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From Wikipedia

Propellant

A propellant is a material that is used to move ("propel") an object. The material is usually expelled by gas pressure through a nozzle. The pressure may be from a compressed gas, or a gas produced by a chemical reaction. The exhaust material may be a gas, liquid, plasma, or, before the chemical reaction, a solid, liquid or gelled.

Common chemical propellants consist of a fuel; like gasoline, jet fuel, rocket fuel, and an oxidizer.

Aerosol sprays

In aerosol spray cans, the propellant is simply a pressurized gas in equilibrium with its liquid (at its saturated vapour pressure). As some gas escapes to expel the payload, more liquid evaporates, maintaining an even pressure. (See aerosol spray propellant for more information.)

Propellant used for propulsion

Technically, the word propellant is the general name for chemicals used to create thrust. For vehicles, the term propellant refers only to chemicals that are stored within the vehicle prior to use, and excludes atmospheric gas or other material that may be collected in operation.

Amongst the English-speaking laymen, used to having fuels propel vehicles on Earth, the word fuel is inappropriately used. In Germany, the word Treibstoff—literally "drive-stuff"—is used; in France, the word ergols is used; it has the same Greek roots as hypergolic, a term used in English for propellants which combine spontaneously and do not have to be set ablaze by auxiliary ignition system.

In rockets, the most common combinations are bipropellants, which use two chemicals, a fuel and an oxidiser. There is the possibility of a tripropellant combination, which takes advantage of the ability of substances with smaller atoms to attain a greater exhaust velocity, and hence propulsive efficiency, at a given temperature.

Although not used in practice, the most developed tripropellant systems involves adding a third propellant tank containing liquid hydrogen to do this.

Solid propellant

In ballistics and pyrotechnics, a propellant is a generic name for chemicals used for propelling projectiles from guns and other firearms.

Propellants are usually made from low explosive materials, but may include high explosive chemical ingredients that are diluted and burned in a controlled way (deflagration) rather than detonation. The controlled burning of the propellant composition usually produces thrust by gaspressure and can accelerate a projectile, rocket, or other vehicle. In this sense, common or well known propellants include, for firearms, artillery and solid propellant rockets:

Propellants that explode in operation are of little practical use currently, although there have been experiments with Pulse Detonation Engines.

Grain

Propellants are used in forms called grains. A grain is any individual particle of propellant regardless of the size or shape. The shape and size of a propellant grain determines the burn time, amount of gas and rate produced from the burning propellant and consequently thrust vs time profile.

There are three types of burns

Fertilizer

Fertilizers (or fertilisers) are substances that supply plant nutrients or amend soil fertility. They are the most effective (30 -80 per cent increase in yields) means of increasing crop production and of improving the quality of food and fodder. Fertilizers are used in order to supplement nutrient supply in the soil, especially to correct yield-limiting factors.

Fertilizers are applied to promote plant growth; the main nutrients present in fertilizer are nitrogen, phosphorus, and potassium (the 'macronutrients') and other nutrients ('micronutrients') are added in smaller amounts. Fertilizers are usually directly applied to soil, and can also be sprayed on leaves as a foliar feeding.

Organic fertilizers and some mined inorganic fertilizers have been used for many centuries, whereas chemically synthesized inorganic fertilizers were only widely developed during the industrial revolution. Increased understanding and use of fertilizers were important parts of the pre-industrial British Agricultural Revolution and the industrial Green Revolution of the 20th century.

Inorganic fertilizer use has also significantly supported global population growth— it has been estimated that almost half the people on the Earth are currently fed as a result of artificial nitrogen fertilizer use.

Fertilizers typically provide, in varying proportions:

The macronutrients are consumed in larger quantities and are present in plant tissue in quantities from 0.2% to 4.0% (on a dry matter weight basis). Micronutrients are consumed in smaller quantities and are present in plant tissue in quantities measured in parts per million (ppm), ranging from 5 to 200 ppm, or less than 0.02% dry weight.

The nitrogen-rich fertilizer ammonium nitrate is also used as an oxidizing agent in improvised explosive devices, sometimes called fertilizer bombs, leading to sale regulations.

Labeling

Macronutrient fertilizers

Macronutrient fertilizers are labeled with an NPKanalysis (also "N-P-K-S" in Australia).

Fertilizer is described by a three number designator; for example, 20-20-10. These numbers are percentages of three elements: nitrogen, phosphorus, and potassium, respectively. Therefore, 20-20-10 fertilizer contains 20% nitrogen, 20% phosphorus, and 10% potassium by weight.

