Why Agrochemicals?September 20, 2013, 9:13 pm
By Dr. C.S. Weeraratna
During the last few weeks, issues related to agrochemicals were highlighted in the print and electronic media. This article attempts to throw some light on the present state of knowledge on agrochemicals.
Fertilizers, pesticides (insecticides, herbicides, fungicides etc.) are categorized as agrochemicals. In the 20th Century and before, in Sri Lanka and in many other tropical countries, a system of shifting cultivation was practiced. This system was very cost effective and did not cause any damage to the environment as synthetic chemicals were not used. However, with the increase in the population, the need for specific crops in large extents increased and crop/s belonging to the same species was cultivated in the same piece of land (monocropping eg. rice, wheat) . Also, with the availability of irrigation facilities, farmers were able to cultivate crops two seasons a year. Availability of feed materials throughout the year saw the pest population increase, and the problems diseases arose. Among these pests are weeds, some families of insects, microorganisms such as bacteria, fungi, viruses etc. In Ireland, during the 1840s the potato crop was affected by a fungus Phytopthora infestans which resulted in severe food shortages and a few millions lost their lives. In Sri Lanka, in 1870 the coffee crop was lost hence, the growing use of pesticides.
When crops are continuously cultivated without keeping the land fallow, large amounts of plant nutrients such as nitrogen, phosphorus, potassium etc, are removed from the land resulting in the soil becoming deficient in these nutrients. Hence there is a need to apply fertilizers.
With the advancement of science and technology, scientists developed a large number of synthetic organic compounds which were able to control pests and diseases. One of the first synthetic pesticide discovered was DDT( Dichloro Diphenyl Tricholoethane) which was used to control the anopheles mosquito, which caused malaria, a disease widely prevalent in the 1940s, causing death to millions in the developing world. At present in Sri Lanka, around 12,000 tonnes? of pesticides are used. Of this total about 65% are herbicides, 22% are insecticides and 13% are fungicides. These pesticides, including those used to control household pests such as flies, are toxic compounds. Their degree of toxicity, indicated by their LD50 values, vary widely. Some are toxic and have low LD 50 values. However, most of the present day pesticides have very high LD 50 values indicating that they are less toxic. Pesticides used sometime ago are recalcitrant but those used at present decompose in a short period.
There are a number of advantages in the use of synthetic chemicals as pesticides. These compounds are more effective in the control of pests and diseases when they occur in a large scale. When there is an insect or a fungal attack on a large scale, application of an insecticide or fungicide is more effective. Many farmers use herbicides to control weeds. Manual control of weeds is far better, but the high cost of labour and sometimes the non-availability of labour prevent the farmers from manual weed control, and they resort to using herbicides.
Bio-pesticides, which are naturally occurring organic compounds also can be used to control some pests. Bio-pesticides are of low toxicity and have no effect on the environment. The organic compound Azadirachtin obtained from Neem (kohomba) is an example. Neem extract is a natural insect repellent and pesticide, There are many such plant species which can be used to manufacture pesticides. However, in our country no attempt appear to have been taken to develop and use such pesticides. Integrated Pest Management (IPM) systems can also be used to control some insects and pests. To control fungi it may be necessary to apply a fungicide. Crop varieties which are resistant to some of the pests have been developed but the continuous evolution of pests reduce the resistance of these crops. We could give up the use of these synthetic toxic chemicals to control pests if IPM methods are developed and applied . In countries such as India, pests are controlled mostly using IPM. However, in Sri Lanka IPM is little used.
When crops are cultivated, considerable amounts of nutrients such as nitrogen (N) phosphorus (P) and potassium (K) are removed from the soil. These have to be replaced if we are to sustain yields. Hence, the use of inorganic fertilizers increased in the mid ‘60’s .With the advancement of technology scientists developed numerous fertilizers such as urea, ammonium sulphate, super phosphate etc. Fertilizers such as potassium chloride, .magnesium sulphate etc. are simply extracted from huge deposits found in some parts of the world . Phosphatic fertilizers such as super phosphate are manufactured from rock phosphate (eg. Eppawela apatite) found in the ground. These fertilizers have a higher content of plant nutrients than the organic fertilizers such as green manure, compost etc. Inorganic fertilizers although they provide more plant nutrients do not improve the physical properties of soils such as soil texture, structure etc. Hence, organic fertilizers also need to be applied to maintain the fertility of soils.
Bio-fertilizers can replace inorganic fertilizers to some extent. Microbial processes such as atmospheric nitrogen brought about by bacteria found in the nodules of legumes, can be used to supply a part of the N requirement of plants but, for yields to be sustained the application of fertilizers may be necessary.
Toxicity of agrochemicals:
As indicated above, toxicity of pesticides indicated by their LD 50 values vary. Most of the pesticides and all fertilizers have very high LD 50 values. The toxicity of most of the present day pesticides decrease after exposure to the atmosphere. When these pesticides enter the soil they are absorbed, or form insoluble complex compounds with soil components, or decomposed by the soil organisms. Some are of the view that we should not use pesticides and fertilizers. But, if there is a pest attack or a nutrient deficiency, what is the alternative? The current yala paddy crop was seriously affected by the delay in applying fertilizers.
Of course, indiscriminate use of fertilizers and pesticides should be avoided. At present fertilizers are applied without appropriate soil analysis. As a result over-fertilization is possible. People can be exposed to pesticides by a number of different routes including air, water and food . Some farmers mix 2 or 3 insecticides hoping to get better results. Pesticide containers are not disposed properly, which may cause pollution of water bodies. Some farmers apply pesticides just before harvesting to increase the quality of their produce. Such undesirable practices cause toxic effects. It is extremely important that only the recommended amounts of fertilizers and agrochemical are applied.
There are reports to say that some farmers have been ale to obtain high yields without using any fertilizers and pesticides. Some have resorted to spiritual methods. It would be desirable if the Agric. Dept. could study these methods used by these farmers and popularize them if they are proved to be effective.
Chronic Kidney Disease:
Chronic Kidney Disease (CKD) is prevalent in some parts of the country. It can be caused by many factors such as diabetes, hypertension etc. Hence, all kidney diseases cannot be attributed to pesticides. The exact cause of a CKD prevalent in some parts of North Central Province is not known, and hence it is called CKD of unknown etiology (CKDu). Some attribute CKDu to pesticides and fertilizers. If so, it is not possible to explain why CKDu is not prevalent in other areas where fertilizers and pesticides are applied in large amounts. Even in NCP, the occurrence of CKDu is considerably higher in some DS divisions. For example, in 2010, 2378 CKDu cases were reported in Padaviya DS division but only 55 cases in Rajangana DS divisions, although both are agricultural areas where fertilizers and pesticides are used widely.
DCD in milk: Dicyandiamide (DCD ) is an agrochemical used to inhibit nitrification in soils. It will reduce formation of nitrates, thereby reducing evolution of nitrous oxide to the atmosphere which has an effect on the ozone layer. Although, milk containing insignificant amounts of DCD (less than 2-3 ppm) was banned, there are no reports to indicate that DCD is toxic. If it is toxic the cows that ingest DCD should also get affected. This issue needs to be clarified.
Last Updated Feb 27 2017 | 10:56 pm