An equinox is regarded as the instant of time when the plane of Earth's equator passes through the center of the Sun. This occurs 23 September. In other words, it is the moment at which the center of the visible Sun is directly above the Equator; the word is derived from aequus and nox. On the day of an equinox and nighttime are of equal duration all over the planet, they are not equal, due to the angular size of the Sun, atmospheric refraction, the changing duration of the length of day that occurs at most latitudes around the equinoxes. Long before conceiving this equality primitive cultures noted the day when the Sun rises due East and sets due West and indeed this happens on the day closest to the astronomically defined event. In the northern hemisphere, the equinox in March is called the Spring Equinox; the dates are variable, dependent as they are on the leap year cycle. Because the Moon cause the motion of the Earth to vary from a perfect ellipse, the equinox is now defined by the Sun's more regular ecliptic longitude rather than by its declination.
The instants of the equinoxes are defined to be when the longitude of the Sun is 0° and 180°. Systematically observing the sunrise, people discovered that it occurs between two extreme locations at the horizon and noted the midpoint between the two, it was realized that this happens on a day when the durations of the day and the night are equal and the word "equinox" comes from Latin Aequus, meaning "equal", Nox, meaning "night". In the northern hemisphere, the vernal equinox conventionally marks the beginning of spring in most cultures and is considered the start of the New Year in the Assyrian calendar and the Persian calendar or Iranian calendars as Nowruz, while the autumnal equinox marks the beginning of autumn; the equinoxes are the only times. As a result, the northern and southern hemispheres are illuminated. In other words, the equinoxes are the only times when the subsolar point is on the equator, meaning that the Sun is overhead at a point on the equatorial line; the subsolar point crosses the equator moving northward at the March equinox and southward at the September equinox.
When Julius Caesar established the Julian calendar in 45 BC, he set 25 March as the date of the spring equinox. Because the Julian year is longer than the tropical year by about 11.3 minutes on average, the calendar "drifted" with respect to the two equinoxes – so that in AD 300 the spring equinox occurred on about 21 March, by AD 1500 it had drifted backwards to 11 March. This drift induced Pope Gregory XIII to create the modern Gregorian calendar; the Pope wanted to continue to conform with the edicts of the Council of Nicaea in AD 325 concerning the date of Easter, which means he wanted to move the vernal equinox to the date on which it fell at that time, to maintain it at around that date in the future, which he achieved by reducing the number of leap years from 100 to 97 every 400 years. However, there remained a small residual variation in the date and time of the vernal equinox of about ±27 hours from its mean position all because the distribution of 24-hour centurial leap days causes large jumps.
This in turn raised the possibility that it could fall on 22 March, thus Easter Day might theoretically commence before the equinox. The astronomers chose the appropriate number of days to omit so that the equinox would swing from 19 to 21 March but never fall on 22 March; the dates of the equinoxes change progressively during the leap-year cycle, because the Gregorian calendar year is not commensurate with the period of the Earth's revolution about the Sun. It is only after a complete Gregorian leap-year cycle of 400 years that the seasons commence at the same time. In the 21st century the earliest March equinox will be 19 March 2096, while the latest was 21 March 2003; the earliest September equinox will be 21 September 2096 while the latest was 23 September 2003. Vernal equinox and autumnal equinox: these classical names are direct derivatives of Latin; these are the universal and still most used terms for the equinoxes, but are confusing because in the southern hemisphere the vernal equinox does not occur in spring and the autumnal equinox does not occur in autumn.
