Fertilizer is a substance added to soil to improve plants'
growth and yield. First used by ancient farmers, fertilizer technology
developed significantly as the chemical needs of growing plants were
discovered. Modern synthetic fertilizers are composed mainly of nitrogen,
phosphorous, and potassium compounds with secondary nutrients added. The use of
synthetic fertilizers has significantly improved the quality and quantity of
the food available today, although their long-term use is debated by
environmentalists. Like all living organisms, plants are made up of cells.
Within these cells occur numerous metabolic chemical reactions that are
responsible for growth and reproduction. Since plants do not eat food like
animals, they depend on nutrients in the soil to provide the basic chemicals
for these metabolic reactions. The supply of these components in soil is
limited, however, and as plants are harvested, it dwindles, causing a reduction
in the quality and yield of plants.
Calcium, magnesium, and sulfur are also important materials
in plant growth. They are only included in fertilizers in small amounts,
however, since most soils naturally contain enough of these components. Other
materials are needed in relatively small amounts for plant growth. These
micronutrients include iron, chlorine, copper, manganese, zinc, molybdenum, and
boron, which primarily function as cofactors in enzymatic reactions. While they
may be present in small amounts, these compounds are no less important to
growth, and without them plants can die. Many different substances are used to
provide the essential nutrients needed for an effective fertilizer. These
compounds can be mined or isolated from naturally occurring sources. Examples
include sodium nitrate, seaweed, bones, guano, potash, and phosphate rock.
Compounds can also be chemically synthesized from basic raw materials. These
would include such things as ammonia, urea, nitric acid, and ammonium
phosphate. Since these compounds exist in a number of physical states,
fertilizers can be sold as solids, liquids, or slurries.
The process of adding substances to soil to improve its
growing capacity was developed in the early days of agriculture. Ancient
farmers knew that the first yields on a plot of land were much better than
those of subsequent years. This caused them to move to new, uncultivated areas,
which again showed the same pattern of reduced yields over time. Eventually it
was discovered that plant growth on a plot of land could be improved by
spreading animal manure throughout the soil. Over time, fertilizer technology
became more refined. New substances that improved the growth of plants were discovered.
The Egyptians are known to have added ashes from burned weeds to soil. Ancient
Greek and Roman writings indicate that various animal excrements were used,
depending on the type of soil or plant grown. It was also known by this time
that growing leguminous plants on plots prior to growing wheat was beneficial.
Other types of materials added include sea-shells, clay, vegetable waste, waste
from different manufacturing processes, and other assorted trash.
Organized research into fertilizer technology began in the
early seventeenth century. Early scientists such as Francis Bacon and Johann
Glauber describe the beneficial effects of the addition of saltpeter to soil.
Glauber developed the first complete mineral fertilizer, which was a mixture of
saltpeter, lime, phosphoric acid, nitrogen, and potash. As scientific chemical
theories developed, the chemical needs of plants were discovered, which led to
improved fertilizer compositions. Organic chemist Justus von Liebig
demonstrated that plants need mineral elements such as nitrogen and phosphorous
in order to grow. The chemical fertilizer industry could be said to have its
beginnings with a patent issued to Sir John Lawes, which outlined a method for
producing a form of phosphate that was an effective fertilizer.
Raw
Materials
The fertilizers outlined here are compound fertilizers
composed of primary fertilizers and secondary nutrients. These represent only
one type of fertilizer, and other single nutrient types are also made. The raw
materials, in solid form, can be supplied to fertilizer manufacturers in bulk
quantities of thousands of tons, drum quantities, or in metal drums and bag
containers. Primary fertilizers include substances derived from nitrogen,
phosphorus, and potassium. Various raw materials are used to produce these compounds.
When ammonia is used as the nitrogen source in a fertilizer, one method of
synthetic production requires the use of natural gas and air. The phosphorus
component is made using sulfur, coal, and phosphate rock. The potassium source
comes from potassium chloride, a primary component of potash. Secondary
nutrients are added to some fertilizers to help make them more effective.
