A substance which provides plant nutrients when added to soil. The term normally refers to inorganic chemicals containing one or more of the basic plant nutrients: nitrogen, phosphorus, or potash. It may also refer to compounds containing trace elements such as boron, cobalt, copper, iron, manganese, molybdenum, and zinc; to lime, which is used to correct acidity; or to a concentrated organic substance such as dried blood and bonemeal. The term is also popularly used in a general sense to include organic materials, such as manure and compost. Fertilizers are added to the soil in granular, crystalline, powder, or liquid forms and may be injected directly into the ground or broadcast on the surface. Nitrogen usually comes in the form of nitrates or ammonium compounds, phosphorus in the form of phosphates, and potash in the form of potassium chloride or sulphate. Since the Second World War the use of inorganic fertilizers has risen in response to the huge increases in grain production. Between 1945 and 1995 inorganic fertilizers have increased from 50 to 325 kg per hectare. High levels of fertilizer application, especially of nitrogenous compounds, can cause pollution of watercourses and drinking water supplies. In some countries, legal limitations are imposed on the total quantity of fertilizer which may be added to the land during each season.
Fertilizers can be organic (composed of organic matter, i.e.Fertilizers typically provide, in varying proportions, the three major plant nutrients (nitrogen, phosphorus, and potassium), the secondary plant nutrients (calcium, sulfur, magnesium), and sometimes trace elements (or micronutrients) with a role in plant nutrition: boron, chlorine, manganese, iron, zinc, copper and molybdenum.
Inorganic fertilizers (Mineral Fertilizer)
Examples of naturally-occurring inorganic fertilizers include Chilean sodium nitrate, mined "rock phosphate" and limestone (a calcium source, but mostly used to reduce soil acidity). Examples of manufactured or chemically-synthesized inorganic fertilizers include ammonium nitrate, potassium sulfate, and superphosphate, or triple superphosphate.Macronutrients and micronutrients
Fertilizers can be divided into macronutrients or micronutrients based on their concentations in plant dry matter. There are six macronutrients: nitrogen, potassium, and phosphorus, often termed 'primary macronutrients' because their availability is often managed with NPK fertilizers, and the 'secondary macronutrient', and calcium, magnesium, and sulfur, which are required in similar quantities but whose availability is often managed as part of liming and manuring practices rather than fertilizers.
Macronutrient fertilizers
Synthesized materials are also called artificial , and may be described as straight, where the product predominantly contains the three primary ingredients of nitrogen (N), phosphorus (P) and potassium (K), which are known as N-P-K fertilizers or compound fertilizers when elements are mixed intentionally.
In general, the mass fraction (percentage) of elemental phosphorus, [P] = 0.436 x [P2O5]
and the mass fraction (percentage) of elemental potassium, [K] = 0.83 x [K2O]
(These conversion factors are mandatory under the UK fertiliser-labelling regulations if elemental values are declared in addition to the N-P-K declaration.)
An 18−51−20 fertiliser therefore contains, by weight, 18% elemental nitrogen (N), 42% elemental phosphorus (P) and 16% elemental potassium (K).
Agricultural versus Horticultural Fertilizers
In general, agricultural fertilizers contain only one or two macronutrients. Agricultural fertilizers are intended to be applied infrequently and normally prior to or along side seeding. Examples of agricultural fertilizers are granular triple superphosphate, potassium chloride, urea, and anhydrous ammonia.
Horticultural or specialty fertilizers, on the other hand, are formulated from many of the same compounds and some others to produce well-balanced fertilizers that also contain micronutrients. Horticultural fertilizers may be water-soluble (instant release) or relatively insoluble (controlled release). Controlled release fertilizers are also referred to as sustained release or timed release. Many controlled release fertilizers are intended to be applied approximately every 3-6 months, depending on watering, growth rates, and other conditions, whereas water-soluble fertilizers must be applied at least every 1-2 weeks and can be applied as often as every watering if sufficiently dilute. Unlike agricultural fertilizers, horticultural fertilizers are marketed directly to consumers and become part of retail product distribution lines.
Justus von Liebig
Chemist Justus von Liebig (in the 19th century) contributed greatly to understanding the role of inorganic compounds in plant nutrition and devised the concept of Liebig's barrel to illustrate the significance of inadequate concentrations of essential nutrients. It can also be used in the Odda Process to produce compound fertilizers such as 15-15-15.