The equivalent common language English terms spring equinox and autumn equinox are more ambiguous. It has become common for people to refer to the September equinox in the southern hemisphere as the Vernal equinox. March equinox and September equinox: names referring to the months of the year in which they occur, with no ambiguity as to which hemisphere is the context, they are still not universal, however, as not all cultures use a solar-based calendar where the equinoxes occur every year in the same month. Although the terms have become common in the 21st century, they were sometimes used at least as long ago as the mid-20th century. Northward equinox and southward equinox: names referring to the appare
Horticulture has been defined as the culture of plants for food and beauty. A more precise definition can be given as "The cultivation and sale of fruits, vegetables, ornamental plants, flowers as well as many additional services", it includes plant conservation, landscape restoration, soil management and garden design and maintenance, arboriculture. In contrast to agriculture, horticulture does not include large-scale crop production or animal husbandry. Horticulturists apply their knowledge and technologies used to grow intensively produced plants for human food and non-food uses and for personal or social needs, their work involves plant propagation and cultivation with the aim of improving plant growth, quality, nutritional value, resistance to insects and environmental stresses. They work as gardeners, therapists and technical advisors in the food and non-food sectors of horticulture. Horticulture refers to the growing of plants in a field or garden; the word horticulture is modeled after agriculture, comes from the Latin hortus "garden" and cultūra "cultivation", from cultus, the perfect passive participle of the verb colō "I cultivate".
Hortus is cognate with the native English word yard and the borrowed word garden. The major areas of Horticulture include: Arboriculture is the study of, the selection, plant and removal of, individual trees, shrubs and other perennial woody plants. Turf management includes all aspects of the production and maintenance of turf grass for sports, leisure use or amenity use. Floriculture includes the marketing of floral crops. Study of flower cultivation. Landscape horticulture includes the production and maintenance of landscape plants. Olericulture includes the marketing of vegetables. Pomology includes the marketing of pome fruits. Viticulture includes the marketing of grapes. Oenology includes all aspects of winemaking. Postharvest physiology involves maintaining the quality of and preventing the spoilage of plants and animals. Horticulture has a long history; the study and science of horticulture dates all the way back to the times of Cyrus the Great of ancient Persia, has been going on since, with present-day horticulturists such as Freeman S. Howlett and Luther Burbank.
The practice of horticulture can be retraced for many thousands of years. The cultivation of taro and yam in Papua New Guinea dates back to at least 6950–6440 cal BP; the origins of horticulture lie in the transition of human communities from nomadic hunter-gatherers to sedentary or semi-sedentary horticultural communities, cultivating a variety of crops on a small scale around their dwellings or in specialized plots visited during migrations from one area to the next. In the Pre-Columbian Amazon Rainforest, natives are believed to have used biochar to enhance soil productivity by smoldering plant waste. European settlers called it Terra Preta de Indio. In forest areas such horticulture is carried out in swiddens. A characteristic of horticultural communities is that useful trees are to be found planted around communities or specially retained from the natural ecosystem. Horticulture differs from agriculture in two ways. First, it encompasses a smaller scale of cultivation, using small plots of mixed crops rather than large fields of single crops.
Secondly, horticultural cultivations include a wide variety of crops including fruit trees with ground crops. Agricultural cultivations however as a rule focus on one primary crop. In pre-contact North America the semi-sedentary horticultural communities of the Eastern Woodlands contrasted markedly with the mobile hunter-gatherer communities of the Plains people. In Central America, Maya horticulture involved augmentation of the forest with useful trees such as papaya, cacao and sapodilla. In the cornfields, multiple crops were grown such as beans, squash and chilli peppers, in some cultures tended or by women. Since 1804 The Royal Horticultural Society, a UK charity, leads on the encouragement and improvement of the science and practice of horticulture in all its branches and shares this knowledge through its community and learning programmes, world class gardens and shows; the oldest Horticultural society in the world, founded in 1768, is the Ancient Society of York Florists. They still have four shows a year in York, UK.
The professional body representing horticulturists in Great Britain and Ireland is the Institute of Horticulture. The IOH has an international branch for members outside of these islands; the International Society for Horticultural Science promotes and encourages research and education in all branches of horticultural science. The American Society of Horticultural Science promotes and encourages research and education in all branches of horticultural science in the Americas; the Australian Society of Horticultural Science was established in 1990 as a professional society for the promotion and enhancement of Australian horticultural science and industry. The National Junior Horticultural Association was established in 1934 and was the first organisation in the world dedicated to youth and horticulture. NJHA programs are designed to help young people obtain a basic understanding of, develop skills in, the ever-expanding art and science of horticulture; the New Zealand Horticulture Institute. The Global Horticulture Initiative (GlobalHo
Cloning is the process of producing genetically identical individuals of an organism either or artificially. In nature, many organisms produce clones through asexual reproduction. Cloning in biotechnology refers to the process of creating clones of organisms or copies of cells or DNA fragments. Beyond biology, the term refers to the production of multiple copies of digital media or software; the term clone, invented by J. B. S. Haldane, is derived from the Ancient Greek word κλών klōn, "twig", referring to the process whereby a new plant can be created from a twig. In botany, the term lusus was traditionally used. In horticulture, the spelling clon was used until the twentieth century. Since the term entered the popular lexicon in a more general context, the spelling clone has been used exclusively. Cloning is a natural form of reproduction that has allowed life forms to spread for hundreds of millions of years, it is the reproduction method used by plants and bacteria, is the way that clonal colonies reproduce themselves.