Calcium is obtained from limestone, which contains calcium carbonate, calcium
sulphate, and calcium magnesium carbonate. The magnesium source in fertilizers
is derived from dolomite. Sulfur is another material that is mined and added to
fertilizers. Other mined materials include iron from ferrous sulfate, copper,
and molybdenum from molybdenum oxide.
Nitrogen fertilizer
component
The
Manufacturing Process
Fully integrated factories have been designed to produce
compound fertilizers. Depending on the actual composition of the end product,
the production process will differ from manufacturer to manufacturer.
• Ammonia is one nitrogen fertilizer component that can be
synthesized from in-expensive raw materials. Since nitrogen makes up a
significant portion of the earth's atmosphere, a process was developed to
produce ammonia from air. In this process, •natural gas and steam are pumped
into a large vessel. Next, air is pumped into the system, and oxygen is removed
by the burning of natural gas and steam. This leaves primarily nitrogen,
hydrogen, and carbon dioxide. The carbon dioxide is removed and ammonia is
produced by introducing an electric current into the system. Catalysts such as
magnetite (Fe 3 O 4 ) have been used to improve the speed and efficiency of
ammonia synthesis. Any impurities are removed from the ammonia, and it is
stored in tanks until it is further processed.
• While ammonia itself is sometimes used as a fertilizer, it
is often converted to other substances for ease of handling. Nitric acid is
produced by first mixing ammonia and air in a tank. In the presence of a
catalyst, a reaction occurs which converts the ammonia to nitric oxide. The
nitric oxide is further reacted in the presence of water to produce nitric
acid.
•Nitric acid and ammonia are used to make ammonium nitrate.
This material is a good fertilizer component because it has a high concentration
of nitrogen. The two materials are mixed together in a tank and a
neutralization reaction occurs, producing ammonium nitrate. This material can
then be stored until it is ready to be granulated and blended with the other
fertilizer components.
Phosphorous
fertilizer component
To isolate phosphorus
from phosphate rock, it is treated with sulfuric acid, producing phosphoric
acid. Some of this material is reacted further with sulfuric acid and nitric
acid to produce a triple superphosphate, an excellent source of phosphorous in
solid form.
•Some of the phosphoric acid is also reacted with ammonia in
a separate tank. This reaction results in ammonium phosphate, another good
primary fertilizer.
Potassium fertilizer
component
• Potassium chloride is typically supplied to fertilizer
manufacturers in bulk. The manufacturer converts it into a more usable form by
granulating it. This makes it easier to mix with other fertilizer components in
the next step.
Granulating and
blending
• To produce fertilizer in the most usable form, each of the
different compounds, ammonium nitrate, potassium chloride, ammonium phosphate,
and triple superphosphate are granulated and blended together. One method of
granulation involves putting the solid materials into a rotating drum which has
an inclined axis. As the drum rotates, pieces of the solid fertilizer take on
small spherical shapes. They are passed through a screen that separates out adequately
sized particles. A coating of inert dust is then applied to the particles,
keeping each one discrete and inhibiting moisture retention. Finally, the
particles are dried, completing the granulation process.
•The different types of particles are blended together in
appropriate proportions to produce a composite fertilizer. The blending is done
in a large mixing drum that rotates a specific number of turns to produce the
best mixture possible. After mixing, the fertilizer is emptied onto a conveyor
belt, which transports it to the bagging machine.
Bagging
• Fertilizers are typically supplied to farmers in large
bags. To fill these bags the fertilizer is first delivered into a large hopper.
An appropriate amount is released from the hopper into a bag that is held open
by a clamping device. The bag is on a vibrating surface, which allows better
packing. When filling is complete, the bag is transported upright to a machine
that seals it closed. The bag is then conveyored to a palletizer, which stacks
multiple bags, readying them for shipment to distributors and eventually to
farmers.
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