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Fertilizers and Environmental Pollution
By Mohammad Ali Khaskheli
Agriculture Officer Sanghar
Fertilizers are organic or inorganic substances, either
natural or synthetic, used to supply elements (such as
nitrogen, phosphate and potash) essential for plant growth.
They are the most effective means of increasing crop
production and of improving the quality of food and fodder.
With them, food for more people can be produced than this
planet would otherwise support.
 Fertilizer
use is most effective (for obtaining high crop yields) on
soils with high natural or improved fertility, but even on low
fertility soils crop growth can be substantially improved.
Fertilizers are needed for all types of long-term crop
production in order to achieve yield levels which make the
effort of cropping worthwhile. Modern fertilizer practices,
first introduced more than a century ago and based on the
chemical concept of plant nutrition, have contributed very
widely to the immense increase in agricultural production and
have resulted in better quality food and fodder. As a
beneficial side-effect, the fertility of soils has been
improved resulting in more stable yield levels, as well as in
a better (nutrition-induced) resistance to some diseases and
climatic stress. Furthermore, the farmer's economic returns
have increased due to more effective production.
Over-application is a common problem in Asian countries.
Farmers want to ensure good yields, and apply so much
fertilizer that much of it is wasted. The problem is
particularly acute with cash and other horticultural crops,
which are high-value and produced very intensively in most of
Asia. In tropical and sub-tropical climates, provided there is
enough soil moisture, vegetable fields may bear as many as ten
successive crops a year, all of which may receive heavy
applications of fertilizer. In Pakistan fertilizer consumption
has increased threefold during the past 30 years. It reached
one million nutrient tonnes in 1980/81, two million tonnes in
1992/93 and three million tonnes in 2002/03. Nitrogen accounts
for 78 percent of the total nutrients, phosphate for 21
percent and potash for less than one percent. Like many other
developing countries, Pakistan does not apply high rate of
fertilizer, and therefore the danger of environmental
pollution is not yet high. However, with a development rate as
that at present, Pakistan will soon become the country using
high fertilizer application rate per area unit in the world.
Chemical fertilizer in the form of salts, when added to soils
gets converted into ionic forms after dissolving in the soil
solution. They are relatively safer than pesticides which
exhibit toxic properties on living systems. However, all the
quantities of fertilizers applied to the soil are not fully
utilized by plants. About 50 per cent of fertilizers applied
to crops are left behind as residues. Though, inorganic
fertilizers are not directly toxic to man and other life
forms, they have been found to upset the existing ecological
balance. The nutrients escape from the fields and are found in
excessive quantities in under ground water, rivers, lakes and
coastal waters.
Fertilizers can become a source of pollution when they are
used in excess. Among the three macro (N-P-K) fertilizers
being used at present, only potassium fertilizer is not yet
considered a source of environmental pollution. The other
substances like nitrogen (urea or calcium ammonium nitrate)
and phosphorus (DAP or MAP) fertilizers, if used unreasonably,
can cause environmental pollution and mainly through
accumulation of nitrate in agricultural products, drinking
water, eutrophication of water sources and accumulation of
cadmium.
Much of the concern about nitrogen in the environment is due
to the potential movement of unused or excess nitrate-N
through the soil profile into groundwater (leaching). Because
of its negative charge, nitrate-nitrogen is not attracted to
the various soil fractions. Rather, it is free to leach as
water moves through the soil profile. Soil type has an
influence on the amount and speed with which nitrate nitrogen
moves through a soil profile with movement greater on sandy as
compared to clay soils. In this concern the nitrogenous
fertilizers cause more harm then other fertilizers.
Nitrogenous fertilizers like urea under go extra cellular
enzymatic decomposition to form ammonium compounds, which are
either absorbed by the plant roots or converted to nitrates,
which are absorbed or lost in leaching or converted to gases
in the nitrogen cycle. The high nitrate rates can increase the
nitrate content of drinking water, vegetables, fruit juices...
and it is the direct cause of blue baby disease. The reason is
that when digested, nitrates (NO3) may be reduced to nitrites
(NO2) changing Oxyhaemoglobine (oxygen-carrying substance in
blood) to Methaemoblobine, a non-active substance, which at
high rates will influence activities of endocrine and form
tumours. Nitrite when entered in to human body may react with
amine to form Nitrosoamine, a cancer-forming substance.
Pregnant women should also avoid drinking water high in
nitrate. Recent studies suggest connections between
high-nitrate water and birth defects or miscarriages.