Examples of these organisms include blueberry plants, hazel trees, the Pando trees, the Kentucky coffeetree and the American sweetgum. Molecular cloning refers to the process of making multiple molecules. Cloning is used to amplify DNA fragments containing whole genes, but it can be used to amplify any DNA sequence such as promoters, non-coding sequences and randomly fragmented DNA, it is used in a wide array of biological experiments and practical applications ranging from genetic fingerprinting to large scale protein production. The term cloning is misleadingly used to refer to the identification of the chromosomal location of a gene associated with a particular phenotype of interest, such as in positional cloning. In practice, localization of the gene to a chromosome or genomic region does not enable one to isolate or amplify the relevant genomic sequence. To amplify any DNA sequence in a living organism, that sequence must be linked to an origin of replication, a sequence of DNA capable of directing the propagation of itself and any linked sequence.
However, a number of other features are needed, a variety of specialised cloning vectors exist that allow protein production, affinity tagging, single stranded RNA or DNA production and a host of other molecular biology tools. Cloning of any DNA fragment involves four steps fragmentation - breaking apart a strand of DNA ligation - gluing together pieces of DNA in a desired sequence transfection – inserting the newly formed pieces of DNA into cells screening/selection – selecting out the cells that were transfected with the new DNAAlthough these steps are invariable among cloning procedures a number of alternative routes can be selected; the DNA of interest needs to be isolated to provide a DNA segment of suitable size. Subsequently, a ligation procedure is used; the vector is linearised using restriction enzymes, incubated with the fragment of interest under appropriate conditions with an enzyme called DNA ligase. Following ligation the vector with the insert of interest is transfected into cells.
A number of alternative techniques are available, such as chemical sensitivation of cells, optical injection and biolistics. The transfected cells are cultured; as the aforementioned procedures are of low efficiency, there is a need to identify the cells that have been transfected with the vector construct containing the desired insertion sequence in the required orientation. Modern cloning vectors include selectable antibiotic resistance markers, which allow only cells in which the vector has been transfected, to grow. Additionally, the cloning vectors may contain colour selection markers, which provide blue/white screening on X-gal medium; these selection steps do not guarantee that the DNA insert is present in the cells obtained. Further investigation of the resulting colonies must be required to confirm that cloning was successful; this may be accomplished by means of PCR, restriction fragment analysis and/or DNA sequencing. Cloning a cell means to derive a population of cells from a single cell.
In the case of unicellular organisms such as bacteria and yeast, this process is remarkably simple and only requires the inoculation of the appropriate medium. However, in the case of cell cultures from multi-cellular organisms, cell cloning is an arduous task as these cells will not grow in standard media. A useful tissue culture technique used to clone distinct lineages of cell lines involves the use of cloning rings. In this technique a single-cell suspension of cells that have been exposed to a mutagenic agent or drug used to drive selection is plated at high dilution to create isolated colonies, each arising from a single and clonal distinct cell. At an early growth stage when colonies consist of only a few cells, sterile polystyrene rings, which have been dipped in grease, are placed over an individual colony and a small amount of trypsin is added. Cloned cells are transferred to a new vessel for further growth. Somatic-cell nuclear transfer, known as SCNT, can be used to create embryos for research or therapeutic purposes.
