Fertilizer chemicals: Pollutants in environment

Md. Rafiqul Islam

After nineteen seventy's many of the changes in our agriculture have been made possible through commendable technological advances such as, by using modern varieties of fertilizers and pesticides which result in the increase of agricultural production. On the contrary, the accelerating pace of such unconstrained technological application has already become a threat to our environment. Environment refers to the air we breathe, the water we drink and the food we take to sustain ourselves. Some of the fertilizer chemicals have adverse impact on such life supporting resources in environment. Among them nitrogen and phosphorus are the major nutrients of concern that are frequently applied in our crop field. In fact, crops are not enough efficient to use these and consequently a greater proportion of the chemicals is lost and/or some undesirable toxic substances are released in the soil, water and gaseous systems. Inappropriate use of fertilizer chemicals aggravates the problem resulting in deterioration of soil, water and food quality and in gaseous imbalance in the atmosphere. Therefore, fertilizer management in the crop field is the crucial issue for maintaining the quality of our life-supporting resources to meet the demand of the future generation.

Basis of fertilization: The fertilization is based on the principle that one has to supply as much as plant nutrient as the plant removes. This follows the rules of the law of minimum and the law of diminishing returns. In fact, the inherent supplies of readily available nutrients for Bangladesh soils are limited. The crops absorb nutrients from these insufficient supplies and the soils become exhausted. So, soils need addition of chemical fertilizers for supporting the growth and development of crops. However the accumulation pattern, recovery and losses of added chemicals in soils are of great concern in environmental pollution.

Proportion of fertilizers: In Bangladesh, despite manifold increases in fertilizer consumption, the rate of its use is still much lower than in Japan, Korea or China. But it is slightly higher than in India mainly for higher cropping intensity in Bangladesh. The actual application of various fertilizers in the country is disproportionate to standard nitrogen, phosphorus and potassium ratio of 1: 0.5: 0.5. Fertilizer use at farmers' level is most alarming to soil degradation and environmental pollution because nitrogen alone constitutes about 70 per cent of the total nutrient use in the country whereas, use of phosphorus and potassium is limited to about 20 per cent and 10 per cent, respectively. The recovery of fertilizer nitrogen in the crop field is only 30 per cent. In USA, the crops recover about 50 per cent of the total nitrogen applied. In Bangladesh, the fertilizer use efficiency is found much lower at farmers' field for all crops due to its non-judicious use, although the efficiency is comparatively higher in modern varieties of crops.

Off-site pollution: Plants require 16 essential elements of which nitrogen, phosphorus, potassium, zinc and sulfur have potential use in Bangladesh soils. The crops with high nutrient requirements are frequently deficit in nitrogen content, which is entirely met by urea fertilizer in the country. Ammonium sulfate has limited use particularly in tea growing areas. These are quite soluble and the crops do not use up greater portion of applied fertilizers due to loss in soil systems. Nitrogen containing fertilizers are converted to ammonium and nitrate in soil. The crops, especially wetland rice readily absorbs ammonium but most of the nitrates are transported with water down the soil profile. Fertilizers containing phosphorus use to increase algal growth in surface water and mosquitoes in shallow water into which they are washed. The process is called eutrophication that alters aquatic ecosystem by depleting oxygen in water. Thus nitrate and phosphate ion contamination of water from nitrogen and phosphorus content fertilizers causes off-site environmental pollution.

Soil pollution: Pollutants come from all types of human activity. These pollutants become a problem when they exist at high concentration or at high potency in low concentration. Manmade threats to our soil quality include improper cultivation, inadequate recycling of added fertilizers, accelerated erosion particularly in the hilly and terrace soils, accumulation of salinity in the coastal soils and addition of toxic agricultural chemicals like pesticides. Soil pollution often relates to diffuse sources. Leaching of nitrate, volatilization of ammonia, sediments and dissolved other fertilizer chemicals in runoff are all diffuse sources of pollution. Increased intake of fertilizers is continuously putting additional poisonous ingredients into the soil and causing serious pollution problem.

Contamination of water: Heavy rain or irrigation following nitrogen application causes runoff and leaching of widely used nitrogen fertilizer compounds. The source of soluble nitrogen in soil water are humus (37%), human and animal manure (22%), fixation by soil bacteria and algae (18%), rainfall (9%) and 13 percent may come from fertilizer added to soils. So, soils heavily fertilized above levels recommended are potential source of nitrate contamination in ground or runoff water. Phosphorus fertilizers are partly responsible in increase of water contamination. The major source of phosphorus contamination in surface water comes from direct damping of wastes and from eroded suspended solids from urban and agricultural land. About 21 per cant of total phosphorus supply in water may come from fertilized land.

Fertilizer nutrients and green-house effect: Earth is warming up and it is predicted that it will continue and accelerate in future. The cause of this warming is thought to be the so-called greenhouse effect. Water management and fertilizer application decisively influence the magnitude of greenhouse gas emissions including carbon dioxide, nitrous oxide and methane. Nitrous oxide content in the atmosphere has increased by about 25 per cent over the previous century. About one third of this increase is thought to be due to agricultural practices. The nitrous oxide is released during the process of de-nitrification, which is fueled by the presence of nitrates in the soil and in the organic matter cover. Heavy nitrogen fertilizer contributes a lot of these nitrates that undergo de-nitrification. The methane content of the upper atmosphere has doubled in the past hundred years, and there is no consensus as to the reasons for this increase. However, the soil is a known source of methane. Adding urea fertilizer to soil generally enhances methane emission, whereas sulfate content fertilizers depress emission.

Adverse health effect: Nitrate levels in drinking water upto 45 ppm are considered safe for human health. Many of the wells have been tested that exceed this value. Considerable quantities of nitrates and nitrite are present in food and their levels were found as high as 100 mg/kg. The acute dosages of nitrate for cattle are 50 mg/kg. Chronic intakes of 100-150 ppm nitrate may cause degeneration of vascular tissue of the brain, lungs, heart, liver and kidneys. Certain vegetables, including cabbage, lettuce, potatoes, several root vegetables and especially spinach contain relatively higher levels of nitrates. Comparisons of analytical values of nitrate found in crops in the past with that found recently give no evidence that crops today have higher nitrate contents as a result of using chemical fertilizers, although total nitrate concentration in the environment has increased. However the farmers are using excessive amount of urea fertilizer particularly in vegetable production. This may aggravate the problem of increasing nitrate in food as well as in drinking water and this needs investigation.

All the means and ways of environmental pollution from fertilization are decisively important for a probe. But a question is raised: "Can we supply the nitrogen need for high level of agriculture production without loosing nitrate to the environment?" To solve this and other related problems of fertilization researchers are contending for increasing nitrogen recovery by crops and decreasing loss of fertilizer elements to the environment. Extensive soil conservation and good management practices may bring such intended results. Soil test and crop response based fertilizer application with efficient use in respect of proportion, time and methods reduce the nutrient loss, runoff and leaching. Government effort is being carried on with the aim of conserving soil health. Soil Resources Development Institute (SRDI) under the ministry of agriculture is rendering an advisory soil testing service of cropping pattern based fertilizer recommendation throughout the country by 15 Laboratories and two soil-testing mobile vans. Other agricultural institutes and non-government organizations are also involved in such programme. Strengthening of this programme and farmers' awareness to adopt leguminous crop in the cropping pattern and green manuring practice with recommended dose of chemical fertilizer for maintaining soil fertility are essential for a comparatively pure and healthy environment for future.

Md. Rafiqul Islam is a Ph.D Fellow, BSMR Agricultural University and Scientific Officer, SRDI

Source: The Daily Star, Wednesday, August 8, 2001


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