At present many people misunderstand that only chemical
nitrogen fertilizers are sources of nitrate pollution. In
fact, nitrate can be formed from soil organic matter, farmyard
manure, and agricultural by-products. Similarly the
underground water may be the source of nitrate (such as in
Runnels Texas State (USA) 3000 mg NO3/litre; where as the
WHO's standard is 50 mg NO3/litre) where excess chemical
fertilizers are not used. The main reason is due to
decomposition of organic matter after incorporation of crop
residues. Researches with N15 conducted by PPI (1996) have
also found that most of leached NO3 were not originated
directly from applied nitrogen fertilizer, but from organic
matter. Research results conducted at Rothamsted Station
(United Kingdom) have come to the similar conclusion: The
leached NO3 source came mostly from organic matter and crop
residues. Nitrogen coming from these sources in decomposing
cycle is easily leached and longer accumulated compared with
that coming from mineral fertilizer. Thus, the application of
organic fertilizers, crop residues or agricultural
by-products, crop increase in condition of high temperature,
high rainfall will be a big source supplying NO3.
Soils of Pakistan are poor in organic matter (below one per
cent) and the use of organic fertilizers is not common,
moreover the rates of chemical fertilizer use have not
increased the recommended levels. Therefore, under such
conditions the nitrate pollution in drinking water can not be
expected. However, safety measures and appropriate management
practices must be adopted at farm level, because water once
polluted with nitrate can not be reclaimed.
Another hazard associated with excessive use of nitrogenous
fertilizers is the gaseous loss of nitrogen, into the
atmosphere. High doses of carbon dioxide and ammonia that
escape into the atmosphere both from fertilizer manufacturing
plants and soils affect human health. Further the oxides of
nitrogen have been reported to adversely affect the ozone
layer, which protects the earth from UV radiation and heating
up of earth. The oxides of nitrogen cause respiratory diseases
like asthma, lung cancer and bronchitis. Arsenic, ammonia are
waste stream components of nitrogen manufacturing plants while
fluoride, cadmium, chromium, copper, lead and manganese are
waste stream components of phosphatic fertilizer industry. If
these waste streams of components are not properly disposed
they cause harm to human beings and animals with contamination
of air and water.
Phosphorus has been associated with environmental pollution
through the eutrophication of lakes, bays and non-flowing
water bodies. The symptoms are algal blooms, heavy growths of
aquatic plants and deoxygenation. The eutrophication of water
sources accelerates growth of algae causing competition for
oxygen with fish and other useful aquatic organisms.
Eutrophication of water bodies due to higher nitrate and
phosphate concentrations, increasing levels of nitrates in
drinking water sources, accumulation of heavy metals such as
lead and cadmium in soils and water resources are the
principal causes of environmental concerns due to irrational
fertilizer use in agriculture. There is no serious problem of
eutrophication in Pakistan at this stage, except some
localized problem due to mismanagement.
Cadmium accumulation in agricultural products is also an
important problem of pollution. This element is contained in
phosphorus fertilizers. At Rothamsted Experimental Station
(United Kingdom), with high rates of phosphorus fertilizer
being applied for hundreds of years, inconsiderable
accumulation of cadmium in agricultural products has been
found.
Phosphate fertilizers may contain cadmium when sedimentary
rock phosphate is used as raw material. Cadmium is also added
to the soil by aerial deposition. Soil chemistry also
influences cadmium mobility and uptake by plants. As with
other metals, low pH increases mobility. Absorption/desorption
of cadmium is about 10-fold more rapid than for lead. Chronic
cadmium exposures result in kidney damage, bone deformities,
and cardiovascular problems. A major human poisoning occurred
in Japan during World War II due to industrial contamination
of rice paddies. Since phosphate fertilizers can contain
significant cadmium concentrations, cadmium can accumulate in
crops, and human health problems can result from crop cadmium
contamination.
The use of phosphatic fertilizers in our country is still so
low (i.e. 12-13 kg P2O5/acre approx.), that the current
fertilizer usage presents no immediate hazard. However,
cadmium should be removed when raw materials are processed,
wherever possible and research should be undertaken to assess
cadmium build-up with continuous use of phosphatic
fertilizers.
Suggestions
A soil test should be performed before the purchase or
application of any "special purpose" fertilizers. It is not
possible to make a blanket statement that one fertilizer is
best for all crops every where. It is true that different
crops use different nutrients at different rates. The unknown
is the reserve of nutrients already in your soil. This changes
with every soil type, location and previous fertilizer history
of the soil and crop.
Lack of knowledge is widespread and is usually due to poor
coordination between those working in research and those in
the field working as extension officers. Local research work
is required into soil and crop conditions, balanced
fertilization, whether macro and micro-nutrients are required,
the use of animal manure and compost, the use of improved
seed, better cultivation and harvesting techniques, and the
economics of fertilizer use. Extension workers must make use
of demonstrations, preferably on farmers’ fields and keep up a
constant flow of information by farm visits and by the use of
radio and television. The availability of proper fertilizers,
according to soil type and crop demand should be assured at
right time.
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