The most purpose for this is to produce embryos for use in stem cell research. This process is called "research cloning" or "therapeutic clonin
In biology, histones are alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes. They are the chief protein components of chromatin, acting as spools around which DNA winds, playing a role in gene regulation. Without histones, the unwound DNA in chromosomes would be long. For example, each human diploid cell has about 1.8 meters of DNA. When the diploid cells are duplicated and condensed during mitosis, the result is about 120 micrometers of chromosomes. Five major families of histones exist: H1/H5, H2A, H2B, H3, H4. Histones H2A, H2B, H3 and H4 are known as the core histones, while histones H1/H5 are known as the linker histones; the core histones all exist as dimers, which are similar in that they all possess the histone fold domain: three alpha helices linked by two loops. It is this helical structure that allows for interaction between distinct dimers in a head-tail fashion; the resulting four distinct dimers come together to form one octameric nucleosome core 63 Angstroms in diameter.
Around 146 base pairs of DNA wrap around this core particle 1.65 times in a left-handed super-helical turn to give a particle of around 100 Angstroms across. The linker histone H1 binds the nucleosome at the entry and exit sites of the DNA, thus locking the DNA into place and allowing the formation of higher order structure; the most basic such formation is the 10 nm fiber or beads on a string conformation. This involves the wrapping of DNA around nucleosomes with 50 base pairs of DNA separating each pair of nucleosomes. Higher-order structures include the 30 nm fiber and 100 nm fiber, these being the structures found in normal cells. During mitosis and meiosis, the condensed chromosomes are assembled through interactions between nucleosomes and other regulatory proteins. Histones are subdivided into canonical replication-dependent histones that are expressed during the S-phase of cell cycle and replication-independent histone variants, expressed during the whole cell cycle. In animals, genes encoding canonical histones are clustered along the chromosome, lack introns and use a stem loop structure at the 3’ end instead of a polyA tail.
Genes encoding histone variants are not clustered, have introns and their mRNAs are regulated with polyA tails. Complex multicellular organisms have a higher number of histone variants providing a variety of different functions. Recent data are accumulating about the roles of diverse histone variants highlighting the functional links between variants and the delicate regulation of organism development. Histone variants from different organisms, their classification and variant specific features can be found in "HistoneDB 2.0 - Variants" database. The following is a list of human histone proteins: The nucleosome core is formed of two H2A-H2B dimers and a H3-H4 tetramer, forming two nearly symmetrical halves by tertiary structure; the H2A-H2B dimers and H3-H4 tetramer show pseudodyad symmetry. The 4'core' histones are similar in structure and are conserved through evolution, all featuring a'helix turn helix turn helix' motif, they share the feature of long'tails' on one end of the amino acid structure - this being the location of post-translational modification.
Archaeal histone only contains a H3-H4 like dimeric structure made out of the same protein. Such dimeric structures can stack into a tall superhelix onto which DNA coils in a manner similar to nucleosome spools. Only some archaeal histones have tails, it has been proposed that histone proteins are evolutionarily related to the helical part of the extended AAA+ ATPase domain, the C-domain, to the N-terminal substrate recognition domain of Clp/Hsp100 proteins. Despite the differences in their topology, these three folds share a homologous helix-strand-helix motif. Using an electron paramagnetic resonance spin-labeling technique, British researchers measured the distances between the spools around which eukaryotic cells wind their DNA, they determined the spacings range from 59 to 70 Å. In all, histones make five types of interactions with DNA: Helix-dipoles form alpha-helixes in H2B, H3, H4 cause a net positive charge to accumulate at the point of interaction with negatively charged phosphate groups on DNA Hydrogen bonds between the DNA backbone and the amide group on the main chain of histone proteins Nonpolar interactions between the histone and deoxyribose sugars on DNA Salt bridges and hydrogen bonds between side chains of basic amino acids and phosphate oxygens on DNA Non-specific minor groove insertions of the H3 and H2B N-terminal tails into two minor grooves each on the DNA moleculeThe basic nature of histones, aside from facilitating DNA-histone interactions, contributes to their water solubility.
