Fertilizers are chemical compounds applied to
promote
plant growth. Typically, fertilizers are applied either
to the soil (for uptake by
plant roots) or by
foliar feeding (for uptake through leaves).
Fertilizers can be placed
into the categories of
organic and
inorganic fertilizers (composed of simple
chemicals and minerals). Organic fertilizers are
'naturally' occurring compounds manufactured through
a natural processes. Inorganic fertilizers are
manufactured through chemical processes using naturally occurring
deposits, while chemically altering them.
Organic fertilizers
contain essential nutrients to improve the health
and productivity of soil and encourage plant growth. Organic nutrients increase
the abundance of soil organisms by providing organic
matter and micronutrients for organisms such as
fungal
mycorrhiza, which aid plants in absorbing
nutrients. Chemical fertilizers may have long-term
adverse impact on the organisms living in soil and a detrimental long term
effect on soil productivity of the soil.
Definitions of 'organic'
There can be confusion as to the veracity of the
term 'organic' when applied to agricultural systems
and fertilizer. The problem is related to the
marketing and colloquial uses of the term. Minerals such as mined
rock phosphate,
sulfate of potash and
limestone are considered organic fertilizers,
although they contain no organic (carbon) molecules.
This is but one of many ambiguities in the usage of
the term organic.
Synthetic fertilizers, such as
urea and
urea formaldehyde, are considered organic in the
sense of the
organic chemistry, and can be supplied
organically (agriculturally), but when
manufactured as a pure chemical is not organic
under organic certification standards[28][29].
Naturally mined powdered
limestone,
mined
rock phosphate and
sodium nitrate, are inorganic (in a chemical
sense) in that they contain no carbon molecules, and
are difficult to
harvest, but are approved for organic
agriculture in minimal amounts.
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), as well as being
naturally-occurring (as is, or via natural
biological processing) This is a contradictory
stance however, because high-concentrate plant
nutrients (in the form of salts) obtained from dry
lake beds by farmers for centuries in a very minimal
fashion are excluded from consideration by most
organic enthusiasts and many governmental
definitions of organic agriculture. One of the main tenants of organic lifestyle
marketing is that organic fertilizers are
completely different than chemical fertilizers.
No such dichotomy exists. There is substantial
overlap between the two.
Health and sustainability issues
Many inorganic fertilizers do not replace
trace mineral elements in the soil which become
gradually depleted by crops. This depletion has been
linked to studies which have shown a marked fall (up
to 75%) in the quantities of such minerals present
in fruit and vegetables. Deficiencies in
zinc,
copper,
manganese,
iron and
molybdenum can limit the
growth of broad acre crops and pastures.
There are concerns regarding
arsenic,
cadmium and
uranium accumulating in fields treated with
synthetic fertilizers. The phosphate minerals contain trace
amounts of these elements and if no cleaning step
is applied after mining, the continuous use of
phosphate fertilizers leads towards an accumulation
of these elements in the soil. High levels of lead and cadium can also be found in many manures or sewage sludge.
Phosphate fertilizers replace inorganic
arsenic naturally found in the soil, displacing
the heavy metal and causing accumulation in
runoff. Eventually these heavy metals
can build up to unacceptable levels.
Another problem with inorganic
fertilizers is that they are now produced in ways
which cannot be continued indefinitely. Potassium
and phosphorus come from mines and such resources are limited.
Nitrogen sources are effectively unlimited (forming
over 70% of
atmospheric gases), however, nitrogen
fertilizers are presently made using
fossil fuels such as
natural gas and
coal, which are limited.
Nitrogen fertilizer is often synthesized using
the
Haber-Bosch process, which produces
ammonia. This ammonia is then used to produce
other compounds (notably
anhydrous
ammonium nitrate and
urea) which can be applied to fields. These
concentrated products may be used as fertilizer or
diluted with water to form a concentrated liquid
fertilizer,
UAN. Ammonia can also be used in combination with rock phosphate
and potassium fertilizer to produce compound
fertilizers.
The production of ammonia currently consumes
about 5% of global
natural gas consumption, which is somewhat under
2% of world energy production.
Natural gas is overwhelmingly used for the
production of ammonia, but other energy sources,
together with a hydrogen source, can be used for the
production of nitrogen compounds suitable for
fertilizers. The cost of natural gas makes up about
90% of the cost of producing ammonia.
The price increases in natural gas in the past
decade, along with other factors such as increasing
demand, have contributed to an increase in
fertilizer price.