Example of labeling

The fertilizer potash(potassium carbonate) is composed of 1:1 potassium to carbonate by volume (47:53 by wt., owing to differences inmolecular weight between potassium and carbonate).

Traditional analysis of 100g of potassium chloride (KCl) would yield 60g of potassium oxide (K2O). The percentage yield of K2O from the original 100g of fertilizer is the number shown on the label. A potash fertilizer would thus be labeled 0-0-60, and not 0-0-52.

History

The modern understanding of plant nutrition dates to the 19th century and the work of Justus von Liebig, among others. Management of soil fertility, however, has been the pre-occupation of farmers for thousands of years.

Forms

Fertilizers come in various forms. The most typical form is granular fertilizer (powder form). The next most common form is liquid fertilizer; some advantages of liquid fertilizer are its immediate effect and wide coverage. There are also slow-release fertilizers (various forms including fertilizer spikes, tabs, etc.) which reduce the problem of "burning" the plants due to excess nitrogen.

Finally, organic fertilizer is on the rise as people are resorting to environmental friendly (or 'green') products. Although organic fertilizer usually contain less nutrients, some people still prefer organic due to natural ingredients.

Inorganic fertilizer (synthetic fertilizer)

Fertilizers are broadly divided into organic fertilizers (composed of enriched organic matter—plant or animal), orinorganic fertilizers (composed ofsynthetic chemicals and/or minerals).

Inorganic fertilizer is often synthesized using the Haber-Bosch process, which produces ammonia as the end product. This ammonia is used as a feedstock for other nitrogen fertilizers, such as compost, manure), or naturally-occurring mineral deposits (e.g. saltpeter).

Naturally-occurring organicfertilizers includemanure, slurry, worm castings, peat, seaweed, humic acid, and guano. Sewage sludge use in organic agricultural operations in the U.S. has been extremely limited and rare due to USDA prohibition of the practice (due to toxic metal accumulation, among other factors).

Processed organic fertilizers include compost, bloodmeal, bone meal, humic acid, amino acids, and seaweed extracts. Other examples are natural enzyme digested proteins, fish meal, and feather meal. Decomposing crop residue (green manure) from prior years is another source of fertility.

Discussion of the term 'organic'

There used to be a distinction between the term "organic" and the term "pesticide free". Organic simply dealt with the use of fertilizer types. Once the term "organic" became regulated, many other factors were added. "Pesticide-free" is not at all related to fertilization (plant nutrition), but has become a legal inclusion.

Likewise, in scientific terms, a fish emulsion can be a good organic fertilizer :), but in some jurisdictions fish emulsion must be certified "dolphin safe" to be considered "organic".

Natural sourcing

Animal-sourced Urea and Urea-Formaldehyde (from urine), are suitable for application organic agriculture, while pure synthetic forms are not deemed, however, pure (synthetically-produced) urea is not. The common thread that can be seen through these examples is that organic agriculture attempts to define itself through minimal processing (e.g. via chemical energy such as petroleum—see Haber process), as well as being naturally-occurring or via natural biological processes such as composting.

Cover crops are also grown to enrich soil as a green manure through nitrogen fixation from the atmosphere; as well as phosphorus (through nutrient mobilization) content of soils.

Powdered limestone, mined rock phosphate and Chilean saltpeter, are inorganic chemicals in the technical (organic chemistry) sense of the word, but are considered suitable for organic agriculture in limited amounts..

Advantages

Although the density of nutrients in organic material is comparatively modest, they have many advantages. The majority of nitrogen supplying organic fertilizers contain insoluble nitrogen and act as a slow-release fertilizer. By their nature, organic fertilizers increase physical and biological nutrient storage mechanisms in soils, mitigating risks of over-fertilization. Organic fertilizer nutrient content, solubility, and nutrient release rates are typically much lower than mineral (inorganic) fertilizers. A University of North Carolina study found that potential mineralizable nitrogen (PMN) in the soil was 182–285% higher in organic mulched systems, than in the synthetics control.

Organic fertilizers also 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 structure
  • Helping to prevent topsoil erosion (responsible for desertfication and the Dust bowl

Organic fertilizers also have the advantage of avoiding certain problems associated with the regular heavy use of artificial fertilizers:

  • The necessity of reapplying artificial fertilizers regularly (and perhaps in increasing quantities) to maintain fertility
  • Extensive runoff of soluble nitrogen and phosphorus, leading to eutrophication of bodies of water (which causes fish kills)
  • Costs are lower for if fertilizer is locally available

According to the PPI institute website, it is widely thought that organic fertilizer is better than inorganic fertilizer. However, balanced responsible use of either or both can be just as good for the soil.