Histones are subject to post translational modification by enzymes on their N-terminal tails, but in their globular domains. Such modifications include methylation, acetylation, phosphorylation, SUMOylation, ADP-ribosylation; this affects their function of gene regulation. In general, genes that are active have less bound histone, while inactive genes are associated with histones during interphase, it a
Propagation of grapevines
The propagation of grapevines is an important consideration in commercial viticulture and winemaking. Grapevines, most of which belong to the Vitis vinifera family, produce one crop of fruit each growing season with a limited life span for individual vines. While some centenarian old vine examples of grape varieties exist, most grapevines are between the ages of 10 and 30 years; as vineyard owners seek to replant their vines, a number of techniques are available which may include planting a new cutting, selected by either clonal or mass selection. Vines can be propagated by grafting a new plant vine upon existing rootstock or by layering one of the canes of an existing vine into the ground next to the vine and severing the connection when the new vine develops its own root system. In commercial viticulture, grapevines are propagated from seedlings as each seed contains unique genetic information from its two parent varieties and would, theoretically, be a different variety than either parent.
This would be true if two hermaphroditic vine varieties, such as Chardonnay, cross pollinated each other. While the grape clusters that would arise from the pollination would be considered Chardonnay any vines that sprang from one of the seeds of the grape berries would be considered a distinct variety other than Chardonnay, it is for this reason that grapevines are propagated from cuttings while grape breeders will utilize seedlings to come up with new grape varieties including crossings that include parents of two varieties within the same species or hybrid grape varieties which include parents from two different Vitis species such as the Armagnac grape Baco blanc, propagated from the vinifera grape Folle blanche and the Vitis labrusca variety Noah. In viticulture, a clone is single vine, selected from a "mother vine" to which it is identical; this clone may have been selected deliberately from a grapevine that has demonstrated desirable traits and propagated as cuttings from that mother vine.
Varieties such as Sangiovese and Pinot noir are well known to have a variety of clones. While there may be slight mutations to differentiate the various clones, all clones are considered genetically part of the same variety. A selection massale is the opposite of cloning, where growers select cuttings from the mass of the vineyard, or a field blend. A color mutation is a grape variety that while genetically similar to the original variety is considered unique enough to merit being considered its own variety. Both Pinot gris and Pinot blanc are color mutations of Pinot noir. A crossing is a new grape variety, created by the cross pollination of two different varieties of the same species. Syrah is a crossing of two French Vitis vinifera species, Dureza from the Ardèche and Mondeuse blanche from Savoie. Theoretically, every seedling if its pollinated by a member of the same grape variety, is a crossing as any vine that results from the seed being planted will be a different grape variety distinct from either parent.
A hybrid is a new grape variety, produced from a cross pollination of two different grape species. In the early history of American winemaking, grape growers would cross the European Vitis vinifera vines with American vine varieties such as Vitis labrusca to create French-American hybrids that were more resistant to American grape diseases such as downy and powdery mildew as well as phylloxera; when the phylloxera epidemic of the mid to late 19th century hit Europe, some growers in European wine regions experimented with using hybrids until a solution involving grafting American rootstocks to vinifera varieties was found. The use of hybrids in wine production declined with their use formally outlawed by European wine laws in the 1950s; as commercial winemakers want to work with a desired grape variety that dependably produces a particular crop, most grapevines are propagated by clonal or massal selection of plant material. This can be accomplished in one of three ways. Cuttings—This involves a shoot taken from a mother vine and planted where the shoot will sprout a root system and regenerate itself into a full-fledged vine with trunk and canopy.
New cuttings will be first planted in a nursery where it is allowed to develop for a couple years before being planted in the vineyard. Grafting—This involves removing the canopy and most of the trunk of an existing vine and replacing it with a cutting of a new vine, sealed by a graft union; this technique, better known as head grafting, is a quick and inexpensive means of changing over a vineyard as the new cutting is able to take advantage of an existing root system and is able to start producing a crop by the next growing season. Layering—In established vineyards where only a few vines need to be replaced within a row, a new vine can be propagated by bending a cane from a neighboring vine into the ground and covering it with dirt; this segment of vine will soon begin sprouting its own independent root system while still being nourished by the connecting vine. The connection between the two vines are severed allowing each vine to grow independently; each cutting, taken from a mother vine, is a clone of that vine.
