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Fertilizers and Environmental Pollution

  • 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.

    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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