Disadvantages

Organic fertilizers have the following disadvantages:

  • As a dilute source of nutrients when compared to inorganic fertilizers, transporting large amount of fertilizer incurs higher costs, especially with slurry and manure.
  • The composition of organic fertilizers tends to be more complex and variable than a standardized inorganic product.
  • Improperly-processed organic fertilizers may contain pathogens from plant or animal matter that are harmful to humans or plants. However, proper composting should remove them.
  • More labor is needed to compost organic fertilizer, increasing labor costs. Some of this cost is offset by reduced cash purchase.

Conventional farming application

In non-organic farming a compromise between the use of artificial and organic fertilizers is common, often using inorganic fertilizers supplemented with the application of organics that are readily available such as the

Aerenchyma

Aerenchyma is an air channel in the roots of some plants, which allows exchange of gases between the shoot and the root. The channel of large air-filled cavities provides a low-resistance internal pathway for the exchange of gases such as oxygen and ethylene between the plant above the water and the submerged tissues.

Aerenchyma form in roots subject to anoxia such as what occurs during flooding of plants and soil . For example, Blom et al. (1994) investigated the adaptive responses of plants to flooding along the banks of the Rhine River, which included such morphological changes such as aerenchyma formation.

Aerenchyma formation

In maize, an aerenchyma is formed from highly selective cell death and dissolution in the root cortex during anoxia in the roots . When plant roots are submerged or the surrounding soil flooded, hypoxia develops, as soil microorganisms consume oxygen faster than diffusion occurs. Nitrification is inhibited as low oxygen occurs and toxic compounds are formed, as anaerobic bacteria use nitrate, manganese, and sulfate as alternative electron acceptors . The reduction-oxidation potential of the rhizhosphere decreases and metal ions such as iron and manganese precipitate .

In general, low oxygen stimulates trees and plants to produce ethylene . Yet Visser et al.., in 1997, found that ethylene slows down primary and adventitious root elongation and formation. Thus, in addition to supplying root tissues with oxygen, aerenchymas assist in diffusing the accumulation of ethylene in order to prevent elongation inhibition (Visser et al. 1997).

Formation of Aerenchyma

Aerenchymas are formed by cell differentiation and collapse (lysigenous aerenchyma) or by cell separation without collapse (schizogenous aerenchyma). The differentiation or separation forms large continuous air spaces that allow diffusion of oxygen from shoot to root . Different experiments defined how cell collapse occurs. Cell death was blocked by antagonists of phospholipid metabolism, of cytolsolic Ca2+ or Ca-calmodulin, and of protein kinases. By contrast, reagents that activate G-proteins raise cytolsolic Ca2+ or inhibit phosphatases-promoted cell death (two references He et al.. 1996). An enzyme that was linked to this process is cellulase, which assists in cell wall breakage. In maize, a protein that is homologous to the enzyme XET (a protein that breaks the β-1,4 links between glucans and xyulosyl, the cross-linking molecule in plant cell walls) was found..

Advantages of aerenchyma

The large air-filled cavities provide a low-resistance internal pathway for the exchange of gases between the plant organs above the water and the submerged tissues. Some of the oxygen transported through the aerenchyma leaks through root pores into the surrounding soil. The resulting small rhizosphere of oxygenated soil around individual roots support microorganisms that prevent the influx of potentially toxic soil components such as sulfide, iron, and manganese. Nitrifying bacteria provide the roots with a favourable nitrogen source .

During drought, aerenchymas allow plant roots to grow deeper for water, even through compacted layers; thick and tough roots are formed. As the roots dieback and decay, the resulting voids are paths in which new roots can grow and elongate when resources are available.

Disadvantages of Aerenchyma

Not all plants are able to develop aerenchymas.