The way that a vine grower selects these cuttings can be described as either clonal or massal selection. In clonal selection, an ideal plant within a vineyard or nursery
Vegetative reproduction is any form of asexual reproduction occurring in plants in which a new plant grows from a fragment of the parent plant or a specialized reproductive structure. Many plants reproduce this way, but it can be induced artificially. Horticulturalists have developed asexual propagation techniques that use vegetative plant parts to replicate plants. Success rates and difficulty of propagation vary greatly. Monocotyledons lack a vascular cambium and therefore are harder to propagate. Plant propagation is the process of plant reproduction of a species or cultivar, it can be sexual or asexual, it can happen through the use of vegetative parts of the plants, such as leaves and roots to produce new plants or through growth from specialized vegetative plant parts. While many plants reproduce by vegetative reproduction, they exclusively use that method to reproduce. Vegetative reproduction is not evolutionary advantageous. Vegetative reproduction is favored when it allows plants to produce more offspring per unit of resource than reproduction through seed production.
In general, juveniles of a plant are easier to propagate vegetatively. Although most plants reproduce sexually, many can reproduce vegetatively, or can be induced to do so via hormonal treatments; this is because meristematic cells capable of cellular differentiation are present in many plant tissues. Vegetative propagation is considered a cloning method. However, root cuttings of thornless blackberries will revert to thorny type because the adventitious shoot develops from a cell, genetically thorny. Thornless blackberry is a chimera, with the epidermal layers genetically thornless but the tissue beneath it genetically thorny. Leaf cutting propagation of certain chimeral variegated plants like the snake plant, will produce nonvariegated plants. Grafting is not a complete cloning method because seedlings are used as rootstocks. In that case, only the top of the plant is clonal. In some crops apples, the rootstocks are vegetatively propagated so the entire graft can be clonal if the scion and rootstock are both clones.
Apomixis is a type of reproduction. In flowering plants, unfertilized seeds plantlets that grow instead of flowers. Hawkweed, some citrus and many grasses such as Kentucky bluegrass all use this form of asexual reproduction. Bulbils are sometimes formed instead of the flowers of garlic. Meristem tissue makes the process of asexual reproduction possible, it is found in stems and tips of stems and roots and consists of undifferentiated cells that are dividing allowing for plant growth and give rise to plant tissue systems. The meristem tissue's ability to continuously divide allows for vegetative propagation to occur. Another important ability that allows for vegetative propagation is the ability to develop adventitious roots which arise from other vegetative parts of the plants such as the stem or leaves; these roots allow for the development of new plants from body parts from other plants. There are several advantages of vegetative reproduction that the produced offspring are clones of their parent plants.
If a plant has favorable traits, it can continue to pass down its advantageous genetic information to its offspring. It can be economically beneficial for commercial growers to clone a certain plant to ensure consistency throughout their crops. Vegetative propagation allows plants to avoid the costly process of producing sexual reproduction organs such as flowers and the subsequent seeds and fruits. For example, developing an ace cultivar is difficult, so, once farmers develop the desired traits in apple, they use grafting and budding to ensure the consistency of the new cultivar and its successful production on a commercial level. However, as can be seen in many variegated plants, this does not always apply, because many plants are chimeras and cuttings might reflect the attributes of only one or some of the parent cell lines. Vegetative propagation allows plants to circumvent the immature seedling phase and reach the mature phase faster. In nature, that increases the chances for a plant to reach maturity, commercially, it saves farmers a lot of time and money as it allows for faster crop overturn.