Inorganic fertilizers sometimes do not replace trace mineral elements in the soil which become gradually depleted by crops grown there. Since this time these trace elements are routinely added to inorganic fertilizers used in Agriculture in this state.
In many countries there is the public perception that inorganic fertilizers "poison the soil" and result in "low quality" produce. When used appropriately, inorganic fertilizers enhance plant growth, the accumulation of organic matter and the biological activity of the soil, while reducing the risk of water run-off, overgrazing and soil erosion. The nutritional value of plants for human and animal consumption is typically improved when inorganic fertilizers are used appropriately.
Organic fertilizers
Examples of naturally occurring organic fertilizers include manure, slurry, worm castings, peat, seaweed and guano. Naturally occurring minerals such as mine rock phosphate, sulfate of potash and limestone are also considered Organic Fertilizers. Examples of manufactured organic fertilizers include compost, dried blood, bone meal and seaweed extracts.Some ambiguity in the usage of the term 'organic' exists because some of synthetic fertilizers, such as urea and urea formaldehyde, are fully organic in the sense of organic chemistry.
Although the density of nutrients in organic material is comparatively modest, they have some advantages. Since the majority of nitrogen supplying organic fertilizers contain insoluble nitrogen and are slow release fertilizers their effectiveness can be greater than conventional nitrogen fertilzers. They re-emphasize the role of humus and other organic components of soil, which are believed to play several important roles:
Mobilizing existing soil nutrients, so that good growth is achieved with lower nutrient densities while wasting less Releasing nutrients at a slower, more consistent rate, helping to avoid a boom-and-bust pattern Helping to retain soil moisture, reducing the stress due to temporary moisture stress Improving the soil structureOrganics also have the advantage of avoiding certain long-term problems associated with the regular heavy use of artificial fertilizers:
the possibility of "burning" plants with the concentrated chemicals (i.e. the necessity of reapplying artificial fertilizers regularly (and perhaps in increasing quantities) to maintain fertility the cost (substantial and rising in recent years) and resulting lack of independenceOrganic fertilizers also have their disadvantages:
As acknowledged above, they are typically a dilute source of nutrients compared to inorganic fertilizers, and where significant amounts of nutrients are required for profitable yields, very large amounts of organic fertilizers must be applied. The composition of organic fertilizers tends to be highly variable, so that accurate application of nutrients to match plant production is difficult. Hence, large-scale agriculture tends to rely on inorganic fertilizers while organic fertilizers are cost-effective on small-scale horticultural or domestic gardens.In practice a compromise between the use of artificial and organic fertilizers is common, typically by using inorganic fertilizers supplemented with the application of organics that are readily available such as the return of crop residues or the application of manure.
It is important to differentiate between what we mean by organic fertilizers and fertilizers approved for use in organic farming and organic gardening by organizations and authorities who provide organic certification services. Some approved fertilizers may be inorganic, naturally occurring chemical compounds, e.g.
Environmental effects of fertilizer use
Over-application of chemical fertilizers, or application of chemical fertilizers at a time when the ground is waterlogged or the crop is not able to use the chemicals, can lead to surface runoff (particularly phosphorus) or leaching into groundwater (particularly nitrates).
It is also possible to over-apply organic fertilizers. However: their nutrient content, their solubility, and their release rates are typically much lower than chemical fertilizers, partially because by their nature, most organic fertilizers also provide increased physical and biological storage mechanisms to soils.
The problem of over-fertilization is primarily associated with the use of artificial fertilizers, because of the massive quantities applied and the destructive nature of chemical fertilizers on soil nutrient holding structures. The high solubilities of chemical fertilizers also exacerbate their tendency to degrade ecosystems.
Storage and application of some fertilizers in some weather or soil conditions can cause emissions of the greenhouse gas nitrous oxide (N2O). Ammonia gas (NH3) may be emitted following application of inorganic fertilizers, or manure or slurry.
For these reasons, it is recommended that knowledge of the nutrient content of the soil and nutrient requirements of the crop are carefully balanced with application of nutrients in inorganic fertiliser especially. By careful monitoring of soil conditions, farmers can avoid wasting expensive fertilizers, and also avoid the potential costs of cleaning up any pollution created as a byproduct of their farming. Uranium is another example for impurities of fertilizers
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