Aerenchymous roots may experience the following problems

  • Water and nutrient uptake may be less efficient; large intercellular spaces decrease the tissue available to transport water and nutrients from the root surface to the root xylem (Visser et al.. 1996, 2000a).
  • Large root diameters reduce biomass-to-surface ratio, resulting in less uptake of water and nutrients and the reduced opportunity to explore all microzones for nutrients.
  • Some roots with aerenchymas are less likely to resist the physical strain of compacted soils. Those roots that penetrate and survive dense and compact drained soils have a higher bulk density and a strongly lignified layer of cells surrounding the aerenchyma, which strengthens the root. This dense, lignified layer prevents radial leakage of oxygen from the aerenchyma and may block some water and nutrient uptake (Colmer et al.. 1998; Visser et al.. 2000).
  • During drought, roots with aerenchyma may be less tolerant to water stress as the open structure of the cortex is probably a low-resistance pathway for water vapor, as well as for air, thereby increasing the susceptibility of the root to water loss.


From Yahoo Answers

Question:

Answers:The advantages of water chlorination: 1. Controls Disease-Causing Bacteria: Disease-causing bacteria may enter the well during construction, repair, flooding or as a result of improper construction. Proper chlorination will kill these bacteria. 2. Controls Nuisance Organisms: Chlorine treatment will control nuisance organisms such as iron, slime and sulfate-reducing bacteria. Iron bacteria feed on the iron in the water. 3. Mineral Removal: Large amounts of iron can be removed from water by adding chlorine to oxidize the clear soluble iron into the filterable reddish insoluble form. Chlorine helps remove manganese and hydrogen sulfide in the same way. The disadvantages of water chlorination: 1. No Nitrate Removal: Chlorine will not remove nitrates from water. The claims of some water treatment firms imply that nitrates can be removed by chlorination. This is not true. Adding chlorine may prevent nitrates from being reduced to the toxic nitrite form; however, nitrates are not removed from water by chlorination. 2. Causes Smell and Bad Taste: Chlorine in water is not poisonous to humans or animals. However, if the concentration is great enough the water will taste bad so consumption may be reduced. Some people object to the smell and/or taste of very small amounts of chlorine. In those cases an activated carbon or charcoal filter may be used to remove the chlorine from the drinking water. 3. Trihalomethanes (THMs) are organic chemicals that may form when chlorine is used to treat water supplies that contain humic compounds. Humic compounds form as a part of the decomposition of organic materials such as leaves, grass, wood or animal wastes. Because THMs are very seldom associated with groundwater, they are primarily a concern where surface water supplies are used. Lifetime consumption of water supplies with THMs at a level greater than 0.10 milligrams per liter is considered by the Environmental Protection Agency to be a potential cause of cancer. THMs can be removed from drinking water through use of an activated carbon filter.

Question:

Answers:The main difference is that organic fertilizers contain the substances that when degraded will nourish the soil with what has been depleted over the seasons. The organic fertilizer's composition comes from the soil and returns to the soil. Inorganic nitrate fertilizers contain many salts that are retain by the soil and if you use the fertilizers over and over again the salts will build up in the soil and make it less and less capable of growing crops.

Question:For example, the Haber's process has contributed to the production of nitrate-based fertilizers, increasing crop yields. Runoffs from fertilized fields is a source of water pollution. The high costs of chemical fertilizers has driven some farmers into debt. Thanks!

Answers:Fields must be fertilized either with chemicals or with manure/mulch. If you don't feed the land it can't produce a crop. Manure, sewage sludge, and mulch are heavy and expensive to haul, and hard to apply. It does add tilth and other water absorbing properties that chemical can't. It includes microbes in with all the salt and herbicides and so on that are in the waste materials. It does include trace minerals and so on, but not always in the quantities and ratios needed. There is just no way to tailor the contents of cow poop or hog water. It also contains weed seeds and rocks and trash and other problems that require additional work, expense and time. Chemical fertilizer can be specifically mixed to meet the needs of a particular crop to be grown in a particular place in a particular kind of soil. Through soil testing, you can micro manage the trace and mass nutrients in the soil and make sure balances are working and everything is exactly available as needed. And yeah, that's expensive. Many people do a general broadcast in an effort to control cost and make a stab at managing the proper nutrients, using a standard blend. That's cheaper, but less effective. And it does need to be applied more than once - especially in wet areas where nutrients can be lost in run off. If the nutrients run off, they can't be used by the crop and that's a waste of money, so farmers try not to do that. And before you blame farms for all the run off pollution, know that the heaviest fertilizer users are golf course, parks and lawns. The amounts farmers use is tiny compared to the pounds per acre poured on by golf course keepers, park managers and home owners. And that all ends up in the sewers and water ways. But farmers take the blame. Nobody ever talks about the tons and tons of chemicals that come off golf courses, lawns and parks. Too many people would throw a fit. But if it's farmers, then it's just a few people and that's okay.