Vegetative reproduction offers research advantages in several areas of biology and has practical usage when it comes to afforestation. The most common use made of vegetative propagation by forest geneticists and tree breeders has been to move genes from selected trees to some convenient location designated a gene bank, clone bank, clone-holding orchard, or seed orchard where their genes can be recombined in pedigreed offspring. A major disadvantage of vegetative propagation is that it prevents species genetic diversity which can lead to reductions in crop yields; the plants are genetically identical and are all, susceptible to pathogenic plant viruses and fungi that can wipe out entire crops. Natural vegetative propagation is a process found in herbaceous and woody perennial plants, involves structural modifications of the stem, although any horizontal, underground part of a plant can contribute to vegetative reproduction of a plant. Most plant species that survive and expand by vegetative reproduction would be perennial by definition, since specialized organs of ve
Soil is a mixture of organic matter, gases and organisms that together support life. Earth's body of soil, called the pedosphere, has four important functions: as a medium for plant growth as a means of water storage and purification as a modifier of Earth's atmosphere as a habitat for organismsAll of these functions, in their turn, modify the soil; the pedosphere interfaces with the lithosphere, the hydrosphere, the atmosphere, the biosphere. The term pedolith, used to refer to the soil, translates to ground stone in the sense "fundamental stone". Soil consists of a solid phase of minerals and organic matter, as well as a porous phase that holds gases and water. Accordingly, soil scientists can envisage soils as a three-state system of solids and gases. Soil is a product of several factors: the influence of climate, relief and the soil's parent materials interacting over time, it continually undergoes development by way of numerous physical and biological processes, which include weathering with associated erosion.
Given its complexity and strong internal connectedness, soil ecologists regard soil as an ecosystem. Most soils have a dry bulk density between 1.1 and 1.6 g/cm3, while the soil particle density is much higher, in the range of 2.6 to 2.7 g/cm3. Little of the soil of planet Earth is older than the Pleistocene and none is older than the Cenozoic, although fossilized soils are preserved from as far back as the Archean. Soil science has two basic branches of study: pedology. Edaphology studies the influence of soils on living things. Pedology focuses on the formation and classification of soils in their natural environment. In engineering terms, soil is included in the broader concept of regolith, which includes other loose material that lies above the bedrock, as can be found on the Moon and on other celestial objects as well. Soil is commonly referred to as earth or dirt. Soil is a major component of the Earth's ecosystem; the world's ecosystems are impacted in far-reaching ways by the processes carried out in the soil, from ozone depletion and global warming to rainforest destruction and water pollution.
With respect to Earth's carbon cycle, soil is an important carbon reservoir, it is one of the most reactive to human disturbance and climate change. As the planet warms, it has been predicted that soils will add carbon dioxide to the atmosphere due to increased biological activity at higher temperatures, a positive feedback; this prediction has, been questioned on consideration of more recent knowledge on soil carbon turnover. Soil acts as an engineering medium, a habitat for soil organisms, a recycling system for nutrients and organic wastes, a regulator of water quality, a modifier of atmospheric composition, a medium for plant growth, making it a critically important provider of ecosystem services. Since soil has a tremendous range of available niches and habitats, it contains most of the Earth's genetic diversity. A gram of soil can contain billions of organisms, belonging to thousands of species microbial and in the main still unexplored. Soil has a mean prokaryotic density of 108 organisms per gram, whereas the ocean has no more than 107 procaryotic organisms per milliliter of seawater.
Organic carbon held in soil is returned to the atmosphere through the process of respiration carried out by heterotrophic organisms, but a substantial part is retained in the soil in the form of soil organic matter. Since plant roots need oxygen, ventilation is an important characteristic of soil; this ventilation can be accomplished via networks of interconnected soil pores, which absorb and hold rainwater making it available for uptake by plants. Since plants require a nearly continuous supply of water, but most regions receive sporadic rainfall, the water-holding capacity of soils is vital for plant survival. Soils can remove impurities, kill disease agents, degrade contaminants, this latter property being called natural attenuation. Soils maintain a net absorption of oxygen and methane and undergo a net release of carbon dioxide and nitrous oxide. Soils offer plants physical support, water, temperature moderation and protection from toxins. Soils provide available nutrients to plants and animals by converting dead organic matter into various nutrient forms.
A typical soil is about 50% solids, 50% voids of which half is occupied by water and half by gas. The percent soil mineral and organic content can be treated as a constant, while the percent soil water and gas content is considered variable whereby a rise in one is balanced by a reduction in the other; the pore space allows for the infiltration and movement of air and water, both of which are critical for life existing in soil. Compaction, a common problem with soils, reduces this space, preventing air and water from reaching plant roots and soil organisms. Given sufficient time, an undifferentiated soil will evolve a soil profile which consists of two or more layers, referred to as soil horizons, that differ in one or more properties such as in their texture, density, consistency, temperature and reactivity; the horizons differ in thickness and gene