogression is due to soil erosion and corresponds to a phenomenon where succession reverts the land to its natural physical state. Degradation is an evolution, different from natural evolution, related to the local climate and vegetation, it is due to the replacement of primary plant communities by the secondary communities. This replacement modifies the humus composition and amount, affects the formation of the soil, it is directly related to human activity. Soil degradation may be viewed as any change or ecological disturbance to the soil perceived to be deleterious or undesirable. At the beginning of soil formation, the bare rock out crops is colonized by pioneer species, they are succeeded by herbaceous vegetation and forest. In parallel, the first humus-bearing horizon is formed, followed by some mineral horizons; each successive stage is characterized by a certain association of soil/vegetation and environment, which defines an ecosystem.
After a certain time of parallel evolution between the ground and the vegetation, a state of steady balance is reached. This stage of development is called climax by "natural potential" by others. Succession is the evolution towards climax. Regardless of its name, the equilibrium stage of primary succession is the highest natural form of development that the environmental factors are capable of producing; the cycles of evolution of soils have variable durations, between tens, hundreds, or thousands of years for evolving soils to more than a million years for developing soils. The same soil may achieve several successive steady state conditions during its existence, as exhibited by the Pygmy forest sequence in Mendocino County, California. Soils reach a state of high productivity, from which they degrade as mineral nutrients are removed from the soil system, thus older soils are more vulnerable to the effects of induced degradation. There are two types of ecological factors influencing the evolution of a soil.
These two factors are significant to explain the evolution of soils of short development. A first type of factor is the average climate of an area and the vegetation, associated. A second type of factor is more local, is related to the original rock and local drainage; this type of factor explains appearance of specialized associations. The destruction of the vegetation implies the destruction of evoluted soils, or a regressive evolution. Cycles of succession-regression of soils follow one another within short intervals of time or long intervals of time; the climate role in the deterioration of the rocks and the formation of soils lead to the formulation of the theory of the biorhexistasy. In wet climate, the conditions are favorable to the deterioration of the rocks, the development of the vegetation and the formation of soils. In dry climate, the rocks exposed are subjected to mechanical disintegration which produces coarse detrital materials: this is referred to as rhexistasy; when the state of balance, characterized by the ecosystem climax is reached, it tends to be maintained stable in the course of time.
The vegetation installed on the ground provides the humus and ensures the ascending circulation of the matters. It protects the ground from erosion by playing the role of barrier. Plants can reduce erosion by binding the particles of the ground to their roots. A disturbance of climax will cause retrogression, but secondary succession will start to guide the evolution of the system after that disturbance. Secondary succession is much faster than primary because the soil is formed, although deteriorated and needing restoration as well. However, when a significant destruction of the vegetation takes place, the disturbance undergone by the ecosystem is too important. In this latter case, erosion is responsible for the destruction of the upper horizons of the ground, is at the origin of a phenomenon of reversion to pioneer conditions; the phenomenon can be partial or total. For example, the clearing of an inclined ground, subjected to violent rains, can lead to the complete destruction of the soil.
Man can modify the evolution of the soils by direct and brutal action, such as clearing, abusive cuts, forest pasture, litters raking. The climax vegetation is replaced and the soil modified. Retrogression is related to old human practices. Soil erosion is the main factor for soil degradation and is due to several mechanisms: water erosion, wind erosion, chemical degradation and physical degradation. Erosion is related to human activity. For example, roads which increase impermeable surfaces lead to ground loss. Agriculture accelerates soil erosion. Meadows are in regression to the profit of plowed lands. Spring cultures surfaces leave the ground naked in winter. Sloping grounds are colonized by vine. Lastly, use of herbicides leaves the ground naked between each crop. New cultural practices, such as mechanization increases the risks of erosion. Fert