Nitrogen-based fertilizers are most commonly used
to treat fields used for growing
corn,
barley,
wheat,
canola,
soybean, etc. One study has shown that
the application of nitrogen fertilizer on off season
cover crops can increase the
biomass of these crops, while having a
beneficial effect on soil nitrogen levels for the
cash crop planted during the summer season.
Benefits of organic fertilizer
However, by their nature, organic fertilizers
provide increased physical and biological storage
mechanisms to soils, mitigating risks of
over-fertilization. Organic fertilizer nutrient
content, solubility, and nutrient release rates are
typically much lower than mineral (inorganic)
fertilizers.
One study found that over a 140-day period, after 7
leachings:
- Organic fertilizers had released between 25%
and 60% of their nitrogen content
- Controlled release fertilizers(CRFs) had a
relatively constant rate of release
- Soluble fertilizer released most of its
nitrogen content at the first leaching
Disadvantages of organic fertilizer
It is difficult to chemically distinguish between
urea of biological origin and those produced
synthetically. Like inorganic fertilizers,
it is possible to over apply organic fertilizers
and, as a result, a water soluble soil test should
be conducted to determine optimum application
rates. Release of the nutrients may
happen quite suddenly depending on the type of
organic fertilizer used. Organic fertilizers from treated sewage, composts
and sources can be quiet variable from one batch to
the next. Unless each batch is tested the amounts of
nutrient applied are not precisely known.
Environmental risks of fertilizer use
High application rates of nitrogen fertilizers in
order to maximize crop yields, combined with the
high solubilities of these fertilizers can lead to
increased
leaching of nitrates into groundwater.
The use of
ammonium nitrate in inorganic fertilizers
is particularly damaging, as plants absorb ammonium
ions preferentially over nitrate ions, while excess
nitrate ions which are not absorbed dissolve (by
rain or irrigation) into groundwater.
Nitrate levels above 10 mg/L (10 ppm) in groundwater
can cause 'blue
baby syndrome' (acquired
methemoglobinemia), leading to
hypoxia (which can lead to coma and death if not
treated).
Nitrogen containing inorganic fertilizers in the
form of nitrate and ammonium also cause
soil acidification.
Eventually, nitrate-enriched groundwater makes
its way into lakes, bays and oceans where it
accelerates the growth of
algae, disrupts the normal functioning of water
ecosystems, and kills fish in a process called
eutrophication (which may cause water to become
cloudy and/or discolored—green, yellow, brown, or
red). About half of all the lakes in the United
States are now
eutrophic, while the number of oceanic
dead zones near inhabited coastlines are
increasing.
As of 2006, the application of nitrogen
fertilizer is being increasingly controlled in
Britain and the United States. If eutrophication can
be reversed, it may take decades before the accumulated
nitrates in groundwater can be broken down by
natural processes.
Storage and application of some nitrogen
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
manure/slurry. Besides supplying nitrogen, ammonia
can also increase soil
acidity (lower
pH, or "souring"). Excessive nitrogen fertilizer
applications can also lead to pest problems by
increasing the birth rate, longevity and overall
fitness of certain pests.
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
fertilizer. This process is called
nutrient budgeting. 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.
Hazard of over fertilization
Over-fertilization of a vital nutrient can be as
detrimental as underfertilization.
"Fertilizer burn" can occur when too much fertilizer
is applied, resulting in a drying out of the roots
and damage or even death of the plant.
All organic fertilizers,
and some specially-formulated inorganic
fertilizers are classified as 'slow-release'
fertilizers, and therefore cannot cause nitrogen
burn. If excess nitrogen is present, some plants can
exude the excess through their leaves in a process
called
guttation.
Environmental toxicity of fertilizer
Toxic fertilizers are recycled industrial
waste
that introduce several classes of toxic materials
into farm land, garden soils, and water streams. The
consumption levels of toxic fertilizer are
increasing and this is leading to major environmental problems. The most common
toxic elements in this type of fertilizer are
mercury, lead, and arsenic.[60][61]
Between 1990-1995, 600 companies from 44
different states sent 270 million pounds of toxic
waste to farms and fertilizer companies across the
country.
According to the United States
Food and Drug Administration,
a) "Current information indicates that only a
relatively small percentage of fertilizers is
manufactured using industrial wastes as
ingredients, and that hazardous wastes are used
as ingredients in only a small portion of
waste-derived fertilizers."
b) "The EPA has continually encouraged the
beneficial reuse and recycling of industrial
wastes."
Example:
Steel industry wastes, recycled into fertilizers
for their high levels of
zinc can
include the following toxic metals:
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